Quartz as a Rock-Forming Mineral

 

last modified: Friday, 03-Oct-2014 01:10:52 CEST

Document status: unfinished, but usable  

Introduction

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Quartz is one of the most abundant minerals, and as a major constituent in many rocks it is an important rock-forming mineral. It is estimated that about 12% of the mass of the Earth's crust is made of quartz.

Under conditions at or near the surface, quartz is more stable than most other minerals and assumes a mostly passive role in the geological environment, and this is the reason why the billion years old quartz crystals from Brazil look just as fresh and new as Alpine rock crystals that formed "just" 10 million years ago.

But while it is chemically an almost inert and passive substance at the surface, quartz is a very active agent under conditions deep within the Earth's crust. Here, at higher temperatures and pressures, it participates in many complex chemical reactions during rock and mineral formation.

The image to the right has been made at Zion National Park, Utah, U.S.A., and shows sandstone made of "fossilized" Triassic sand dunes that has weathered into bizarre shapes. These rocks consist almost entirely of quartz.


 

Distribution of Quartz in Different Types of Rocks

Quartz is very unevenly distributed: Some rocks are entirely made of quartz, others are completely void of it. The rocks of the Earth's crust are subdivided into 3 classes (volume data from Okrusch and Matthes, 2005, after Ronov and Yaroshevsky, 1969):

 
Fig.1.1: The Earth's crust is made of 64.7% igneous, 7.9% sedimentary and 27.4% metamorphic rocks.

The largest amount of quartz is contained in igneous rocks, in particular in so-called granitoids, granites and related rocks. When one talks about the Earth's crust as a whole, volcanic rocks are more common at the surface than granitoids, because that encompasses the oceanic crust that is largely made of basalt and gabbro. These rocks have a very low quartz content or are void of it. On the other hand, the continental crust contains large amounts of granitic rocks that are rich in quartz, so almost all quartz is found in the continental crust that covers just about 40% of the Earth's surface[1].

Very high concentrations of quartz can be found in certain sedimentary rocks like sandstone, as well as in alluvial[2] and marine sands and sand dunes. On the other hand, limestones are also very common and typically have a very low quartz content. The abundance of sandstones and sands at the Earth's surface is a bit misleading, as sedimentary rocks cover most of the Earth, but only make up a small part of the Earth's crust volume, so the total amount of quartz in them is rather low.

Metamorphic rocks also show large variations in quartz content. The quartz content often reflects the mineral composition of the precursor rock. Very high concentrations of quartz are found quartzite and certain schists, while rocks derived from limestones and certain igneous rocks may be void of it. The total amount of quartz present in metamorphic rocks is lower than in igneous but higher than in sedimentary rocks. Metamorphosis is the only major process in which quartz is either produced or consumed and disappears from the environment during the formation of new minerals.

Finally, quartz and all the other silica polymorphs are absent from the Earth's upper mantle rocks, and the presence of silicon dioxide polymorphs stishovite and seifertite that are stable under these conditions in the lower mantle is uncertain.


 

Quartz Content and Silica Content of Rocks

Due to the importance of quartz in geochemical processes, rocks are often classified both by their quartz content and by their silica content. Quartz is one form of silica, SiO2, but the terms "quartz content" and "silica content" have a very different meaning.

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For example, a rock that is entirely made of albite, NaAlSi3O8, a feldspar, has 0% quartz content, but contains 77.3% silica (weight percent).

It was once customary to express the chemical composition of a mineral as an oxide formula (mostly because the structural relationships between the elements involved were not precisely known), and it is still useful if one reflects the silica content. So in the case of albite, instead of

   NaAlSi3O8

one could as well write:

   Na2O ⋅ Al2O3 ⋅ 6 SiO2

which reads as 1 part sodium oxide, 1 part aluminum oxide, and 6 parts silicon oxide, silica.
Or, maintaining the number of atoms in the original formula:

   0.5 Na2O ⋅ 0.5 Al2O3 ⋅ 3 SiO2

And in fact, one could take 1 part of Na2O, 1 part of Al2O3 and 6 parts of SiO2, put them in a pot, turn on the heat, and after some time one would get albite.

Finally, there is the term free silica that can have two meanings:


Silica and quartz content in an imaginary rock

To get a feeling for the numbers used in rock classification, I have calculated the silica content of an imaginary rock made of 20% quartz, 60% orthoclase feldspar and 20% muscovite mica, and presented the results in Table 2.1 below. The composition of this rock is a bit simple, but it would qualify as a so called "alkali-feldspar granite".

Regular Formula Oxide
Formula
Molar
Weight
Weight
Percent
Contribution to
Total Composition
Quartz
20%
SiO2 1.0 SiO2 60.0843 100% × 20% = 20% SiO2
Orthoclase
60%
KAlSi3O8 0.5 K2O
0.5 Al2O3
3.0 SiO2
94.1906
101.96128
60.0843
16.9%
18.3%
64.8%
× 60% = 10.1% K2O
× 60% = 11.0% Al2O3
× 60% = 38.9% SiO2
Muscovite
20%
KAl2[(OH)2AlSi3O10] 0.5 K2O
1.5 Al2O3
3.0 SiO2
1.0 H2O
94.1906
101.96128
60.0843
18.0152
11.8%
38.4%
45.3%
4.5%
× 20% = 2.4% K2O
× 20% = 7.6% Al2O3
× 20% = 9.1% SiO2
× 20% = 0.9% H2O
Total
100%
= 12.5% K2O
= 18.6% Al2O3
= 68.0% SiO2
= 0.9% H2O
Table 2.1: Computing the silica content of an imaginary rock.

So if we sum up the oxide components of this imaginary rock, we get 68.0% silica, SiO2, although the rock contains only 20% quartz, or 20% free silica.

Compound Molar
Weight
Weight
Percent
Mol
Percent
K2O
Al2O3
SiO2
H2O
94.1906
101.96128
60.0843
18.0152
12.5%
18.6%
68.0%
0.9%
8.9%
12.2%
75.6%
3.3%
Table 2.2: Mol percent versus weight percent values for the imaginary rock.

Fig.2.1: Oxide composition of the imaginary rock.

Depending on the method used for analyzing the samples, the mineral composition and the chemical composition of a rock (corresponding to the quartz and the silica contents, respectively) is usually given either as weight percent or as volume percent. It is interesting to look at the relative number of molecules in the composition, which is given as mol percent. The translation from weight percents into mol percents is fairly straightforward and the result is shown in Tab.2.2 to the right. Because SiO2 has a relatively low molar weight, the values for weight percents are usually lower than those for mol percent, in this example case 68 weight-% versus 75.6 mol-% for silica.

This is visualized in Fig.2.1, showing pie chart representations of the oxide composition of the example rock. The top row shows the proportions for weight percent, the bottom row for mol percent, in both cases with the overall composition to the left, and the contributions of the minerals in the rock to the right. Obviously the orthoclase feldspar contributes the bulk of the silica content, not the quartz. Although this is just an idealized example, it is not unrealistic, and similar proportions are found in many igneous rocks.

There is another interesting point to note: the "water content" of the rock. Because its molar weight is much lower than that of the other oxides, the contribution of the water to the overall weight is very low (0.9%), and this is a bit misleading, as its molar content is more than 3 times as high (3.3%). Of course, this rock does not contain any free water in the sense it contains free silica (as quartz). The water listed in the oxide composition of the rock is bound as hydroxyl groups (OH) in muscovite. However, upon changes in the environmental conditions, such bound water may play a role in chemical reactions or may be released from a mineral as free water. At high temperatures, water is a very aggressive and a very mobile agent that promotes many chemical reactions between minerals that would otherwise take place at a much lower speed. So even that little water may play a big role in a rock's chemistry. And - equally important, as we will see later - it has great influence on the physical properties of magma.


 

Silica, Silicates and Rock Chemistry

Now that the terms silica content and quartz content have been clarified, we can look at the relation between the silica content and the mineral composition of a rock. Again, we start with a simple example.

If one melts a mixture of quartz SiO2 and magnesium oxide MgO in the right proportions in a pot, once the melt has cooled down and solidified again, one would yield a block of the silicate mineral forsterite, containing isolated SiO4 tetrahedra:

SiO2 + 2 MgO → Mg2SiO4  (Forsterite)    [1]

Fig.3.1: Oxide composition of forsterite

Why did I pick the oxide of magnesium and not some other metal oxide?
Because in the overall composition of the planet Earth, magnesium is a very important component and in many cases an indicator of the origin of a rock: forsterite is the principle component of olivine[3], a very common rock-forming mineral in the upper Earth's mantle. Certain mantle rocks, like peridotites and dunites, contain more than 90% olivine. If we look at the pie chart representation of the silica content of forsterite (Fig.3.1), we see that it is much lower (42.7 weight-% and 33.3 mol-% SiO2) than that of the granite shown above (Fig.2.1).

Of course it is much lower, you may say, that lies within its formula.
The interesting point is this:

Forsterite cannot form in the presence of quartz.
Quartz cannot form in the presence of forsterite.

Both minerals are mutually exclusive in a rock[4]. The reason for this behavior lies in an important property of silica: The SiO4 tetrahedra have a strong tendency to polymerize. They form larger groups by sharing oxygen atoms, either by addition of orthosilicic acid molecules, for example

[SiO4]4- + H4SiO4 → [Si2O7]6- + H2O + 2 H+    [2], a group of 2 tetrahedra

[SiO4]4- + 2 H4SiO4 → [Si3O9]6- + 3 H2O + 2 H+    [3], a ring of 3 tetrahedra

or, under dry conditions, simply by the addition of silica, for example

n [SiO4]4- + n SiO2 → n [Si2O6]4-    [4], a chain of tetrahedra

So when silica reacts with metal oxides to form silicates, it can be incorporated into the new minerals as individual SiO44- groups, like in forsterite (formula [1]). But it might as well form chains, rings, sheets and networks of SiO4 tetrahedra. Whether it actually forms chains or networks, or just remains as individual SiO44- groups depends on many parameters, including the amount of silica present in the rock.

Fig.3.2: Oxide composition of enstatite

We get back to our example of mixing magnesium oxide and quartz in a pot, but this time we use twice the amount of quartz used in formula [1]:

2 SiO2 + 2 MgO → Mg2Si2O6  (Enstatite)    [5]

We get the mineral enstatite, a member of the rock-forming mineral group of pyroxenes. The higher silica content is shown as pie charts in Fig.3.2 (58.9 weight-% and 50 mol-% SiO2). In fact, enstatite and all the other pyroxenes are chain silicates, that is, their SiO4 tetrahedra form infinite parallel chains, just as suggested in formula [4] above.

The same result can be achieved by mixing forsterite with quartz:

SiO2 + Mg2SiO4  (Forsterite) → Mg2Si2O6  (Enstatite)    [6]

So this is the reason why quartz and forsterite do not form together in a rock, they will react to form enstatite. And given what has been said about the abundance of olivines in the Earth's upper mantle, we may conclude that very likely there is no quartz in the upper mantle.

What if we add more quartz to the mixture? In this simple system of just 4 components, nothing: no other mineral than enstatite will form, and any extra amount of quartz will simply be left over in the mixture. Thus one can say: The presence of enstatite is compatible with the presence of quartz in a rock.

Fig.3.3: Oxide composition of anthophyllite

In the presence of water and a bit more silica, the mixture will turn into a different mineral, anthophyllite:

8 SiO2 + 7 MgO + H2O → Mg7(Si4O11)2(OH)2  (Anthophyllite)    [7]

Anthophyllite is a member of the rock-forming mineral group of amphiboles, and like the pyroxenes, these are chain silicates, but while pyroxenes contain single chains of SiO4 tetrahedra, amphiboles contain double SiO4 chains and hydroxyl groups (OH). As shown in the pie charts in Fig.3.3, because water is also added, the silica content of the mineral is not increased as a whole, but relative to the MgO content.

We could do as we did in formula [6] and achieve the same result by mixing enstatite, water and silica:

2 SiO2 + 2 H2O + 7 Mg2Si2O6  (Enstatite) → 2 Mg7(Si4O11)2(OH)2  (Anthophyllite)    [8]

If enstatite and quartz occur together as rock forming minerals in a rock, what does this mean? They probably cannot form together in the presence of free water, for any excess of silica would react with enstatite, and one would instead find a mixture of enstatite and anthophyllite. So the conditions during their formation have probably been rather dry. Of course, this is an idealized picture, and other minerals present in the rock could either promote or inhibit this reaction.


Fig.3.4: Oxide composition of talc

Still more water and more silica, and we will get talc, except that talc will not simply form by mixing and heating MgO, SiO2, and H2O, it is a complex product formed by the alteration of other minerals, so formula [9] is idealized and does not reflect the actual process of its formation:

4 SiO2 + 3 MgO + H2O → Mg3(Si2O5)2(OH)2  (Talc)    [9]

The structure of talc is very different from that of pyroxenes and amphiboles: it is a sheet silicate in which the SiO4 tetrahedra are arranged in two-dimensional infinite sheets and the metal and hydroxyl ions are placed between them. The differences in their structure are reflected in the differences in their physical properties: while forsterite, enstatite and anthophyllite are very hard minerals that may form centimeter-sized crystals, talc is very soft and made of tiny flakes, because the individual SiO4 tetrahedra sheets are almost freely movable relative to each other.

To summarize, because SiO4 tetrahedra tend to polymerize and arrange themselves in large groups, different minerals can form from the same metal oxides and silica, depending on the amount of silica in the rock, but also on other parameters, like the presence of water.



 

Basic Classification of Igneous Rocks

The term "igneous rocks" encompasses both volcanic (extrusive) and plutonic (intrusive) rocks. When classified by their overall chemical composition, most plutonic rocks have a volcanic counterpart of equal composition and vice versa. However, this is not necessarily true for the mineral content of these rocks. While, for example, in a granite the excess free silica is entirely contained in the quartz crystals, in a rhyolite (the volcanic counterpart of granite) all the free silica may be bound in a glassy matrix.

 

Basic Terms Used for the Classification of Igneous Rocks

There are two alternative approaches for classifying igneous rocks:

Which one is preferred is mostly a practical matter. Plutonic rocks are commonly classified by their mineral content, because it is possible to identify minerals and determine their relative content by optical methods, and very often even by visual inspection. Volcanic rocks, on the other hand, often consist of a fine-grained or even a glassy matrix with minerals embedded in them. This makes a visual identification of minerals impossible and one has to turn to chemical methods to determine the type of rock.

Two pairs of antagonistic terms are often used to characterize igneous rocks, and both have to do with the silica content of the rock:

Fig.4.1: Classification of igneous rocks by their silica content

As silica is the anhydride of orthosilicic acid, igneous rocks that are rich in silica (more than 63%) are commonly called acidic rocks, while those with a low silica content are called basic rocks. Rocks with medium silica content are called intermediate rocks, and rocks with less than 45% silica are called ultrabasic (see Fig.4.1). So a rock that is entirely made of orthoclase feldspar (KAlSi3O8) will be called silica rich and acidic, as it contains 64.8% silica (see Tab.2.1). A peridotite rock, composed largely of olivine, (Mg,Fe)2SiO4, is called ultrabasic, as it contains only about 40% silica. Igneous rocks that contain quartz are almost certainly acidic rocks.

The term felsic, a word composed of the first syllables feldspars and silica, refers to generally light-colored minerals in an igneous rock that are solely composed of Si, Al, K, Na, Ca, and O. This includes feldspars and quartz, of course, but also the so-called foids, short for feldspathoids. Foids partially or completely replace feldspars in rocks of low silica content. They are similar in appearance to feldspars, but they differ in their structure and have a lower SiO2 content. The presence of foids in a rock is a safe indication that there is no quartz in this rock.

The term mafic originally referred to minerals rich in magnesium and iron (ferrum in Latin). These minerals are often dark, in particular if they contain a lot of iron. But mafic minerals are more openly defined and include any minerals that are not felsic, like micas, pyroxenes, amphiboles, garnets, oxides and minor components of a rock, like zircon, apatite, etc. Rocks that contain more than 90% mafic minerals are called ultramafic rocks.

Bright-colored rocks dominated by felsic minerals tend to be acidic rocks, and rocks with a low content in felsic minerals (and thus high content of mafic minerals) tend to be basic rocks. But in particular for felsic minerals this relationship is not always true.

 

The QAPF Classification

A generally accepted classification scheme that is based on the mineral content is the QAPF scheme, introduced by Streckeisen in 1974. The classification is based on 5 groups of minerals:

Quartz
Alkali-Feldspar
Plagioclase Feldspar
Foids
Mafic minerals

To classify an igneous rock, only the felsic mineral components are considered and the relative content of Quartz, Alkali-Feldspar, Plagioclase, and Foids within the felsic content is determined, the mafic components are ignored. This scheme is only applied to rocks with at least 10% of felsic minerals (= less than 90% mafic minerals).

For practical reasons, the values are given as volume percent: the rock is analyzed in a petrographic microscope, in which the relative volumes of the various minerals are optically determined. Since the specific density of the felsic minerals is mostly between 2.5 and 3.0, the numbers do not differ too much from weight percentages.

Fig.5.1: Example for the relation between the total composition and the QAPF composition of a rock

The Mafic minerals in the rock can be used for a more detailed classification, and rocks with more than 90% mafic minerals are solely classified by their mafic mineral content, but these classification schemes will not be discussed here.

So if a rock is made of 30% mafic and 70% felsic minerals, and contains 10% quartz, 40% alkali-feldspar and 20% plagioclase, the QAPF values given for quartz, alkali-feldspar and plagioclase are 14%, 57%, and 39%, as shown in Fig.5.1. This would be a granitoid rock, a so-called "quartz-bearing syenite", and the mafic minerals might be various micas, amphiboles, garnets, etc.

Because foids and quartz are mutually exclusive in a rock, the QAPF classification is always based on a maximum of 3 components.


Fig.5.2: The QAPF Diagram for Igneous Rocks

The QAPF values can be plotted into a diamond-shaped coordinate system, the QAPF diagram (Fig.5.2), sometimes called Streckeisen diagram, named after the author. It is made of 2 triangles, so-called ternary diagrams with the corners Q, A, P and F, A, P), adjoined to each other along their A-P edge. A ternary diagram is a coordinate system with three axes: each point in the space within the triangle represents a different proportion of the 3 components. The corners represent cases in which only one component is present, here 100% of quartz, alkali-feldspar, plagioclase or foid. If you are not familiar with these diagrams, check the appendix Ternary Diagrams for a visualization of the idea behind it.

The QAPF diagram is divided into 15 fields that define ranges of mineral compositions for the different classes of rocks. Some of the fields are subdivided (see Table 5.1 below for a partial assignment of rock names to the numbers, and a full list at the appendix The QAPF Diagram).

Every rock whose QAPF composition falls in the range of the upper triangle contains some quartz, every rock type defined in the lower triangle is void of it and contains foids instead. Rocks with compositions along the A-P line contain only feldspars and mafic minerals.

Probably the most important field for the quartz collector is field 3, the field for granites. A granite is defined as follows:
"Any intrusive igneous rock with less than 90% mafic minerals, a content of 20% - 60% quartz and between 90 % and 35% of alkali-feldspar in the remaining volume."

This, of course, is a lengthy definition, and one benefit of using the QAPF diagram is that it is much easier to memorize the locations of the fields in the QAPF diagram than to memorize all the lengthy definitions of 15 different rock types.

The other benefit of the QAPF diagram is that one recognizes systematic neighborhood relationships between the rock types, for example, how a monzonite differs from a syenite or a granite.

Table 5.1 below lists a few important plutonic rocks and their volcanic counterparts. A full list of all rocks defined in the fields in the QAPF diagram is found in the appendix The QAPF Diagram. Rocks that are most common and occur in large volumes at the surface are marked bold. Interestingly the silica rich plutonic rocks granite and granodiorite are more common than their volcanic counterparts rhyolite and dacite, whereas the silica-poor volcanic rocks andesite and basalt are much more common than the plutonic rocks gabbro, diorite and anorthosite.

Some of the fields are assigned to two or three rock types (for example field 10), these rocks are distinguished by their mafic mineral content.


Field Number Plutonic Rocks Volcanic Rocks
1a
3
4
5
7
8
9
10
11
14
Quartzolite
Granite
Granodiorite
Tonalite
Syenite
Monzonite
Monzodiorite / Monzogabbro
Diorite / Gabbro / Anorthosite
Foidsyenite
Foiddiorite / Foidgabbro
-
Rhyolite
Dacite
Plagidacite
Trachyte
Latite
Andesite
Basalt
Phonolite
Tephrite / Basanite
Tab.5.1: Important igneous rocks in the QAPF diagram. The rocks written in bold are most common and occur in large volumes.


 

Igneous Rocks

Quartz occurs as a rock-forming mineral in igneous rocks if the overall silica content of the original magma surpasses 63-65 weight-percent. When the magma cools, different minerals will successively grow in the still liquid environment[5]. While certain ions get concentrated in the mineral grains, the silica and the water content of the residual liquid increases. One of the last minerals to form is quartz, either as its high-temperature polymorph, beta-quartz, or as regular alpha-quartz. Because it simply fills out irregular spaces between already present feldspar and mica crystals, it usually does not develop crystal faces[6]. The longer the cooling of the magma takes, the larger the final mineral grains will be. Large bodies of intrusive igneous rocks that do not appear at the earth's surface cool very slowly and turn into granites and related igneous rocks with their well-known "pepper-and-salt-structure". Extrusive igneous rocks (better known as volcanic rocks) with a similar chemical overall compositions are rhyolites. If the magma cools very quickly, there is not enough time for a differentiation of the magma into different minerals and the formation of crystals, and a natural glass, obsidian, will form that does not contain any quartz.

Intrusive or Plutonic Rocks

Intrusive igneous rocks are easily recognized by their texture: they are composed of irregularly intergrown mineral grains of different sizes and colors. The majority of igneous rocks are of granitoid composition and contain quartz, granites, granodiorites, quartz monzonites and quartz monzodiorites are most common.


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Granite composed of red potassium-rich feldspar orthoclase feldspar, white and creme-colored plagioclase feldspar, dark and shiny flakes of biotite mica, and gray irregular quartz grains of vitreous luster. The green spots are epidote. Its overall hardness, the fact that the mineral grains are tightly interlocked and the fact that most granites lack a preferred direction of cleavage give granite a great strength and make it a good material for buildings and sculptures. From a granite quarry at Priatu, Sardegna, Italy.



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A granodiorite is a granitoid rock with a larger amount of plagioclase feldspar than granite (field 4 in the QAPF diagram). It typically contains more mafic minerals, in this case biotite mica, and thus often looks a bit darker than most granites. This specimen has been picked up at a tunnel excavation at the Gerstenegg facility of the Kraftwerke Oberhasli, Haslital, north of the Grimsel pass, Bern, Switzerland. The local name of this rock is "Grimsel-Granodiorit". This is a fresh rock that has not been subject to weathering, nevertheless one can see that the plagioclase feldspar has partially suffered chemical alteration to epidote and turned slightly green, a phenomenon often seen in rocks containing plagioclase. The rock has been subject to medium grade metamorphosis (greenshist to amphibolite facies), but except for the alteration of plagioclase, the mineral composition and texture did not change very much because it was already subject to very high temperatures at its formation.


Fig.7.1
 
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Figure 7.1 shows the "Gross Spitzkoppe" (1728 m), a granite dome in Namibia. Its peak is more than 700 meters higher than the plain. It is the tip of a large body of granite, a so-called pluton, that intruded into much older metamorphic rocks during the cretaceous age. So the peak has once been covered by softer rocks that could not withstand the erosional forces and that have been eroded with time. The surface of the mountain is not necessarily the boundary of the former intrusion, more likely the outer parts have already weathered away and what one sees now is the more resistant core. A pluton is often formed by several successive intrusions of magma, and is thus not necessarily a homogeneous body. In the middle you can see the roofs of other parts of the pluton that slowly and gradually appear at the surface - the Gross Spitzkoppe is much like an iceberg of granite that swims in a sea of older rocks and its own granite debris.

Fig.7.2
 
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The Brandberg in Namibia (Fig.7.2) is an almost circular aggregate of 7 granitic plutons that is about 30 km in diameter and up to 2574 m high. In its immediate vicinity, but not within its mountains, there are numerous pegmatites that are exploited for gems and rare elements, and that are also famous quartz locations.


Intruded plutons may merge into a very large body that extends over hundreds of kilometers, called a batholith. Batholithes are typically found where an oceanic plate subducts under a continental plate. Complex processes of magmatic differentiation at these plate boundaries lead to an enrichment in silica and water, a lowering of the melting point and a decrease in specific density of the magma. As a consequence, the magma starts to rise along the plate boundary and either intrudes overlying rocks as granite-type rocks or erupts at the surface to form volcanoes.

Fig.7.3
 
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The Sierra Nevada in California is essentially the outcrop of such a batholith, made of granite-type rocks. It has been slightly tilted by tectonic forces, so the Sierra Nevada has a very steep eastern slope, as shown in the Fig.7.3. It is a westward view from Owens Valley, south-east of Lone Pine. The distant peak to the right that is almost free of snow is Mount Whitney (4421 m).


Fig.7.4
 
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The rocks into which a granite body intrudes exert a great pressure on the plutonic body because of their weight. When these rocks erode, this stress is relieved, and while the granite pluton slowly approaches the surface, joints develop in the pluton's outer zone, and the granite parts into onion-like concentric sheets. Since these cracks develop long after the magma has intruded and cooled down, there's hardly any mineralization along these joints. This is the peak of the Schlossberg, east of the city of Weiden in the Oberpfälzer Wald, Bavaria, Germany. On top you see the ruins of Flossenbürg castle.


Fig.7.5
 
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Granite is the epitome of tough, durable rock. But while it is much more durable than limestones, sandstones or marbles, for example, it is not as stable as most people believe, because it is mostly made of minerals that are not very stable at surface-conditions: feldspars. These will slowly weather and form clay minerals that will be important components of soil. Granite is also slightly porous, so moisture can penetrate the rock and dissolve minerals. Quartz and micas are more stable and will remain in the grainy soils.

In dry climates weathered granite often assumes bizarre, rounded shapes that develop because the surface peels off in thin onion-like sheets. The image shows a granite formation near the Gross Spitzkoppe, Namibia. As a scale note the tree to the left.


Fig.7.6
 
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The surface peeling is driven by chemical weathering mediated by the water in the pores of the granite. In dry climates the rock's surface will dry more quickly if it is exposed to the sun, while the weathering will proceed at a greater speed in small crevices and shadowed, lower parts that will stay moist for a longer time. In the long run, this leads to the formation of overhanging walls, small caves or, as shown in the picture, rounded granite boulders that eventually roll downhill. Seen in the north-eastern Erongo Mountains, Namibia.


Fig.7.7
 
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In humid environments chemical weathering dominates and acts primarily along already present joints, forming deep and mostly straight crevices in the rock. The result can be rounded rectangular rocks, but also a jagged landscape, like the one show in the image, a westward view of Costa Paradiso, Sardegna, Italy.



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Granitoid rocks do not always form large rock bodies, they may occur in granite veins that cut through other rocks. Very often these veins show special textures, with transitions to pegmatite veins, or a zonar structure with graphic granite that usually accompanies pegmatites. This specimen shows a graphic granite from a small granitic vein that intruded into gabbro. The left and right part of the specimen is made of epitactically intergrown quartz and cream-colored microcline feldspar crystals that form a finely patterned graphic granite, the central zone consist of larger crystals. In the lower left corner one can see white calcite, probably a late formation that filled out a small cavity. Miarolitic pockets have been frequently found in granite veins at this locality. The large elongated black crystals are biotite. From the "Gabbro" quarry, Bad Harzburg, Harz Mountains, Lower Saxony, Germany.


Volcanic and Subvolcanic Rocks

Fig.7.8
 
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Although volcanic rocks exhibit a great number of different textures, it is mostly the texture that is used to identify and classify a rock as volcanic. In the strict sense, volcanic rocks have to be ejected from a volcano during an eruptions to qualify as extrusive igneous rocks, but the rocks that form from the magma that stays inside the volcano are nevertheless called volcanic, sometimes more specifically subvolcanic. There is a gradual transition from volcanic to subvolcanic to intrusive igneous rocks.

Figure 7.8 shows Damavand, 5610 m, a typical composite volcano in the Alborz mountains north-east of Tehran, Iran. It is made of of intermediate volcanic rocks, mostly trachytes and andesites.



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A subvolcanic rock of andesite composition with a porphyric texture. This texture is only found in volcanic rocks: More or less well-formed crystals are embedded in a fine-grained, often homogeneous matrix, in this case white plagioclase feldspar crystals, black elongated amphibole crystals and black biotite mica flakes. The matrix can also be natural glass, which makes a classification of a rock difficult. Andesites are intermediate rocks with respect to their silica content and may or may not contain quartz. Even if there is no free silica in the fresh rock, it might be released later during alteration or weathering. From the Karkas Mountains at Josheghan Qali village, Isfahan Province, Iran.



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Quartz rarely plays a direct role as a rock-forming mineral in volcanic rocks. One of these cases is rhyolite with a porphyric texture that contains well-formed quartz crystals in a fine-grained matrix. These crystals have been formed in the magma at high temperatures and low pressures (close to the surface in a volcanic vent), so they initially formed as high-quartz and assumed their typical stubby, bipyramidal shape, with a diameter of up to 1 cm in the specimen shown to the right. Because they are translucent, they look dark-gray. Depending on their orientation, their cross-section is either hexagonal or almost square. The gray-green matrix also contains many partially decomposed white feldspar crystals with hematite powder in the tiny cavities. Grosser Auerberg north of Stolberg in the Harz mountains, Germany.


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A small polished slab of the same rhyolite rock from the Grosser Auerberg that much better shows the ideally developed, biterminated, gray to brown quartz crystals embedded in the gray-green matrix next to creme colored feldspars. One can recognize a flow texture in the rhyolitic matrix. The rock has undergone substantial alteration that led to the partial dissolution of the feldspars. Nevertheless the quartz crystals appear internally uneffected. It is typical for quartz grains in a rock that even when the quartz gets attacked and dissolved, the interior of the crystals does not show any signs of alteration, whereas many other minerals show signs of a decomposition inside the grains. The reason is simply that the quartz lattice is very tight and inhibits the inward diffusion of ions from the surrounding fluids, so chemical reactions can only take place at the crystal surface.



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This bright-colored rock has a rhyolitic to dacitic composition (sometimes called "rhyodacite", although that is not an officially approved term). It is mostly made of small feldspar crystals, irregular grains of quartz and black, shiny hexagonal flakes of biotite mica and an unknown yellow mineral, possibly an alteration product of an amphibole. This is a very tough rock, and the locals call it "granite", and in fact its mineral composition is probably that of a granite or granodiorite.

This rock is as acidic (silica-rich) as its granitoid counterparts, so there is a good chance to find quartz varieties in cavities. If these actually form or not also depends on the water content of the magma, or the availibility of hydrothermal brines from other sources, as water promotes the alteration of the rock, the formation of cavities during cooling and the dissolution and transport of silica through the pores of the rock. This rock contained only very few very small cavities with small quartz crystals, it was apparently very dry during its solidification. From a subvolcanic dome in the Karkas Mountains at Josheghan Qali village, Isfahan Province, Iran.

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This is the origin of the rock shown in the previous image, one of numerous small subvolcanic domes made of rhyolitic to dacitic rocks that intruded into overlying sedimentary rocks (limestones and conglomerates), but did not quite make it to the surface. From the distance these domes look exactly like a small granite pluton. The mountains in the background are made of limestones of Miocene age. East of Josheghan Qali village, Isfahan Province, Iran (note people for scale).



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Amygdoidal textures, characterized by small almond-shaped mineral-filled vesicles in a fine-grained or glassy matrix, can only be found in volcanic rocks. The vesicles, former gas bubbles in the lava, are commonly filled by zeolite minerals, opal and chalcedony. These minerals are all typical for low-temperature environments and have formed long after the rock has solidified and cooled, and are a product of hydrothermal alteration. This alteration will release silica even in intermediate and basic volcanic rocks that by themself do not contain any quartz. This rock comes from a locality where andesites, basalts and rhyolites occur, but although it contains opal-filles vesicles, I can't tell which type of rock it is without a chemical analysis of the matrix. From Sibley Volcanic Park, east of Berkeley, Alameda County, California.


A high content of silica has two effects on the behavior of a lava:

As a result, silica-rich lava is tenacious and flows slowly, and also has strong tendency to solidify as a volcanic glass. The glass may be massive, as the well-known obsidian, but the spongy pumice is also a volcanic glass. In line with their high silica content, many obsidians are rhyolitic in composition, and most of them contain a small amount of water, too (as molecular water as well as bound in silanole groups). By contrast, basaltic lava will form volcanic glass of comparable homogeneity only if it is cooled very quickly. For example, the rims of pillow lavas are made of volcanic glass that formed when the lava was quenched by sea water. However, the matrix of intermediate and basic volcanic rocks with a porphyric texture does often contain a large amount of glass or even consist entirely of glass.


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Jewelery grade, shiny black obsidian. Bright lines that run through the rock indicate the direction of flow. Obsidian forms rather short lava flows and does not cover large areas as basalt lava does because the lava is highly viscous. From Davis Creek, Modoc County, California.


Volcanic glass  is generally not a stable rock. At near-surface conditions, forms with a large surface (pumice, for example) tend to weather, while the massive obsidian is more stable. When a volcanic glass stays hot or is heated up by successive lava flows, it may experience devitrification ("un-glassing"). Then minerals (that is, crystals) grow inside the glass matrix, forming gray or white patches and rims. Unaltered obsidian and pumice deposits are generally young, rarely are they older than a few million years.


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Pumice is a volcanic glass with a sponge-like structure and an accordingly very low density. Like obsidian, most pumice is of rhyolitic or dacitic composition (and thus rich in silica), and often very light-colored. Contrary to obsidian, pumice is ejected from the volcano in often violent eruptions and its deposits can cover large areas. Due to the metastable nature of glass and the large surface it weathers readily. From Bend, Oregon.



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In snowflake obsidian parts of the black volcanic glass have started to crystallize. The crystal clusters grew more or less radially from randomly distributed spots into the highly viscous lava, forming gray patches that resemble snowflakes. The crystalline patches usually consist of the silica polymorph cristobalite or feldspars. This phenomenon can only be observed in obsidians of high silica content, like rhyolites. This tumbled obsidian specimen is of unknown provenance, but could be from central Utah.



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Sometimes mineral growth in obsidian causes the formation of spherulites, spheres of radially grown crystals, often with concentric rings (spherulitic growth is also seen in agate, but certainly took place at low temperatures in a watery environment). Some of the spherulites in this specimen are hollow and outlined with small mineral pustules. Among the minerals formed inside obsidians are feldspars and silica polymorphs, most commonly cristobalite. This specimen of shiny black obsidian with spherulites is from a quarry at Rocchia Rosse, Acqua Calda, Lipari, Italy.


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Close-up of the sherulites in the previous specimen. It has been estimated that such structures grow within days to weeks (Watkins et al., 2009).



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This rather unusual ryolite came from a narrow dike-like zone very close to the top of Tillie Hall Peak, an old volcanic vent, south of Mule Creek, Graham County, New Mexico. The dark glassy flakes are irregular quartz crystals that lack crystal faces. They are all surrounded by a "halo" of bright matrix. It is possible that once euhedral quartz crystals were again resorbed in parts by the magma.


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Sedimentary Rocks

Quartz tends to accumulate in deposits of eroded material, both due to its physical and chemical resistance and because it is often formed from silicate minerals during chemical weathering. If you ever traveled through a desert, you might have noted that often the mountains are much darker than the debris surrounding them. The darker components of igneous and metamorphic rocks consist mostly of minerals that easily weather under the influence of carbonic acid, oxygen, and water. Micas and feldspars will slowly decompose and their tiny flakes and grains will be carried away by the wind, leaving quartz behind. Once quartz has formed and appeared at the surface, it will only slowly weather away, if at all. Most of the weathering of quartz is due to physical forces: changes in temperature, erosion, cracking by ice-wedges, and grinding. Not surprisingly, many sedimentary rocks, in particular those that formed on land, contain large amounts of quartz.

Chemical weathering and dissolution of quartz substance only plays a role in tropical semi-arid and savanna climates. Here quartz slowly gets dissolved and washed out of top soil and regolith, and the residual iron and aluminum oxides and hydroxides often form economically important bauxite deposits.



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Most sandstones  consist largely of quartz grains held together by a cement of quartz or of carbonates like calcite. Sandstone with a quartz content of more than 90% is called quartz sandstone.
Sandstone is often a very porous material with a rough fracture surface. Depending on the cement and the shape of the grains, it can be quite tough or almost crumbly.

The photo shows a piece of bright-colored Triassic quartz sandstone from Unteralpfen in the Southern Black Forest, Germany. It is mostly made of millimeter-sized quartz grains and the voids in between are partially filled with yellow iron oxides and calcarine material.



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This sandstone is much more fine-grained than the previous specimen, and also much softer, giving off tiny grains when rubbed. It is from the Rossbode, Äginental, close to the Nufenenpass, Ulrichen, Wallis, Switzerland.


Fig.8.1 1280x1000 258kb
Sandstone is always formed by deposition of sand, both in marine and in terrestrial environments and thus it often shows layering (which is typically absent in chemically formed limestones). These layers do not always lie parallel to each other, as shown in Fig.8.1., either because the sand has been deposited at a slope, for example of a dune, or because the sand has been moved, for example by currents in the water. Same location as Fig.8.2.


Fig.8.2
 
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The durability of quartz sandstone is determined by the minerals that fill out the voids between the grains. If this cement consists mainly of quartz, the rock will be chemically very stable. But very often the voids are filled with calcite and the rock weathers quickly, as this cement dissolves in rain water that percolates the rock. When the layers of the rock differ in the composition of the cement or in grain size, weathering may result in bizarre looking rock formations, like the "Teufelstisch" ("devil's table", Fig.8.2) at Hinterweidenthal, west of Pirmasens in the Pfälzer Wald, Germany. Note that the "stand" is bright red and the plate on top of it is brown, just as the brown rocks in the left foreground that have once been resting on a similar stand that has weathered away.

That the rock is porous can be concluded from the fact that a little tree grows on top of it. It wouldn't if the rock did not store any water from rainfalls. Sandstones are important aquifers.

The soils formed on quartz sandstones are usually rather infertile. They do not contain important trace elements needed as nutrients, and even if they did, they do not contain micas or produce[7] clay minerals that usually serve as a storage and buffer for these elements.


Fig.8.3
 
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The sharp edges and blocky appearance of the red sandstone walls of this gorge indicate that erosion is mostly driven by physical forces, and much less due to chemical weathering. These walls consist mostly of quartz. Kolob Canyons, Zion National Park, Utah.


Fig.8.4
 
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Sandstone can be easily sculptured but is nevertheless strong enough to be used for buildings. Figure 8.4 is a view of one of the eastern towers of the Gothic Münster at Freiburg im Breisgau, Germany. The cathedral has been built between 1200 and 1536 and did surprisingly survive the bombings during World War II. You see a mosaic of differently colored sandstones, because the sandstone slowly weathers and stones need to be replaced occasionally, and because the stones darken with time, as they absorb the dust and smoke of the city. A Gothic church is a permanent construction site. Because the sandstone is porous, all the gargoyles are covered by green moss.


Chert, a microcrystalline quartz variety, can also be a rock-forming material. It may form by alteration of volcanic rocks, for example, from silica-rich ashes and debris in the Onvervacht sedimentary group in the Transvaal province, South Africa ( ->Lowe and Knauth, 1977).

Most cherts are based on biogenic sediments, however. Novaculites, diatomites and radiolarites are rocks of biogenic origin with a very high silica content that form from marine and lake sediments containing opaline skeletons - the same that are also the main silica source of flint nodules in sedimentary rocks. The opaline substance in the rocks is slowly altered under the influence of percolating waters and slowly recrystallizes into other silica polymorphs, like cristobalite, moganite, and ultimately quartz. Thus older cherts typically have been completely transformed into quartz (still cryptocrystalline) and do not contain any opaline substance. This transformation is accompanied by a change in texture: the initially irregular grain borders get straightened out and form a "triple-point texture" (see Knauth, 1994, for a review on chert formation).

Fig.8.5 1434x2160 693kb
Radiolarites form from the opaline skeletons of radiolaria, marine single cell protozoa. These are very dense, hard, tough, and often dark chert-like rocks that weather only slowly in moderate climates. They typically show some layering, with interspersed thin layers of softer marl or slate, reflecting frequent changes in the marine environment. Radiolarite has an angular fracture, forming roughly rectangular edges, on a small scale the fracture is conchoidal. Because of its physical strength, radiolarite is sometimes used for road construction.
The opaline skeletons of marine single-cell organisms were deposited on the ocean floor and later solidified into a fine-grained chert-like rock, sometimes entirely made of chert. Although the skeletons are up to about a millimeter in size, the former skeletons are usually not visible to the naked eye. In radiolarites from Mesozoic and Paleozoic ages the opaline substance has been completely converted to quartz.

Figure 8.5 shows an outcrop of radiolarite in an old radiolarite quarry (now a historic site) at Lautenthal in the Harz Mountains, Germany (the field of view is about 240x360 cm). The radiolarite was formed in a deep marine environment at the Lower Carboniferous age about 350 million years ago. During the Harz orogeny it was subject to intense folding. Interspersed within the radiolarite are a few sheets of slate which can be recognized by their hatched texture and the fact that they have eroded more than the radiolarite.



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This dark gray radiolarite is from a river bed close to the locality shown in Fig.8.5. The folding of the rock led to the formation of a lot of small and often straight quartz veins that run criss-cross through the rock.


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While the raw, unpolished radiolarite from the locality shown in Fig.8.5. looks homogeneously black, one can see very fine parallel sedimentary layers in this close-up of a polished slice. At high magnification (note the scale), elliptical casts of former radiolarian tests are visible. They were originally circular and hollow but were deformed during the compaction of the sediments and filled up with chalcedony.


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Another view of the same polished slice, showing radiolarian test that are a little better preserved. Many of them look hollow, but they are filled with translucent chalcedony.



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Radiolarite, Okutama, Japan 



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Chert, Okutama, Japan 


Diatomites are rocks similar to radiolarites that also form from sediments of opaline skeletons, but the skeletons are mostly those of diatoms, single-celled algae. Diatoms occur in marine and in sweet-water environments, so diatomites can also found in former lake beds.

The deposition of large amounts of biogenic opaline silica in deep water marine environments and the formation of diatomites is a relatively new phenomenon in the history of Earth - only since about 50 million years the diatoms flourish, likely because of a general cooling of the climate (Knauth, 1994). In present-day oceans, siliceous ooze can be found in areas of high biological activity, but only at great depths under conditions that - due to high pressure and low temperatures - do not allow the deposition of calcite and calcareous skeletons and shells.

If the diatomite is very young and has not been buried under other rocks at great depths for a long time, it has not yet been compressed and sometimes remains very porous. This type of diatomite is also called diatomaceous earth because of its soft consistence.

Fig.8.6
 
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Figure 8.6 shows an outcrop of diatomite at a road cut at Lompoc, California. The field of view is about 120cm. The layered diatomite in the center is a very soft and light material, the layers of gray rock are dense shales that are cut by thin parallel sheets of brown and black opal. Lompoc is the location of the world's largest diatomite mine. Thanks to its porosity and the fact that it is chemically almost inert and does not react with most substances, diatomite is used as a material for filters and absorbents, but also as an inflammable insulator or as abrasive. If dry, diatomaceous earth is a very light material.

Diatomite varies in composition, it may be almost pure amorphous (opaline) silica, but slowly - on a geological time scale - transforms into chert composed of quartz. Some diatomites contain cristobalite, and prolonged exposure to the fine dust of cristobalite may cause silicosis and lung cancer (e.g., Goldsmith, 1994, Pelucchi et al., 2006).



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This is a sample of diatomaceous earth taken from the outcrop at Lompoc shown in the previous picture. When dry, it is conspiceously light, soft and crumbly, with a tendency to part along the layers. Because it is very porous, it absorbs a lot of water, gaining a lot of weight. Do not inhale the powder the dry specimen readily give off when rubbed (see notes above). I keep this specimen in a sealed plastic bag.



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Another specimen from the same spot in Lompoc, a finely banded diatomite with interspersed layers of translucent dark brown and gray chert or opaline silica (the "or" indicates that I don't know for sure). It came from the gray rock layer in the lower part of Fig.8.5. This rock is not crumbly like the previous specimen, but tough, hard and not porous.



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Novaculites  are light-colored diatomites or - more common - radiolarites. Like other radiolarites, they are very tough rocks. Their fracture resembles that of ceramics. When their structure is uniform and very fine, they are used as a high quality whetstone. Very good novaculite comes from the Ouachita Mountains in Arkansas, U.S.A., like the gray specimen shown to the right.


 

Metamorphic Rocks

The role of quartz in metamorphic rocks is very complex, because the silica content can determine the faith of a rock during metamorphosis to a large extent. During low-grade metamorphosis, at low temperatures and pressures, the quartz content of the resulting rock will not differ greatly from that of the original rock, as quartz is still an almost inert substance under these conditions, but at higher temperatures quartz will start to react with other minerals to form silicates. So while it may coexist with some minerals at certain conditions, it may simply disappear from the rock under different conditions, for example upon prolonged heating. Other factors that determine the reactivity and the fate of quartz in a rock are the water content, the amount of carbon dioxide and oxygen, etc.

Rocks that consist mostly of silica and carbonates shall serve as an example, because the processes in them are simple, at least when compared to processes in silicate rocks. Limestone, a sedimentary rock, is largely made of calcite, CaCO3. Quartz and calcite can coexist over a wide range of conditions, and quartz crystals found in limestones are fairly common. For example, certain types of authigenic quartz form inside limestone. Metamorphosis at moderately high temperatures will turn a limestone into marble, and quartz crystals can also be found in that type of rock, like the famous Carrara diamonds.

However, at temperatures above about 450°C, quartz may start to react with calcite to form the mineral wollastonite. Ca3Si3O9:

3 SiO2 + 3 CaCO3 → Ca3Si3O9 + 3 CO2    [10]

This reaction releases carbon dioxide, and depending on the amount of CO2 present (the partial CO2 pressure, to be exact), the reaction can be inhibited until higher temperatures are reached. As a consequence, that reaction is self-inhibiting. There is also a dependence from the total pressure, as well as the partial H2O pressure, and quartz and calcite can remain stable at temperatures of more than 700°C in the absence of water. The reverse reaction cannot take place if the CO2 has largely escaped from the rock and the quartz will not be formed again when the rock cools. The fact that CO2 is removed from the system and "freezes" the reaction products makes silicate-carbonate reactions and the minerals formed in them important indicators of conditions during metamorphosis.

Of course, whether any quartz remains after high-grade metamorphosis also depends on the initial amount of quartz in the rock. Quartz might have been present in excess, so not all of it has been consumed. One should also keep in mind that these reactions are extremely slow, and many rocks are not "mature", that is, the chemical reactions have not been completed and the mineral components have not reached a chemical equilibrium.

If a rock contains large amounts of dolomite, MgCa(CO3)2, things get more complex. In general, quartz is far less stable in dolomite marbles than in calcite marbles and will start to react with dolomite at about 400°C. A common product of quartz, dolomite and water is talcum, Mg3[(OH)2/SiO4], a sheet silicate:

3 MgCa(CO3)2 + 4 SiO2 + H2O → Mg3[(OH)2/SiO4] + 3 CaCO3 + 3 CO2    [11]

At higher temperatures, the talcum will react with calcite and quartz to form the mineral tremolite, Ca2Mg5[(OH)2/Si8O22], an amphibole, again releasing CO2:

5 Mg3[(OH)2/SiO4] + 4 SiO2 + 6 CaCO3 → 3 Ca2Mg5[(OH)2/Si8O22] + 6 CO2 + 2 H2O     [12]

In a further step tremolite may consume more quartz and calcite to form the mineral diopside, CaMgSi2O6, a pyroxene:

Ca2Mg5[(OH)2/Si8O22] + 2 SiO2 + 3 CaCO3 → 5 CaMgSi2O6 + 3 CO2 + 2 H2O     [13]

It is important to note that the minerals on both sides of formulas [10] to [13] may coexist in a rock, and that there are overlapping areas of stability for most of the minerals involved in the reactions. So one may -depending on the local conditions- find quartz along with diopside and tremolite, or talcum together with quartz and tremolite.

The whole matter gets even more complex in rocks that contain alumosilicates in addition to carbonates and silica, like marls and their derivative metamorphic rocks.


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In this rock a quartz vein cuts through dolomite marble. Between the colorless quartz and the light brown dolomite one can see gray-green talcum with a silky shine. From the Feldbachtal, north of the Binntal, Wallis, Switzerland.


Quartzite  is a metamorphic rock[8] that is made of at least 80% quartz. Quartzites are formed from precursor rocks that are mostly made of quartz, this includes sandstones, cherts and radiolarites. In most cases it is simply a sandstone that has "matured" into a very tough rock composed of tightly interlocked quartz grains and that has undergone a low-grade metamorphosis at low temperatures. In most sandstones individual grains can be identified with the unaided eye, whereas quartzite looks much more homogeneous and its fracture surface does not show a dull, rough grainy structure. Instead, the fracture cuts right through the individual quartz grains and can be a little shiny. While in sandstone individual grains are mostly rounded, the grains in quartzite are more angular and as the former voids in sandstone have been filled with quartz substance during the recrystallization of quartz grains, quartzite is not porous. It is also much less susceptible to weathering in moderate and cold climates and forms steep cliffs and outcrops in a landscape. Quartzite is generally more translucent than sandstone and looks a bit like marble. Being much harder and also very tough, it is easy to distinguish from marble. Quartzite often contains small amounts of muscovite mica.


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The photo shows a quartzite specimen from the Saasbach in the Äginental, Ulrichen, Wallis, Switzerland, north of the Nufenenpass. Note the shiny fracture surface.


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A smaller specimen from the same location. It is difficult to identify individual grains in the tightly interlocked structure. The brown patches are caused by small limonitized pyrite cubes.


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A closeup of the previous specimen, showing the translucency and the sparkling surface. The reflections are not caused by crystal faces, but by the conchoidal fracture that cuts right through the individual grains.


A gneiss is a metamorphic rock that shows signs of layering and alignment of structures parallel to the layering. The classification of a rock as a gneiss is primaly based on its texture and not its composition, but a gneiss should contain a significant amount of feldspars. There is no strict definition for the minimum amount of feldspar required to make it a gneiss (Vinx, 2003). The term "gneiss" subsumes a great variety of rocks and there are gradual transitions to schists as well as intrusive igneous rocks (granitoid rocks). A common denominator of all gneisses is their tendency to part into plates that are thicker than 1 cm (Vinx, 2003), whereas shists and phyllites tend to form thin sheets. Depending on the precursor rock, orthogneiss that is derived from ingeous rocks and paragneiss that is derived from sedimentary rocks are sometimes distinguished. Not all gneisses contain quartz, but most do, since the precursor rocks are often granitoid rocks or quartz-bearing sedimentary rocks.


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A typical gneiss has a banded appearence, nevertheless it does not break into thin sheets but forms massive boulders with only a moderate tendency to part along the layers. The banding does not have to be evenly spaced. The dark mineral is mostly biotite, the bright components are feldspar and little quartz. From the Rossbode, ─ginental, close to the Nufenenpass, Ulrichen, Wallis, Switzerland.



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The image shows an orthogneiss that has been formed from granite from the southern part of the belt of metamorphic rocks (sometimes called "Altkristallin") that enclose the Aar Massive, picked up close to the Hotel Belvedere, Furkapass, Wallis, Switzerland. The rocks in this belt have been subject to strong tectonic shearing forces that turned the granite into gneisses, shists and sometimes even fine-grained phyllites[9]. The rock consists primarily of red orthoclase feldspar, gray quartz and dark biotite. At elevated temperatures quartz is much more susceptible to mechanical deformation than feldspars and will recrystallize to more elongated grains, whereas the feldspar crystals are more stubborn and form large irregular, but rounded grains. The result is a texture in which feldspar crystals are embedded by dark mica minerals to form "eyes" and the corresponding rock is called augengneiss ("Auge" is German for "eye").


Slates, phyllites and mica shists are layered metamorphic rocks that are derived from sedimentary rocks with clay minerals, often from deep water marine environments. Their presence always indicates mountain forming processes (orogenies). Slates are composed of sheet-silicates and quartz and have undergone only a weak, low-grade metamorphosis. In phyllites, which form at low-grade metamorphosis (greenshist facies), the mica crystals are so fine that they can hardly be seen with the unaided eye. They are composed mainly of micas (50% or more), quartz and carbonates. Mica shists form from phyllites during medium grade metamorphosis (amphibolite facies) and look more coarse grained. Their mineral composition is similar, but they also often contain conspicious accessory minerals like garnets and amphiboles.

Many mica shists and phyllites contain large amounts of fine grained quartz. If these rocks or their precursor rocks are subject to tectonic stress, quartz tends to segragate from the rock and accumulates in fine-grained, white lense-shaped bodies or irregular elongated veins that run roughly parallel to the layers of the rock. The quartz is then sometimes called segregation quartz, and its presence is an indication of a long term, weak to medium grade metamorphosis. A few in situ images of segregation quartz are shown in the chapter Occurrence.

During higher grade metamorphosis mica shists will gradually transformed into paragneiss, and now the mineral composition changes at the expense of quartz - the quartz reacts with the mica minerals to form other minerals, for example feldspars and andalusite (or sillimanite, depending on the pressure):

KAl2[(OH)2/AlSi3O10] + SiO2 → KAlSi3O8 + Al2SiO5 + 2 H2O     [14]
muscovite + quartz → K-feldspar + andalusite + water

Which is one of the reasons why many paragneisses have a lower overall quartz content.


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Most mica shists contain large amounts of quartz, but the quartz grains are hidden between the shiny blades of mica. In bright colored mica shists, the mica is often muscovite or paragonite, darker mica shists contain more biotite or phengite mica. From the Mittleberg, Binntal, Wallis, Switzerland.



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Although it looks like a mica shist with the typical segregation quartz lenses, this particular rock did not form from sedimentary rocks, but from granodiorite during extreme tectonic stress that caused grinding and recrystallization of the original rock. Such a rock is called a mylonite (Greek for mill), or, if the grains are invisible to the naked eye, ultramylonite. Mylonite rocks that look much like a phyllite are sometimes called phyllonite, and like in true mica shists and phyllites that are derived from sedimentary rocks, segregation quartz veins and lenses occur frequently. The specimen was picked up at material from tunnel excavations at Kraftwerke Oberhasli, Haslital, north of the Grimsel pass, Kanton Bern, Switzerland.



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 Banded iron formation, 30km north-east of Kuruman, South Africa.



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 Itabirite, Serra Dos Carajas, Pará, Brazil.



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 Itabirite, Serra Dos Carajas, Pará, Brazil.


 

Quartz-Free Environments

Although quartz is one of the most abundant minerals and makes up an estimated 12% of the Earth's continental crust, it is very unevenly distributed, and most of the quartz is concentrated in granites, gneisses, quartzites and sandstones. Certain types of rocks are completely void of quartz and free silica.

There are different ways to interpret this lack of free silica:

- The lack of quartz in an igneous rock might be solely attributed to the bulk chemical composition of the magma it formed from: if the concentration of SiO2 is too low, it does not allow for quartz to form as a mineral, simply because there is no silica left over from the mineral forming processes.

- It can be viewed as being caused by the presence of other minerals in the rock that react with the quartz. This, of course, also depends on conditions other than the bulk chemical composition: quartz might be stable inside a rock at a certain temperature or pressure, but upon changes the silica might be used up in reactions with other minerals that lead to the formation of new silicate minerals. That way the overall chemical composition of the rock does not change, but its mineral composition does and quartz may appear or disappear in it.

These interpretations are not mutually exclusive, they are just different ways to express the same fact.


Sedimentary Rocks

As quartz is almost inert at temperatures and pressures that produce sedimentary rocks, the most important factor that determines if quartz is present in them is the presence of free silica during the rock's formation.

Normally, quartz will not disappear from these rocks. If one chooses a very open definition of the term "sedimentary rock", then there is one notable exception: In tropical climates with pronounced rain and dry seasons, silica (free silica as well as silica bound in silicates) is slowly washed out of the soil and the remaining minerals (aluminum and iron oxides) form a very infertile soil, laterite, that might later assume a hard, brick-like consistence.

However, weathering of minerals contained in sedimentary rocks may release silica.


Igneous Rocks

The amount of silica (including the silica that is chemically bound in silicates) in igneous rocks is one of the parameters that is used for their classification. Above the terms felsic and mafic have already been introduced. Free silica (as quartz) can be expected to be found in rocks with more than about 60-65% silica in their total chemical composition. Well known examples are granite and its volcanic rock counterpart, rhyolite.

Another important classification scheme has also been presented above: the QAPF diagram, which uses the quartz content instead of the silica content of rocks as a parameter for classification. The rocks represented by the lower part of the QAPF diagram are free of quartz and contain characteristic minerals that replace feldspars in these rocks, so-called "feldspathoids" or foids. The presence of foids in a rock is a safe indication that this rock is free of quartz. Note that foids are still called felsic minerals. A well known example of a foid is the blue sodalite, Na4Si3Al3O12Cl, with a silica content of only 37 weight-%.

Fig.10.1: Chemical composition of typical feldspars and foids compared.

Other important and much more common examples are nepheline, Na3K(AlSiO4)4, and leucite, KAlSi2O6.
Fig.10.1 shows pie charts of their chemical composition (in weight percent) in comparison with two common feldspars, orthoclase and albite, NaAlSi3O8. The foids contain less silica, and if they get in contact with quartz, they will be slowly transformed into feldpars:

KAlSi2O6 + SiO2 → KAlSi3O8   [14]
Leucite + Quartz → Orthoclase   
Foid + Quartz → Feldspar   

Quartz crystals might still grow in pockets and veins from silica dissolved in hot waters that percolate a weathered foid-bearing rock, or small amounts of quartz might fill out the voids between the grains of an altered rock (then invisible). So the rule that foids and quartz are incompatible is only true for mature and fresh rocks in chemical equilibrium. If, by some odd coincidence, foids are found next to quartz, the foids will be separated from the quartz by a thin layer of feldspars. However, it is most unlikely to find well formed crystals next to foids.


In addition to these foids, silica-poor rocks often contain large amounts of dark minerals rich in magnesium and iron (ferrum in Latin), mafic minerals. Many of these minerals are either incompatible with the presence of quartz or are loosely associated with silica-poor environments, like pyroxenes and amphiboles (see Silica, Silicates and Rock Chemistry above).

Among the dark minerals in general, micas (like the members of the biotite group) and iron compounds (pyrite, hematite, limonite, magnetite) are fully compatible with the presence of quartz, and may occur in very acidic rocks.



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When this dark and gray basalt was rising from the base of a continental plate, it carried a xenolith to the surface, a piece of rock that does not belong to it but originates in another type of rock. This xenolith is a green peridotite largely composed of bright green olivine and darker pyroxenes (in this case the orthopyroxene "bronzite" and the clinopyroxene chromium diopside). Xenoliths made of large olivine grains are not uncommon in basalt. The specimen is from Smrči near Turnov, Vychodočeský Kraj, Czech Republic.

The peridotite and the basalt (probably a basanite or tephrite that also contains small grains of olivine) both have a very low content of SiO2 compared to that of other igneous rocks like granite, diorite or andesite. A fresh and unaltered peridotite or a basalt containing an olivine fraction will not contain any free silica, and of course, no quartz. If it does, the quartz has certainly entered the basalt later at relatively low temperatures and did not remain in the rock for a long time. If it had been swallowed by a basaltic magma, it would have simply been dissolved in the magma to form new minerals.



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This rock shall serve as an example for an intrusive igneous rock (as opposed to volcanic igneous rocks), that is incompatible with quartz. The rock is coarse grained, just as a granite, but much darker with a conspicuous greenish color. It is a gabbro, a type of rock that consists mainly of plagioclase feldspar and dark minerals like pyroxenes, amphiboles, and olivine. Gabbro is the plutonic counterpart of basalt, it is what basalt would look like if its magma had not appeared at the surface but formed a plutonic intrusion. The normally white color of fresh plagioclase has turned into a gray-green because of chemical alterations, and the rock also contains some biotite that has been formed by weathering of fayalite, an iron-rich olvine. There are gabbros with sufficient amounts of silica in them to form quartz (and these do not contain olivine, of course), but most gabbros, in particular those that are dark, contain no quartz. If present, then the quartz often fills out the spaces between the mineral grains, but does not form grains by itself, so it is essentially invisible to the unaided eye. The specimen is from the "Gabbro" quarry at Bad Harzburg, Harz Mountains, Lower Saxony, Germany.


Metamorphic Rocks

Because the relative amount of SiO2 in a metamorphic rock has a profound influence on the types of minerals that form in them, the presence or absence of certain minerals can indicate if quartz can be found in them. The following minerals do not form at the presence of quartz or sufficient amounts of SiO2 in the originating rock:

- silicates with a low portion of SiO2 and a relatively high concentration of magnesium, Mg, like:
Forsterite, Mg2SiO4, an olivine mineral

- oxides and carbonates of magnesium, like:
Periclase, MgO
Magnesite, MgCO3, at medium- and high-grade metamorphosis
Dolomite, CaMg(CO3)2, at high-grade metamorphosis

- silicates with a low portion of SiO2 and a relatively high concentration of aluminum, Al, like:
Corundum, Al2O3, at medium- and high-grade metamorphosis
Foids in general, like leucite, K[AlSi2O6], analcime, Na[AlSi2O6]⋅H2O, nepheline KNa3[AlSi2O6]4


A mineral that doesn't get along too well with quartz is corundum, Al2O3, better known under the names of its varieties ruby and sapphire. If the rock is in chemical equilibrium, that is, if there has been sufficient time for its components to fully react with each other, corundum and quartz will react to form either sillimanite, andalusite, or kyanite:

SiO2 + Al2O3 → Al2SiO5  (Andalusite / Sillimanite / Kyanite)  [15]

In low-pressure environments quartz can coexist with all the Al2SiO5 polymorphs, but not with corundum. Corundum and quartz can be found together in sedimentary rocks, which is possible because both Al2O3 and SiO2 are not very reactive substances and the reaction takes place at a very low speed.

Although a stability field in the area above 650°C and 3.5 kbars has in been predicted by experimental studies on the system Al2O3, SiO2 and H2O (Aramaki and Roy, 1963), these findings could not be reproduced in a later study (Carr, 1968) and were attributed to the use of amorphous chemicals (silica, aluminum oxide) as starting substances, and until recently there was a general agreement that corundum and quartz never form together and never are in chemical equilibrium.

But in fact there are a few high-grade metamorphic environments in which quartz and corundum, or quartz and spinel (which is almost as "quartz-unfriendly"), are found together (for a review of localities, see Mouri et al., 2004). In most cases, the quartz and corundum grains are separated by a narrow rim of sillimanite or kyanite, but in some cases corundum and quartz are in immediate contact and apparently in chemical equilibrium. These environments are all characterized by very high pressures (between 7 and 17 kbars) and temperatures (between 900°C and 1100°C), so there seems to be a well-defined stability field of the coexistence of quartz and corundum. In fact, the presence of quartz and corundum grains in immediate contact in a metamorphic rock is seen as an indicator of extreme high-pressure and temperature conditions.


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In  this specimen corundum has formed nice red ruby crystals, the larger ones sit inside orthoclase feldspar crystals, smaller pink corundum grains are found next to biotite and an unidentified feldspar (probably also orthoclase) in the matrix of this granulite, a high-grade metamorphic rock. Despite its bright color and its granite-like look, the rock contains no quartz[10]. Biotite is indicative of temperatures well below 900°C, and accordingly quartz is not expected. From the East Ghats Granulite Belt, Mysore, Karnataka, India.



 

Further Information, Literature, Links

C.M.R. Fowler
The Solid Earth - An Introduction to Global Geophysics
Cambridge University Press, Cambridge, 2004
ISBN 0-521-89307-0

Gregor Markl
Minerale und Gesteine - Eigenschaften - Bildung - Entstehung
Elsevier, Spektrum Akademischer Verlag, Heidelberg, 2004
ISBN 3-8274-1495-4

Martin Okrusch, Siegfried Matthes
Mineralogie - Eine Einführung in die spezielle Mineralogie, Petrologie und Lagerstättenkunde.
Springer-Verlag, Berlin Heidelberg New York Tokyo, 2005
ISBN 3-540-23812-3

Roland Vinx
Gesteinsbestimmung im Gelände
Elsevier, Spektrum Akademischer Verlag, Heidelberg, 2003
ISBN 3-8274-1513-6

Hans-Rudolph Wenk, Andrei Bulakh
Minerals - Their Constitution and Origin
Cambridge University Press, Cambridge, 2003
ISBN 0-521-52958-1





Footnotes

1 The oceans cover 70.6% of the Earth, so one would expect that value to be 29.4%. But the continents are larger than they appear, because parts of the continental crust are hidden in the marine continental shelf areas that are covered by water.

2 Material in alluvial rocks and sediments has been transported and deposited by rivers and streams.

3 Olivine is the name for a group of minerals that encompasses forsterite, Mg2SiO4, and fayalite, Fe2SiO4. Olivine is typically made of 85%-95% forsterite and 5-15% fayalite. The gemstone peridote is just a deeply green olivine.

4 Occasionally forsterite or olivine is found next to quartz in a rock, but they did not form together. This happens, for example, when olivine-rich and quartz-rich sediments get mixed and solidify, or when a olivine-rich lava encloses quartz-rich rocks.

5 In most cases, the magma is not a liquid in the common sense, but a mixture of highly viscous melt and small crystals of various minerals.

6 In rare cases one can see the hexagonal outlines of quartz crystals that formed as beta quartz in granites and rhyolites.

7 Clay minerals are produced during the weathering of feldspars.

8 Sometimes the term "quartzite" is also used for some sandstones.

9 The shists and phyllites of that locality are more commonly referred to as mylonites and ultramylonites to indicate that they formed by mechanical grinding and recrystallization of the precursor rocks.

10 The label that came with the rock said it was made of kryptoperthitic K-feldspar, biotite, quartz, and corundum. That did not make sense to me, as this was a high-grade metamorphic rock with a "mature" look and the corundum did not show any signs of alteration. Sebastian Möller at the university of Kiel kindly analyzed a thin section of the specimen using a petrographic microscope and found the rock to be made of perthitic and non-perthitic feldspar, biotite, corundum and small amounts of zircon, but no quartz.


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