Alpine-Type Fissures

 

last modified: Wednesday, 27-Apr-2011 22:58:29 CEST

Document status: work in progress

Fig.1.1 (8/2009) 2160x1376 385kb
Among the geological environments that produce the most beautiful quartz crystals are alpine-type fissures, also called alpine-type clefts. Alpine-type fissures are cavities that have opened during tectonically caused stretching and bending of the rock. The cavities are entirely enclosed by the surrounding host rock. Many of them contain minerals in excellent crystals, and certain parageneses are typical for alpine-type fissures. The term "alpine-type" refers to their formation, which is completely different from that of the similar geodes, vugs, miaroles and pegmatite pockets.

Alpine-type fissures have first been exploited, studied and described in the European Alps, so they are often simply called "Alpine fissures". But they occur in other mountain chains, as well, for example the Himalayas and the Ural mountains. An "Alpine fissure in the Himalayas" sounds very odd, so I refer to them as alpine-type, and not as "Alpine".

For centuries alpine-type fissures have been the main source of quartz crystals for lapidary and ornamental uses. While pegmatite deposits took the lead as major source of rock crystals and pure quartz, the best smoky quartz still comes from alpine-type fissures. To the quartz collector, alpine-type fissure specimen are not only attractive because of their beauty but also because the relationship between the different quartz habits and forms and the growth conditions have been studied in detail.

Fig.1.2 (8/2008) 2160x1434 363kb
This is a typical example of an alpine-type fissure, outlined by quartz crystals. It sits inside a large aplitic dike, but it is not a miarole or pegmatite pocket: There is no gradual transition from the host rock to the crystals and there are also no onion-like zones of different mineralizations and textures around it. Instead, the quartz crystals sit immediately on the host rock. The largest extension is perpendicular to the cut, that is, the fissure extends farther into the host rock. The shape of alpine-type fissures varies with the type of host rock and the conditions of their formation, but most are flat, elongated cavities, like this one. Another typical feature is the presence of green pocket clay (chlorite-group minerals), that may fill a pocket completely, but fortunately has been mostly washed out in this case. To the left one can see a quartz vein (called the "Quarzband" in the German-speaking parts of the Alps) that extends into the host rock and will eventually wedge out at some distance and that also contains some green chlorite near the upper left corner of the image. This Quarzband is not always present in alpine-type fissures.
The photo has been taken in upper Val Giuv, Eastern Aar Massive, Tujetsch, Graubünden, Switzerland.


Fig.1.3 (4/2008) 1520x1007 316kb
This is not an alpine-type fissure, it is a miarole pocket with quartz and microcline crystals that is shown for comparison. It has an irregular shape and shows a gradual increase in grain size from the granite host rock to the crystals that outline the cavity. Seen at Rock Corral, east of Milford, Mineral Mountains, Utah.


Fig.1.4 (8/2009)
 
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This very large pristine alpine-type fissure was found 1974 during tunnel excavations for the Oberaar power plant at Gerstenegg in the Grimsel area, Kanton Bern, Switzerland. It was immediately put under protection and is now open to the public. The field of view is about 2 meters, the fissure is at least 14 meters long. It has a much more irregular shape than the fissure in Fig.1.2 and the dark granodiorite host rock has suffered strong alteration close to the pocket that has caused bleaching and a more spongy texture. The rock walls are covered with large rock crystals, other minerals are green chlorite, calcite, and pink octahedral crystals of fluorite that mostly sit on the quartz crystals. Fig.1.1 is another view of the same pocket.



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This is a small piece of a wall from an alpine-type fissure in mica shist. The fissure has smooth and straight walls and the rock crystals sit on the host rock. In this case, the host rock has not been altered close to the cavity. From Storenuten mountain, north of the Ringedalsvatnet lake at Odda, Hordaland, Norway. Figure 2.12 is an image of the locality.


Fig.1.5 (8/2009) 2048x1434 575kb
Crystals do not always sit directly on the host rock, sometimes they grow in a cavity of milky quartz that fills out most of the fissure (the "Quarzband"). Such a setting is common at very large fissures found in gneisses and granitoid rocks in the Aar Massive in central Switzerland, for example. The crystal-bearing cavity in this freshly opened and partially exploited large alpine-type fissure is about 80cm wide, the floor is covered by green chlorite pocket clay. It first looks like an ordinary hydrothermal quartz vein, only on closer inspection its true nature is revealed: the quartz "vein" wedges out, the fissure is fully enclosed, it runs perpendicular to the orientation of the feldspar crystals in the host rock, neighboring fissures show the same inclination, and the contact between the quartz and the host rock is straight. Upper Val Giuv, Eastern Aar Massive, Tujetsch, GraubŁnden, Switzerland.


 

Formation, Occurrence and Types of Fissures

Alpine-type fissures can only form under special conditions that are not easily met. They form when rocks get folded or sheared at great depths and at relatively high temperatures, between 200° and 600°C. Under these conditions the rocks are still solid, but malleable, so they do not shatter when they get deformed slowly. The rocks must still maintain some degree of rigidity and may not be too ductile, otherwise fissures would not open. It is favorable that the rock has a layered structure like gneiss or schist, but this is not a requirement.

During the deformation of the rocks, fissures open when:

Of course, there are transitional forms, and in the scientific literature very often that distinction is not made and both types are subsumed under the term "extension fissures".

Alpine-type fissures may assume many different shapes, but in general they tend to form flat cavities, with relative dimensions between about 5 by 2 by 1 and about 10 by 3 by 1, so they are long cavities that are about two to three times as wide as high. In igneous rocks the fissures are a bit higher, in mica shists they are often very narrow. Their orientation depends on the direction of the tectonic forces and the direction of the layering of the rocks:

Most fissures do not open completely all of a sudden, they widen gradually, and minerals attached to both walls might develop special growth forms. A typical growth form of quartz that can often be found in alpine-type fissures is faden quartz.

Fig.2.1 (7/2006) 1200x1530 645kb
The alpine-type extension fissure shown in Fig.2.1 is a good example of this behavior. The fissure opened perpendicular to the layers of the host rock made of mica shist. The fissure is about 20-30 cm wide and about 80-100 cm high. At its top one can see a part of a quartz vein. It is located at the eastern slope of Storenuten mountain, north of the Ringedalsvatnet lake at Odda, southwest Norway.

Fig.2.2 (7/2006) 1464x1004 458kb
This is a small shearing fissure - the field of view is just about 80 cm - with its typical sigmoidal (s-shaped) profile. The fissure runs roughly perpendicular to the original layering of the mica schist and also causes a disturbance in the layering pattern. You will note the presence of small quartz lenses and veins to the left and the right of the fissure that run parallel to the layers. These segregation quartz veins predate the fissure. The two narrow quartz veins that run through the rock immediately under the fissure are very likely small alpine-type fissures that were completely filled with quartz and that probably also opened before the main fissure. The photo was also made east of Storenuten mountain, very close to the location of Fig.2.1.

Fig.2.3 (8/2005) 1200x823 206kb
This block has just been removed from an alpine-type fissure, is about 60 cm wide, and covered with smoky quartz crystals that are still partially covered and protected by pocket clay. The crystals sit directly on the syenite host rock, the crystals grew as extensions of quartz grains in the rocks. The white vertical lines on the fresh rock surface on the front are large feldspar crystals, so what we see is in accordance with the general rule that alpine-type fissures run roughly perpendicular to the layering in the rocks. Photo made in Val Giuv, Eastern Aar Massive, Tujetsch, GraubŁnden, Switzerland.


The opening of fissures is promoted wherever the homogeneous structure of the host rock is mechanically disturbed, for example at the border to another type of rock or at aplite dikes and pre-existing quartz veins that run through the rock. For that reason, alpine-type fissures are often closely associated with quartz veins that are older than the fissure.

Fig.2.4 (6/2006)
 
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Figure 2.3 shows a fracture zone that was formed by intense shearing at the boundary between two metamorphosed Jurassic sedimentary rocks of different competence. Rigid rocks that resist deformation are called competent, while ductile rocks are called incompetent. To the left there is rather soft (incompetent) black slate/phyllite, to the right a more rigid (competent) sand-rich limestone with layers of slate. The fissures along the contact of the two rocks that lie roughly parallel to the layers of the rocks have been filled with milky segregation quartz, a few more of these lenses can be seen to the left in the slate. The competent rock to the right contains a few fissures that run perpendicular to the layers and that mark early fractures. The segregation quartz is even more competent than the sand-rich limestone, and if shearing or strain causes "pressure shadows" opposite sides, the softer rock gets pulled away from the quartz and a fissure forms. This has happened immediatly below the center of the image - the hole is an alpine-type fissure. The image was shot at a road cut at the Furka Pass, Wallis, Switzerland. Field of view about 2 meters.

Fig.2.5 (9/2006) 1280x1000 276kb
A close-up of the fissure shown in Fig.2.4, to show the relationship between quartz veins and alpine-type fissures. The pocket in the center is about 20-30 cm wide. In the upper right half you see a large quartz vein, but the pocket is not at its center, it is left to it cutting through the layers of the schist at a right angle. The rigid quartz vein has promoted the opening of the pocket and very likely quartz crystals were growing from the milky quartz into the cleft. To the left you see a few lens-shaped veins of segregation quartz, quartz veins that predate the alpine-type fissures that occur in these rocks.



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This  specimen with shiny and clear rock crystals comes from a fissure not more than 500 m away from the empty one shown in Fig.2.1, placed in its presumable original orientation. It looks much like a crystal-lined pocket in a massive quartz vein. But the "pocket" is not in the center, it is placed next to the wall of the host rock, parts of which can be seen to the left. The massive quartz vein at the right side was actually wider than what can be seen here - until the specimen has been formated for transport. The reason for this asymmetry is that the massive quartz vein is older than the fissure and because it is more competent than the mica schist, fissures opened at the border between schist and vein quartz during folding and shearing of the rock. The fissure did not only yield rock crystal and light-colored smoky quartz, but also anatase crystals, needles of rutile and inclusions of brookite in quartz.


Fig.2.6 (9/2006) 1004x1488 388kb
Extension fissures are often accompanied by so-called boudinage (boudin is French for "sausage"), a phenomenon that is related to what has been described in Fig.2.4. Boudinage occurs when a rock that is composed of layers of different competence gets stretched or sheared by tectonic forces. Upon extensional strain the competent layers will fracture while the ductile layers will simply get thinner.

Fig. 2.6 shows a single large boudin structure that formed in a shist, the diameter of the competent center is about 1 meter. The photo was taken at a road cut between Gletsch and Oberwald, Wallis, Switzerland, an area with a lot of alpine-type fissures. Since rocks of different competence are usually aranged in layers, such boudin structures often form chains.

Fig.2.7 (8/2007) 1280x1007 372kb
The ductile layers will also be drawn into gaps between boudins, which causes an inward pinching of the rock's ductile layers at the gap between two boudins, as seen in the left part of Fig.2.7 taken at the same location as Fig.2.8. The pattern looks a bit like an hourglass, with the neck between two boudins.

Very often a gap will remain between two boudins and alpine-type extension fissure develops. It is very likely that there was once a small pocket at the neck of the hourglass that was opened and exploited while the road was built.

Fig.2.8 (9/2006) 1280x1007 302kb
The hourglass-like inward pinching of the rock's layers can also be observed around a fissure without the obvious presence of rocks of different competence, and this is often also called boudinage. This happens because even though the rock is stretched in one direction, there is still a large pressure acting perpendicular to it, and the weight of the many kilometers of overlaying rock will press the surrounding rocks into the cavity. Figure 2.7 shows an alpine-type fissure with inward pinching vertical layers, in particular on its right and on its lower side, although there is no obvious boudin of competent rock above or below the fissure. The photo has been taken from the entrance of the Hotel Belvedere east of the Rhone Glacier, Wallis, Switzerland, at a road cut of the Furka Pass street.

While boudinage in the strict sense (as shown in Fig.2.6 and Fig.2.7) can be found in many different types of rocks, the inward pinching around an alpine-type fissure can only be observed in rocks with a shist-like structure and layering, it is absent in granitoids or sandstones, for example.


Fig.2.9 (6/2006) 1520x1007 334kb
En echelon fissures are fissures (often shearing fissures) that run parallel through the rock, but are aligned obliquely. They mark a shearing zone in a rock. Figure 2.9 shows small calcite-filled en echelon fissures in a sandy limestone on a hiking trail near Villars-sur-Ollon, Vaud, Switzerland (the lens cap is 7 cm wide). Such parallel arrangements are often found in alpine-type fissures, although they are not as densely packed as these very small fissures.


Fig.2.10 (8/2008) 1664x1328 570kb
An example of two neighboring alpine-type fissures in upper Val Giuv, Graubünden, Switzerland, the lower one is used as a storage for tools now. The elliptical cross section is typical for fissures at this locality.

It might seem a bit far-fetched to claim that the formation of both fissures is linked in some way, but just as shown in Fig.2.9, alpine-type fissures open in localized zones of shearing and extension of rocks, they are not randomly distributed. Local rockhounds often check the relative position of old and emptied fissures and look for such patterns to find zones in the rock that were stretched.


Fig.2.11 (8/2008) 1600x1434 553kb
Two other pockets in a large vertical aplite dike, just 20 meters left of those in Fig.2.10. This time the upper pocket is used as a storage. The lower pocket is smaller and would be more difficult to see if there wasn't the chlorite sand at its entrance. Note the many narrow fissures around the large pockets that run through the rock in parallel.


Fig.2.12 (9/2006) 1200x1000 373kb
If we return to the locality at the Hotel Belvedere shown in Fig.2.8 and "step back", we see another large alpine-type fissure underneath, in accordance with what was said in Fig.2.9 and Fig.2.10. The alpine-type fissure directly above the street is about 75-100 cm wide and about 30 cm high.


Fig.2.13 (7/2006) 1007x1504 430kb
Sometimes fissures are not cut on a rock wall, but on the ground. Then their true extensions can be estimated much more easily. Figure 2.13 shows a system of several aligned alpine-type fissures, each of them measuring a few meters in length. The whole chain of fissures is about 30 meters long. The ground right of the fissure is a about 10-20 cm higher than then ground to the left, because this is a shearing fissure. The fissures do not form a straight line, but are offset slightly, with some additional fissures that run parallel to the main chain at a distance of a few meters. The fissures have a vertical inclination, of course perpendicular to the layers of the mica shist, at the same angle as the fissure seen in Fig.2.1. In fact, they are both at the same locality east of the Storenuten mountain. There is a lot of fresh rubble laying around that makes it difficult to see the pattern, because the fissure system has been worked on by rockhhounds that were after the anatase and rock crystals found here. They noticed the quartz veins that run perpendicular to the rock's layers and the step in the ground level associated with it.

In the Alps, there is a story about two groups of rockhounds who met each other inside a fissure they have been working on from both ends, not knowing of each other's activities. So fissures are generally very narrow, but they can get very long.


 

Localities

Alpine-type fissures are found at many places, but to compile a comprehensive list of localities is very difficult. For a long time alpine-type fissures have been thought to be only found in the European Alps and only in the second half of the 20th century localities outside the Alps were reported. In part this is due to the use of the confusing term "Alpine fissure ". Because of the long heritage of professional rockhounding and the extensive studies of their properties the differences between alpine-type fissures and other types of crystal-bearing "pockets" were already understood in the Alps 50-100 years ago, but this is not the case in most other countries. So probably there are still many localities that will eventually turn out to be alpine-type fissures.

This is an incomplete list of localities with well known or suspected occurrences of alpine-type fissures:
  • European Alps (Austria, France, Italy, Switzerland)
  • Wales
  • Dingle Peninsula (Ireland)
  • Madagascar
  • Gamsberg (Namibia)
  • Urals, Russia
  • Japan
  • Rhodopes (Greece)
  • Pamir, Pakistan
  • Himalayas (Nepal, India)
  • Muzo, Andes, Colombia
  • Quebec, Canada
  • Rheinisches Schiefergebirge and Ardennes (Germany, Belgium)
  • Southern Kaledonies (Norway)
  • Appalachian Mountains (North Carolina)


Fig.3.1 (8/2009) 4320x2868 1831kb
Probably the highest density of alpine-type fissures in the world is found in the central Swiss Alps, in particular in the southern parts of the Aar Massive and the northern parts of the neighboring Gotthardt Massive.

The photo shows a classical locality of alpine-type fissure minerals: The Grimsel area and the central part of the Aar Massive. The Aar Massive, the Gotthard Massive and the Mont Blanc Massive are old crystalline mountain chains made of granitoid rocks that predate the formation of the Alps. The Aar Massive (named after the river Aare that has its source there) stretches roughly in westsouthwest-eastnortheast direction north of the Rhone and Rhine Rivers. In the center of the photo you see the Totesee lake at the Grimsel Pass, the zigzag line is the Grimsel Pass street winding up from the Rhone valley. The highest mountain of the massive is the Finsteraarhorn (4274 m), the deep valley to the left is the Rhone valley, which also marks the southern border of the massive. The photo has been taken from the Tällistock mountain at 2800 m altitude, viewing north-west.

If one looks at the world-wide distribution of alpine-type fissures, one may notice a close relationship to mountain ranges that formed during a collision of continental plates, so called alpine-type mountain ranges. Mountain ranges that formed during the collision of an oceanic and a continental plate, so-called Andes-type mountain ranges seem to be almost void of them. The favorite text book example for alpine-type mountain ranges are not the Alps (which are far too complex to be discussed briefly), but the Himalayas. In the list of known or suspected alpine-type fissure localities only Colombia and Japan are an exception - their mountain ranges are of Andes type. All the others are alpine-type mountain ranges, even if they have been mostly eroded, like the Urals or the Appalachian Mountains. In Andes-type collisions, the thin but heavy oceanic plate gets subducted under the continental plate, and the latter suffers only very little deformation during this process. A continental plate, on the other hand, has a relatively low average specific weight and thus cannot be subducted deeply into the denser mantle. As a consequence, the collision of continental plates leads to deformations at a large scale and intense folding and shearing of the continental crust, parts of which may even be overturned completely. One geomorphological feature is typical for alpine-type mountain ranges: the formation of nappes. Nappes are sheets of rock that have been pushed from their original position over other rocks where they for a continuous cover. This displacement is a response to intense folding and shearing of the crust.

 

Alpine-type Fissure Mineralizations

The distribution and mineralogy of alpine-type fissures has most extensively been studied and documented in Switzerland. Johann G. Koenigsberger has meticulously classified alpine-type fissures and distinguished about 115 different alpine-type fissure parageneses that occured in about 20 different types of deposits. His work has been summarized in the book "Die zentralalpinen Minerallagerstätten" ("The Central Alpine Mineral Deposits"), published in 1940, as part of the threepart book "Die Mineralien der Schweizer Alpen" by Niggli, Koenigsberger and Parker.

Different from ore veins, in which minerals often precipitate from brines that originate in distant rocks of different composition, the minerals found in alpine-type fissures basically reflect the composition of the surrounding rock. For example, fluorite can only be found in alpine-type fissures that are surrounded by rocks that contain at least small amounts of fluorine. If certain minerals have been found in an alpine-type fissure, then other fissures in the same rock at that locality are likely to contain the same minerals. Which is not to say that the full set of minerals of a locality will be present in each fissure. Smaller fissures typically contain only the dominant minerals of a certain paragenesis, in particular the more "exotic" ones will be missing. In a sense this is sometimes also true for the different types of quartz that can be found at a locality: the bigger the fissure, the more likely gwindels are found, for example.

The rock composition has only a short-range influence on the mineralization. The upper Val Giuv, a valley in Graubünden, Switzerland, is a good example: the area is largely composed of an igneous rock locally called "Giuvsyenit" (actually much of it would be classified as a quartz monzonite in the QAPF diagram, and not as a syenite) that is cut by numerous aplitic dikes in random directions. Both contain alpine-type fissures, but the minerals in them differ: one can find actinolite and titanite in the pockets in the syenite rock, but not in the aplitic dike pockets, which instead sometimes contain the beryllium mineral milarite. If a fissure opens up between the aplite and the syenite, it may contain both types of minerals. Both will contain adularia feldspar and quartz, the latter tends to be much darker in the syenite because it contains more radioactive trace elements.

Although the host rock strictly determines the paragenesis of the fissures, the minerals found in the pocket are not necessarily the same as those in the rock. Val Giuv is again a good example: The "Giuvsyenit" is rather poor in quartz (on average about 5%, sometimes 0%). Nevertheless the paragenesis of the fissures is clearly dominated by quartz. The material for its growth was in part released during the alteration of the plagioclase feldspar which is not very stable at near-surface conditions and tends to decompose and which also suffered from the greenshist facies metamorphosis during the Alpine orogeny. Even rocks that lack quartz altogether may contain quartz crystals in the fissures, as long as the rocks are not undersaturated with respect to silica and do not contain minerals that would react with silica to form other minerals, like foids or olivine.

Fig.4.1 (9/2006) 2048x1918 947kb
The rock that encloses an alpine-type fissure is very often altered because the minerals that make up the rock get dissolved and recrystallize within the fissure. The image shows a narrow fissure in a mica schist at the Mittlebärg in the eastern Binntal, Wallis, Switzerland. The walls of the fissure are bleached on both sides. The fissure is about 10 cm and the bleached zone about 2 cm wide. Alpine-type fissures do not have to contain quartz crystals, and this one apparently did not contain any.


Fig.4.2 (8/2008) 1856x1434 732kb
A close-up of the upper fissure shown in Fig.2.9. The cavity is little higher than the natural pocket, because parts of the upper wall have been removed during the work on the pocket. At the lower right corner there is some of the "Quarzband" with lots of dark green chlorite left and the floor of the pocket is still covered with chlorite sand. The host rock is locally called "Giuvsyenite", a slightly metamorphosed igneous rock of quartz monzonite to quartz-bearing syenite composition. It contains large K-feldspar crystals which are oriented roughly vertically due to alignment to the direction of flow in the magma, but there is no layering. Nevertheless the fissure opened perpendicular to the orientation of the feldspar crystals, just as fissures do relative to the layers in schists and gneisses.
The area around the pocket is altered and looks darker than the fresh Giuv syenite. The darker color is probably due to weathering of plagioclase that tends to decompose into green minerals like chlorite and epidote (for comparison, see an altered granodiorite). Close to cavity the rock turns yellow-green and gets softer.



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This is a piece of a fissure wall in Giuv syenite. The host rock is only very slightly altered and has a few small patches of a yellow-gray fine-grained material that has probably formed during the decomposition of some plagioclase. One can see that the mafic mineral is not biotite, like in many granites, but actinolite, which is typical for syenites. The fissure runs through the rock perfectly straight, so in a side view the wall looks even.

Although numerous stubby smoky quartz crystals cover the wall, there is no macroscopically visible quartz in the host rock. Beside quartz, only very short, dark green actinolite fibers have grown into the fissure opening as a continuation of already present actinolite fibers in the rock. The white feldspar crystals were fractured when the fissure opened, like the actinolite, but only a few feldspar crystals grew a few millimeters into the pocket as adularia, in most cases the fracture surfaces have just healed to form regular crystal faces. This was probably a narrow fissure and it probably opened rather late, when temperatures where already too low to promote the dissolution and redisposition of the matrix feldspar to form adularia. In general, the rock walls around small alpine-type fissures tend to show very little alterations. Rock walls at narrow branches of larger fissures are usually just as altered as in wider parts of the fissure, though.

Something else is interesting, but difficult to see on the photo: Almost all quartz crystals sit on the host rock obliquely. Their c-axes are inclined around 40°-50°, but they point in random directions. This is a strong indication that the quartz crystals grew epitactically on the underlying elongated feldspar crystals that were aligned during the influx of the viscous syenite magma into the surrounding rocks. From the Terziana Muotta, Val Giuv.



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Like the previous specimen, this piece is from a fissure wall, but it has a very different consistence. It has a spongy, crumbly matrix whose minerals have partially been dissolved. In the first of the two images this is most obvious in the lower right corner.

The second image is a close-up of the upper left corner, viewed from the left side. There are numerous cracks in the rock wall, some of them are filled with quartz, and all the dark minerals of the granite are gone. Found at an old fissure south of Piz Pali mountain, Val Mila, Tavetsch, Graubünden, Switzerland.

A little side note: Alpine quartz crystals show large variations in their look that may reveal their provenance to the initiated, in particular when they come with a matrix. When I showed this specimen to professional strahler Dosi Venzin, he just asked: "Val Mila?"



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Quartz crystals in alpine-type fissure environments typically form at moderate to elevated temperatures, low-temperature minerals are usually not found as inclusions deep inside the crystals, but as superficial inclusions or as crusts covering the crystals. The most common example is chlorite, which is very often only found as superficial inclusion or crust. Zeolites are eve better examples of minerals that form at a late stage, but they are not so common in alpine-type fissures. A classical locality for zeolites is the Caschle mountain between Val Mila and Val Strem, Graubünden, Switzerland. The specimen shows shiny smmoky quartz crystals associated by the white zeolite mineral stilbite from that locality. Found by Reto Bearth in 2009.



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Rock crystals and creme-colored adularia grew on the wall of a fissure that was running through mica shist. Adularia is hydrothermally grown K-feldspar, and it is not common in pegmatite and miarolitic pockets but typical for alpine-type fissure environments. The crystal with the metallic shine in the lower right corner is an anatase. From Storenuten mountain, north of the Ringedalsvatnet lake at Odda, Hordaland, Norway. Figure 2.12 is an image of the locality.


Fig.4.3 (9/2005) 1200x852 227kb
This is a freshly opened narrow fissure that is outlined by transparent smoky quartz crystals, but as the fissure is filled with green chlorite, the crystals are hardly visible. Secunda Muotta, Val Giuv, Graubünden. Many fissures are completely filled with so-called pocket-clay made of clay minerals.


 

Quartz from Alpine-Type Fissures

It is difficult to narrow down the types of quartz to be expected in alpine-type fissures, and easier to count off what does not occur in them or is rare. One of the problems is that this compilation is mainly based on what is known from the European Alps, and there is very little literature on localities outside Europe. Another thing thing to keep in mind that some types of quartz are found in the Alps, but not in alpine-type fissures, for example the amethyst that occurs in so-called "Teiser Kugeln", geodes in volcanic rocks.


Varieties

Rock crystal is very common and only absent where it is replaced by smoky quartz.

Still the best smoky quartz comes from alpine-type fissures. It is very common in igneous rocks and derived metamorphic rocks.

In the European Alps amethyst is only found at few localities, the best of them in the Zillertal Alps in Austria. Amethyst from alpine-type fissure environments (including those from outside the European Alps) seems to occur exclusively as scepter quartz and related growth forms, often in combination with skeleton growth forms.

Citrine (with irradiation-induced color centers) is extremely rare in the European Alps. It has been found at very few locations in Austria and is otherwise unknown, and finds of citrine in the Alps are celebrated in collectors magazines. So far the color of Alpine citrine specimen has been found to be very pale.

Rose quartz and pink quartz are formed in pegmatite environments and do not occur in alpine-type fissures.

Ferruginous quartz that is evenly and deeply colored by inclusions of iron oxides or hydroxides is apparently absent in alpine-type fissures. Yellow crystals with thin superficial layers of goethite inclusions are occasionally found and have sometimes been mistaken for citrine.

Cryptocrystalline quartz varieties like chalcedony are definitely a rarity in alpine-type fissures. These are low temperature products that are associated with a chemically more unrest environment. The rocks around alpine-type fissures that cool to the appropriate temperatures for chalcedony formation have suffered a retrograde metamorphosis and the chemical reactions that could release the silica needed for chalcedony to form are steadily slowing down the colder it gets.


Growth Forms

Only one quartz growth form is apparently found exclusively in alpine-type fissures: the gwindel. Even here it is rare, and only found in certain environments, mostly weakly metamorphosed igneous rocks.

Since alpine-type fissures typically open and widen slowly, faden quartz is found, mostly in weakly metamorphosed sedimentary rocks like slates and phyllites.

Scepter quartz has already been mentioned as the typical growth form of amethyst from alpine-type fissures. Interestingly, smoky quartz scepters or smoky/amethyst scepters have not been found in the Alps, as far as I know.

Skeleton quartz is found at different localities. Amethyst scepters frequently show skeleton growth features, and rock crystal scepters with skeleton quartz growth forms on Tessin habit crystals have been found in the Binntal, for example. One locality is very famous for skeleton quartz, the Val D'Illiez at Monthey, Vallais, north of the Mont Blanc Massive, and the neighboring region around Bex, Vaud, north-east of the Val D'Illiez. The fissures occur in sandstones in Flysch rock, a series of sedimentary rocks with varying amounts of calcite, clay minerals and quartz. Similar specimen have been found in the Helvetian sedimetary rocks at the northern boundary of the central Swiss Alps, for example the Kiental valley, Bern, also mostly in Flysch rock (Stalder and Touray, 1970).

Split growth, both as sprouting or as artichoke quartz, is uncommon, in particular artichoke quartz seems to be absent. Artichoke and sprouting quartz do occur in the European Alps, for example the Val Bedretto and the Binntal valley, but come from hydrothermal veins, often associated with needle quartz.


Crystal Habits

Quartz from alpine-type fissures can occur in very different habits, and these seem to be in part dependent on the surrounding rocks and the conditions during formation. In Switzerland, for example, crystals of normal habit are found in the Aar and Mont Blanc Massif, while Tessin habit and transitional habit crystals occur in a zone south of the Rhone and Rhine valleys, mostly in phyllites, gneisses and mica shists (so-called Bündnerschiefer). Transitional habits seem to be more common in the Austrian Alps. Quartz from fissures in shists in the eastern areas of Graubünden contain Dauphiné habit and Muzo habit crystals that are absent in the areas where Tessin and normal habit crystals can be found. The "original" Muzo habit crystals from Colombia also came from alpine-type fissures.

Needle quartz is occassionaly found as individual crystals along with normal habit crystals that are slightly elongated, for example in alpine-type fissures at Tipling, Nepal. But aggregates of needle quartz that outline a cavity do not occur, as this type of needle quartz is indicative of fast growth, and crystals in alpine-type fissures grow very slowly. It should be noted that there is needle quartz from the Alps (the best known locality is the Val Bedretto in Ticino), but these crystals occur in a large hydrothermal vein.

Pseudocubic habit and Cumberland habit are apparently not found in alpine-type fissures.


Inclusions


20mm 
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The probably most common inclusion found in quartz crystals from alpine-type fissures are chlorite group minerals. These may occur as irregularly distributed vermicular inclusions, as phantoms or evenly distributed in the crystals.

The photo shows a group of elongated, Dauphiné habit crystals with creme-colored pericline feldspar crystals on matrix. The chlorite inclusions are mostly confined to the lower part of the crystals, and the smaller crystals at the base are completely filled with it. Interestingly, the feldspar crystals show no chlorite inclusions. From the Tipling Mine in the Dhading-District, Himalayas, Nepal.


Quartz crystals from alpine-type fissures grew over a very long time (sometimes millions of years) and fluids were included in them while the environment slowly changed. This chemical record makes them interesting to geologist, and a lot of the knowledge about the Alpine orogeny is based on the study of these fluids.


Size


approx. 20cm 
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Crystals that are larger than 20 cm are rare in the European Alps and demand a high price if they are of good quality. The largest, meter-sized crystals of the European Alps have been found in the Hohe Tauern in Austria and the Aar Massive in Switzerland. Until 2008 the record was held by crystals found between 1996 and 2007 by strahlers Franz von Arx and Paul von Känel at the Planggenstock mountain, Aar Massive, measuring more than 100 cm. This record was overtopped in 2008 by a crystal measuring more than 120 cm, found by Franz von Arx and Elio Müller in the same fissure. The group with the largest crystals is shown on the photo to the right, taken at an exhibition at Flüelen, Switzerland. The remarkably clear crystals have been worked out of a quartz vein inside a very large fissure that measures 39 meters and that has been opened between 1996 and 2008 (Flüelen exhibition booklet, 2007). As of November 2009, the end of that fissure is still not reached.

Much larger crystals of more than 2 m length have been found in the Central and Polar Urals in Russia, but I do not know if these really came from the alpine-type fissures that occur in that mountain range.


 

Strahlen

In the German-speaking parts of Switzerland "Strahlen" (pronounced "shtrah-len", with an "a" like in "car") is the term for the century old Alpine profession of rock hounding. It refers to rock crystals, which are also sometimes called "Strahlen" (German for "rays"). The people who do this, either professionally or as a hobby, are called Strahler.

In Austria the term is a bit more down to earth, Steinsucher, or Stoasucha in dialect, which translaters to "stone searchers". In France and the French-speaking parts of Switzerland the term is cristalliers, in Italy and the Italian-speaking parts of Switzerland it is cristallieri, again the reference to quartz is obvious. The Romansh term is cavacristallas.

A Strahler explores by "reading the rocks" to find signs of hidden alpine-type fissures. In the Alps, Strahlen has been a source of income for many centuries, so all alpine-type fissures that are easy to access and easy to open have already been exploited. An open pocket is hardly ever found, although exceptional finds are made occasionally, as erosion continues.

In some areas you need a permit for rockhounding. For example, in many communities in Switzerland you need a so-called Strahlerpatent, which can be obtained for a fee at the local authorities, and some municipalities do not allow strahlen at all. These regulations are subject to change and one is well advised to check the legal situation before a rockhounding trip, as the fines can be very high. It should be understood that digging in a forest or on cultivated land is generally forbidden. The use of tools and machinery is also strictly regulated and some municipalities and provinces do not allow anything but simple manual tools.
In Austria there are no patents and rock hounding is not generally forbidden. However, many of the mineralogically interesting areas are inside a National Park. These consist of a core zone ("Kernzone") where mineral collecting is strictly forbidden and an outer zone ("Auβenzone") where only collecting with hand tools (small hammer and chisel) is allowed. In Italy the regulations are also rather complex, and one is well-advised to ask about the current legal situation at mineralogical clubs.

Fig.6.1 (8/2005) 2000x2560 919kb
There are only a handful of professional Strahlers, Dosi Venzin from Switzerland is one of them. Fig.6.1 shows him at work on a narrow fissure in Val Giuv. It can be a lot of work to open a fissure and one may need to use heavy tools to do it - the yellow tool in the front is a gasoline-driven drill. This is very much dependent on the type of rock and the local conditions of the rock, of course, and sometimes a hammer, chisels and a special prybar, the "Strahlstock", will be sufficient.

Fig.6.2 (8/2005) 1800x1800 438kb
Dosi Venzin carefully inspects a small specimen just removed from the fissure. As you can see, opening a fissure with heavy tools does not guarantee a yield of large crystals. The crystals found in this area are usually not very big, but of exceptional clarity and very homogeneous color.

Fig.6.3 (8/2005) 1200x840 212kb
The photo shows smoky quartz crystals on syenite matrix that have just been removed from an alpine-type fissure and that are still partially covered by pocket clay. The photo was shot at the same location in Val Giuv, the field of view is about 30 cm.

Fig.6.4 (8/2007) 1200x938 313kb
Strahlen is a slow business, and sometimes people work on a pocket for many years. Fissures in shists are generally smaller and easier to work on, and are often exploited within a day, sometimes an hour or less. The image shows a large alpine-type fissure in "Giuv syenite" rock in upper Val Giuv, at an altitude of about 2800m. It has first been exploited at the end of the 19th century, and has been worked on again in the 1960s. I haven't made accurate measurements, so I can only estimate it is about 1-2 meters wide, 1 meter high, and 5-8 meters long. To the left there is a bright aplitic dike cutting vertically through the syenite, and many fissures at Val Giuv lie in the vicinity of aplitic dikes and quartz veins.

Fig.6.5 (8/2007) 1520x1007 278kb
A view into the old alpine-type fissure shown in Fig.6.1. The cavity is partially filled with water, a couple of years ago it would simply have been filled with ice. The cavity was not that large initially, and most of the walls are fresh rock because the strahlers removed most parts of the original wall from the pocket, either parts of a massive quartz vein or matrix specimen. The aplite dike already shown in Fig.6.1 is visible at the left wall. There's a large rusty metal rod used as a prybar in the water and you can see a few wooden shelves that have been used to push the heavy rocks and the specimen out of the cavity. At the end of the alpine-type cleft one can still see small parts of the old fissure walls covered with brown smoky quartz crystals.

One of the difficulties is that alpine-type fissures are often not associated with striking geological features in the landscape, like a big vein or an obvious zonation around a pegmatite. They are simply to small to influence the morphology of the surrounding rocks in a way that would make their presence obvious from a distance. To find them, you have to patiently walk along the rocks. They often appear to be randomly distributed in a homogeneously formed rock[6.1]. A pocket might be just half a meter away inside the rock but someone not acquainted with the art of Strahlen would not note anything special[6.2]. The signs that lead one to a pocket are numerous, but rather subtle, and are not the same in all the different host rocks. So one needs a lot of patience and experience to find an alpine-type fissure and even more energy to open one. If someone inexperienced on a rockhounding trip to the Alps simply follows the well-known "good signs" for alpine-type fissures in an area that is reachable with the physical condition of an office employee, he will likely find only emptied pockets.

The following story might give you an idea of the skills of Strahlers: in 2003 I and a group of other rock hounds were attending a guided tour in the Feldbachtal, a side valley of the Binntal, Switzerland. At the end of the day, we and one of our guides, Ruedi, were heading back to our camp to meet the others for dinner. We were quickly walking down a slope covered by rubble, dirt and a little vegetation. Suddenly Ruedi stopped and took out his pick. We couldn't see anything suspicious, but he showed us a narrow band in the dirt that was a bit more yellow and started to dig into the ground. Two minutes later he had pulled several rock crystal groups out of a pocket that none of us would have found.

Fig.6.6 (8/2009) 2160x1434 354kb
A problem strahlers in the Alps frequently have to deal with is that fissures are rarely dry. Very few fissures are empty cavities outlined by crystals (like the one shown in Figs.1.1 and 1.4), often the crystals lie in tough mud, and at high altitudes the fissures may be filled with ice or frozen mud, pocket clay and rubble. Ice in pockets is not only a problem because it is difficult to remove, crystals exposed to the stress of growing ice may also crack. Fissures that are opened deep inside a mountain tend to have less damaged crystals than fissures close to the surface. Figure 6.6 is a view inside the pocket in upper Val Giuv shown in the Fig.1.5. A gas heater is placed in the pocket and left there over night to melt the ice that encloses the crystals. The pocket has been opened at an altitude of 2900 m in permafrost area.


Fig.6.7 (8/2008) 1434x2160 540kb
The last image shows Dosi Venzin drilling a borehole for a small amount of explosive into the tough Giuv syenite rock. In areas that have been searched for centuries the use of heavy tools and sometimes explosives is the only way to quickly open new pockets. The use of explosives requires a special permit and is allowed only a few areas. The amount of explosives used is very small, mostly around 80-200 g. The idea is to loosen the host rock, without damaging the fragile crystals, and then move most of the cracked rock out of the way manually. This is the same location in Val Giuv as in Fig.6.1, just shot 3 years later.



Footnotes

6.1 Earlier I have pointed out that alpine-type fissures often occur in groups and are associated with zones of stretching, but this is not so easy to see on a jagged rock wall.

6.2 Which is not to say that I am acquainted with the art of Strahlen. I am slowly getting better at it, but I have very little practical experience.



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