Flint and Chert

 

last modified: Thursday, 16-Feb-2012 23:23:59 CET

Document status: complete

Flint and chert are dense, cryptocrystalline varieties of quartz, slightly translucent to almost opaque. Firestone, Hornstone, and Silex are other names for flint and chert. Some authors use "chert" as the more general term, with flint being a dark variant. Others use the term "flint" for nodules and "chert" for large bodies of rocks. Both terms are more often used very broadly. There's apparently no clearly defined line that separates flint from chert and that people agree on.

Note on classifying flint and chert
This chapter is called "Flint and Chert", but after looking at the matter again and again, I'm getting less and less convinced that chert and flint should be treated together in a chapter, although some authorities on that matter (e.g. Knauth, 1994) refer to flint as "nodular chert".
As a general rule, one should primarily categorize rocks according to what they are, and not to how they formed. Otherwise the nomenclature will turn quickly into a inconsistent mess, once old assumptions about the formation must be revised. In that sense, Knauth did the right thing to call flint "nodular chert", because he considers them equal in their basic physical properties.
However, I will use the term "flint" for the nodular forms and the term "chert" for the rock-forming type. This is done not so much out of respect for the "tradition", but because I'm not sure how "special" flint is in its properties, and how broadly the term chert is defined with respect to structural and physical properties.
It is possible that once I've been working through the literature more deeply I change my mind and lump chert, flint (and possibly jasper) together.

 

Specific Properties

Flint does not have a specific color, but is often dark gray with shades of brown, red, or yellow, and sometimes white. Brighter or more colorful variants are sometimes called chert by some people. If a large body of rock is entirely made of dense, dull cryptocrystalline quartz, it is generally called chert, regardless of its color. Flint may show color banding, but this is not a concentric banding as seen in agate.

The color can be caused by inclusions of organic compounds (black), metal sulfides (black), and various metal oxides and hydroxides (yellow, orange, brown, reddish, etc.). It is slightly translucent to almost opaque, sometimes only thin chips are translucent at the edges. On freshly broken surfaces the luster is dull, at best waxy, but because it is very hard, flint takes a good polish and assumes a glassy luster, just like agate.

Flint is not a chemically very pure quartz variety, the large amounts of impurities and its fine-grained structure can make it dull and almost opaque. Some people would say that flint and chert are technically spoken not minerals, but rocks. It is a textural variety of quartz that shares some properties with jasper. It contains considerable amounts of other silica modifications, mostly moganite (Heaney and Post, 1992), perhaps opal. Like jasper, it has a very irregular, grainy structure, whereas agates - also a cryptocrystalline quartz variant - consist of regularly intergrown tiny quartz crystals that give them a "fibrous" structure. Jasper is almost opaque and typically its colors are more intense, while flint is often a bit translucent. The size of the grains in flint is between 0.5 to 20 micrometers (Knauth, 1994).

Small cavities lined with small quartz crystals (usually less than a millimeter in size) are not uncommon in large flint nodules. The walls of the cavities are often made of gray, white or blue, translucent chalcedony.

Flint freshly removed from chalk contains a few percent of water. After a couple of years they have mostly dried out and get more brittle. Flint will crack in fire because of the water in it, sometimes so badly that small flint chips fly around.

Flint concretions from sedimentary rocks forms irregular nodules that are often surrounded by a thin white layer, sometimes called cortex. Although it has a powder-like consistence and can sometimes be partially rubbed off, it is not chalk, but quartz (you cannot dissolve it in acids).

The most interesting physical property of flint is the way it splits. Flint has a conchoidal fracture like rock crystal or glass, but its fracture surfaces are not as uneven and curved. It's easier to control the direction of the splitting, and the edges are more straight. This depends a little bit on the amount of impurities, "purer" flint behaves more like glass. Chips coming off a flint can have razor sharp edges, making it suitable as a cutting tool (it's a good idea to wear safety goggles if you work on flint).

Flint is easy to spot in a gravel pit: often it is covered by a thin white layer, and - in contrast to the other pebbles - it has an irregular shape.
The latter is due to four reasons:

Gangue quartz - which is harder than flint - has a more uneven fracture and a more inhomogeneous structure that other rocks can more easily work on, and in the end you get a white rounded pebble.

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The image to the right shows an eastward view of a beach at the Baltic Sea near Boltenhagen, Mecklenburg-Vorpommern, Germany. The beach is covered with rounded boulders of igneous and metamorphic rocks from Scandinavia that were deposited by southward moving glaciers during the ice-ages. In addition, a large number of flint nodules can be found which originate from Cretaceous chalk cliffs in Denmark and Germany that mostly have been eroded away. Because the flint nodules and chips are irregular, the waves can move them more easily than rounded rocks and the flint accumulates at the upper part of the beach.

Below is an image of the gravel in the upper part of the beach. Roughly half of all the rocks are flint. Many of them have a white cortex, others can be recognized by their irregular shape and glass-like fracture pattern. The field of view of that image is about 80 cm.


In many pieces of flint from sedimentary rocks one can find tiny white marine fossils, embedded silica skeletons of radiolaria, foraminiferes, diatoms, or sponges. Sometimes the fossils are quite big, like the silicified sea urchins that are occasionally found in the chalk cliffs at the island of Rügen in the Baltic Sea, or the Cretaceous chalk of Dorset in southwestern England.


 

Microstructure


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Occurrence

Flint occurs as irregularly shaped concretions in sedimentary rocks, usually in chalk, a soft limestone, but also as fillings in cracks in other types of rock. Flint also forms thin layers between the beds of the sedimentary rocks. The platy specimen from such layers are just a few millimeters or centimeters thick.

In sedimentary rocks, the silica in flint is usually of biogenic origin: countless skeletons of tiny marine organisms, like radiolaria, foraminiferes, or diatoms that have sedimented on the ocean floor and are slowly buried under more chalk, silica and organic ooze. The silica skeletons consist of opaline silica (amorphous silica with some water) and are much easier dissolved in alkaline solutions than quartz. The process that concentrates dissolved silica locally to form concretions (the flint nodules nodules) is apparently related to the decay of organic material from the dead animals under anaerobic conditions (absence of oxygen). The altered chemical composition of the surrounding watery solutions[1] causes the silica to accumulate around the dead body, and occasionally a larger fossil, like the skeleton of a sea urchin, can be found inside a flint nodule.

I don't know much about other places, but in Europe flint can be found in large quantities weathering out of the Cretaceous chalk cliffs of the Baltic Sea and the English Channel (e.g. the "Cliffs of Dover"). It forms "in situ soap deposits" where the chalk has been washed away and has been widely distributed by glaciers that moved over the Baltic Sea southward during the ice ages. When you hike in the glacially transformed landscape of Northern Germany, most of the pebbles you see are flint, the rest are pebbles of igneous and metamorphic rocks from Scandinavia. There is actually a "firestone line", the southern border of the distribution of flint which coincides roughly with the southern border of the ice shield during the last ice age.

In France, flint can be found in Cretacean limestones in the Isle de France, and it used to be mined in that area during the stone ages. Similar flint deposits are found in Jurassic limestones of the Fränkische Alb in Bavaria, Germany.

Chert as a rock-forming material is described in the chapter Quartz as a Rock-Forming Mineral.

 

"The Flintstones"

We know from findings in the Olduvai Gorge in Kenia (where the remains of "Lucy", a female Australopithecus, have been discovered) and many other classic locations of anthropology, that flint, along with obsidian and crystalline quartz has been used as a raw material for tools as early as 1.5 million years ago.

In the neolithic age (the late stone age) flint was one of the first minerals to be mined and traded. Flint tools have been found several hundred kilometers away from the origin of the raw material. Important sources were Syria and Mesopotamia (about 6000-5000 b.c.), or France and England during the megalith age (about 4000-2000 b.c). The Grimes Graves Mines in Norfolk, England, had 360 shafts (Source: ->"Past Worlds - Times Atlas of Archeology"). The many collapsed mining shafts have turned the area into a strange looking landscape of craters that have puzzled people for centuries. In some churches in southern England you can see flint used as ornamental stone or even as a principal building material.

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Another famous site and a U.S. National Monument are the Alibates Flint Quarries in Texas, where colorful chert has been mined by American Indians from prehistoric times till about 1870. The flint was used for arrowheads, knifes and other tools, and has been traded between the different Indian people. In the Panhandle of northern Texas, a layer of bright dolomite cap rock that formed in the Permian Age covers large areas. At steep slopes in river valleys like that of the Canadian River that runs through the Alibates National Monument and feeds the neighboring Lake Meredith, the dolomite layer gets cut and dolomite boulders tumble down the slopes of older reddish rocks, as shown in the photo shot at Diamond Point made at the National Monument. Nodules of colorful flint slowly weather out of the dolomite and can be collected from the ground, but the flint has also been actively mined by the Indians in quarries worked into the dolomite.


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These colorful pieces of flint were scattered on the ground on the slope of the butte that is shown in the foreground in the photo above.


 

Flintknapping

Some people still work on flint today as a hobby, a practice called "flintknapping". Flintknapping involves basically 2 techniques:
1. striking the rock with a tool (like a hammer, rock, or wood)
2. applying pressure with a relatively soft tool (wood, bone, copper) for the fine work to chip off tiny flakes.
This is also the way the flint has been crafted into arrow heads, axes, and knives in the stone age or by American Indians in historic times. Flintknappers can work on many materials, like glass, obsidian, or agate, but good quality flint is still one of their favorites. Sometimes the flint is heat treated to make it more workable.

 

Firestone and Firearms

Flint was used to ignite gunpowder in firearms until the early 19th century. Flint alone will not do it, though: the flint is used to hit pyrite (FeS2), which will give of sparks, as the energy of the strike is converted into heat energy. If you hit one flint with another flint really hard, you can sometimes also see some sparks, but this is not a very reliable way of producing them. When flint hits pyrite, the heat energy will ignite tiny grains coming off the pyrite that will burn in air to form iron oxides and sulfur dioxide, SO2 (that's why it smells "like sulfur").



 

Locations and Specimen

Belgium


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A cross section of a tubular flint nodule that weathered out of a Cretaceous marl. Numerous sand grains are embedded in the flint matrix, evidence that the calcite in the marl has been metasomatically replaced by chalcedony during diagenesis. Picked up in the underground mine Caestert, Pietersberg, Visé, Liège Province, Belgium.


Denmark


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This  small fossil sea urchin, Galerites sp., is entirely made of flint and has weathered out of Cretacean chalk cliffs. It is from Stensmark beach at Grenaa, 60 km north-east of Århus.


Germany


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A  part of a tubular flint concretion found at a pebble beach of the Baltic Sea near Dänisch Nienhof, north of Kiel, Germany. Most pebbles at the beaches of the Baltic Sea are flints, and sometimes one can find nodules that weigh 10 kg or more.

The thick white crust, the cortex, is not made of chalk, but of fine-grained opaline silica. The dense and homogeneously colored core is almost black of inclusions of organic compounds.



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Many  flints carry small inclusions of skeletons of small marine organisms, like radiolaria or foraminiferes. Made of opaline silica, these are also the main source of the silica in the flint.

There is also one of the small cavities that can frequently be found inside flint nodules, outlined with tiny quartz crystals. The specimen is from the same location as the one on the first image.



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Sometimes  silica skeletons are not embedded in the flint, but accumulated and preserved in a cavity inside the flint nodule. The skeletons are very small and you need a magnifying glass to appreciate their porcelain-like look. The specimen is from the same location as the one on the first image.



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This  is a much rarer find. It shows a large ball-shaped fossil, the skeleton of a sea urchin (probably Galerites sp.), inside a flint nodule. The skeleton's interior was not completely filled with silica, so a botryoidal layer of blue-gray chalcedony could form and later be overgrown by small sparkling quartz crystals. I found this specimen at the Weissenhäuser Strand (beach) at the Baltic Sea, north of Lübeck, when I was 9 years old. After I got it, I searched the entire beach for quite some time until I slowly realized how much luck I had.



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This type of flint called "Roter Feuerstein" ("red flint") or "Helgoland flint" is occasionally found at the shores of the small island of Helgoland in the North Sea. In particular those with a red core and a black and white cortex are high in demand. The red flint is even used for lapidary arts. Flint from Helgoland has been found at various Neolithical archeological sites on the mainland, so it was already valued for its nice color in the stone ages. Although the red color is simply caused by iron oxides and hydroxides, the formation is not fully understood. Studies by Krüger, 1980, show that while the carbon content in the form of bitumen in the black zone is very high and - just as in all flints from the European Cretacean deposits - is responsible for the dark color, the red core is essentially free of carbon. The iron content does not differ significantly between the black and red parts, but apparently the oxidative state of the iron. Krüger suggests a change in the chemical environment during diagenesis. The silica accumulation is assumed to start in a oxygen rich zone that does not allow the accumulation of carbon and causes the oxidation of iron ions to colorful Fe3+. Later the environment changes to oxygen-poor and thus reducing conditions that protect the embedded bitumen and keep the iron in the lower and less colorful oxidative state as Fe2+. An alternative might be that the initial distribution of both elements and oxidative states were the same accross the flint nodules. When the environment changed to reducing conditions, the carbon compounds got reduced and immobilized in the outer parts while carbon compounds made of small molecules (methane or amines, for example) continued to diffuse out of the core. The initially red iron compounds in the outer parts would have paled under reducing conditions. Which brings up the question of how such color zones can be kept stable in a rather porous rock that has been exposed to oxygen-rich seawater for thousands of years. My guess would be that the reduced carbon compounds in the outer zone act as protective, reducing agents that keep the iron from being oxidized.

If you visit Helgoland you will notice the vertical red sandstone cliffs of Triassic age that make up most of the island. The flint is not from the cliffs but from younger Cretaceous limestones that are below the water line. Flint nodules that got detached after storms also accumulate on the beaches of the small neighboring island Düne. I've never tried it myself but I've heard that one needs to be really patient to find a red flint - the one you see on the photo is bought. More information on Helgoland flint and the location can be found at FlintSource.NET.


U.S.A.


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This  is a piece of banded chert that possibly weathered out of limestones present at that location. The specimen has an interesting internal structure, it seems to be composed of irregular layers, but this banding is completely different from that of an agate. There are a few small cavities that are outlined with tiny quartz crystals, one of them in the upper right half. The piece is slightly translucent. The white crust is the same that is found in the gray European flints, it is made of opaline silica and marks the border to the host rock. This piece of chert was found at Jalama Beach, Santa Barbara, California. I should note that just a few kilometers north of that beach in the Santa Ynez Mountains, there is a large quarry of diatomaceous earth near Lompoc.



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A nice colorful specimen found (and left) at the Alibates Flint Quarries National Monument, Texas. The location is introduced above under The Flintstones.


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A different view of same specimen.



 

Further Information, Literature, Links

L.P. Knauth wrote a review article on ->the petrogenesis of chert that compares the various environmental conditions for chert and flint formation.

The FlintSource.NET web site gives information on flint locations (nicely done with a clickable map), with scans and photos of specimen and localities, and descriptions of flint types.

The Flints from Portsdown Hill covers the local specialties, but also explains the general terms, the formation of flint nodules, and some of the included fossils.

If you are interested in more information about flintknapping, www.flintknapping.com might be a good starting point.

GEMROCKS: Ornamental & Curio Stones includes a list of North American locations, synonyms and a general description of chert and flint.


Footnotes

1 For example, when proteins decay, amines get released. These are alkaline organic nitrogen compounds related to ammonia that will rise the pH of the water. They are also the cause of the bad smell of old fish, by the way.


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