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chert vs.flint

Posted by Norman King  
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Norman King December 24, 2010 01:15AM
I just noticed these discussions on the pages for chert and flint.

Chert: “A cryptocrystalline variety of Quartz - found in layers and nodules in sedimentary rocks, typically light in color. Technically, it is a "rock" - not a mineral - but it is usually classified as a variety of Quartz for convenience's sake, since it is usually composed largely of quartz grains with few other minerals involved. Chert is similar to flint, but light in color, and formed in a somewhat different environment.”

Flint: “A granular cryptocrystalline variety of quartz, typically dark in color. Technically a rock - not a mineral - but it is usually classified as a variety of quartz for convenience's sake, being composed largely of grains of quartz with few other minerals involved. Similar to chert, but dark, and it forms in a somewhat different environment.”

These explanations are oversimplified and in part incorrect. Chert and flint are indeed microcrystalline (not cryptocrystalline) quartz. They are microcrystalline, not granular (that’s two different things). Secondary chert and flint are essentially identical–flint is simply a variety of chert. Chert is found as nodules to layers (layers if the nodules grew so much as to coalesce) in coarse-grained limestone poor in organic matter, so it is light in color and may have various tints of yellow, red, or brown from iron oxide. Flint also forms in limestone, typically in chalk or recrystallized chalk. The limestone is very fine grained and is rich in organic matter and reduced iron, having formed in deep, quiet water. Such a rock is dark colored. The silica replaces carbonate before the rock can become oxidized, and it surrounds and protects the organic matter and reduced iron, even when it is uplifted to near the Earth’s surface where oxidation can occur. Thus, you find black flint nodules in white chalk or limestone.

Much chert in the rock record is primary chert. It forms in deep marine water by accumulation of microscopic skeletal material (radiolarians and diatoms) forming radiolarite and diatomite, which recrystallize to “bedded chert” if buried deeply enough. This is the origin of diatomite (never buried deeply) and bedded chert (at one time buried thousands of meters) in the Monterey Formation of California. Primary chert also seems to have formed in Precambrian times in reducing marine environments close to sea-floor volcanic activity or submarine vents (source of silica), such as the Gunflint Formation bedded cherts in northern Minnesota and Ontario. Thus it forms in conditions unlike secondary chert and the variety of chert called flint.

There is a common (mis)understanding of what flint is. The flint hills of Kansas, for example, are rich in chert but contain no flint at all. In the flint photo gallery at mindat, photos 261907 and 289510 can be defended as flint, although I do not think they are very representative. The example used on the flint mineral data page as an example is very clearly not flint–very clearly, not even close. All other photos in the flint gallery are not flint. They are either chert, or jasper (red, yellow, brown, or orange variety of chert), or agate (interlaminated chert and chalcedony).

I will upload photos of some examples of classic flint shortly.
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Rock Currier December 24, 2010 07:09PM
Norman,
You sound like you know what you are talking about. Why don't you write up the descriptive information for the chert and flint pages. I think what you will write will be better than what we have now.

Rock Currier
Crystals not pistols.
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Norman King December 24, 2010 07:39PM
Rock,

Well, there aren't many areas in mineralogy where I can sound like an expert. Maybe that will change with time, but I've had a good head start with things that are largely sedimentary. Give me some time, and I'll see what I can do.

Thanks. Happy holidays!
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Rock Currier December 24, 2010 08:27PM
Norman,
Why don't you rework what you have written above about chert and flint a little into two separate little articles, cross referencing when you think appropriate, and Ill put that into the flint and chert pages. At least we will have something better than we have now and that will do till you think you have something more extensive and better.

Rock Currier
Crystals not pistols.
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Alfredo Petrov December 25, 2010 06:07PM
Quartz varieties for the most part have never been officially defined, and there is a lot of "fuzziness" here, with different groups using different definitions. Sedimentary petrologists may define "chert" for example as Norman does, but there is much variety in chert usage in other fields. I don't think we here at Mindat have a mandate to specify definitions applicable to all fields, so I would not like to see any wholesale changes to the current Mindat pages on those varieties until some sort of consensus is reached.

Rock's suggestion is probably best: Norman (if he is willing) to write articles on these topics, post them in the articles section (and/or Best Minerals section?), with links to the articles posted on the minerals pages, and open it up for discussion. I anticipate quite a lot of discussion... leading perhaps, eventually, to some more refined definitions on the mineral pages.
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Tomasz Praszkier December 25, 2010 07:21PM
I am not very good in English so I am not sure if it will be easy to understand :-)
Chert is local concentration of silica in sedimentary rock which do not have "sharp borderline" with rock - usually is slowly changes in to the "host rock".
Flint (in Polish meaning :-) is a concrecional form of silica with sharp borderlines with rock.
Both - chert and flint - can have the same origin (usually remobilization from sponge spikules of silica gel), both are a rock. Usually chert has less silica (in %) even in central part than flint.
Difference is in form not in chemistry, origin etc.

Hope that helps
Tomek

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"Spirifer" Geological Society
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David Von Bargen December 25, 2010 08:32PM

There is no sharp mineralogical distinction between flint and chert. The term flint is used principally for the siliceous nodules found chiefly in chalk and marly limestones, such as in the Upper Cretaceous chalk beds of western Europe and other regions. Massive bedded deposits are generally called chert.
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Tomasz Praszkier December 25, 2010 10:13PM
Chert and flint are not mineralogical but petrographical terms so use geological dictionary - there is distinction :-)
This is like you will try to find difference between marble and limestone - mineralogical is the same, but difference is clear and easy. Just look for it in geological dictionary.

Tomek

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"Spirifer" Geological Society
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Norman King December 25, 2010 10:16PM
Rock, Alfredo, and Thomasz,

First, let me say that I am honored to be invited to try to improve the chert/flint discussions. I have in mind what Rock first suggested–that I make a few strategic changes in the current texts that will clarify to anyone, professional sedimentologist or not, how we all can understand the terminology used on mindat, because it will be unambiguous. I know all of the terminology that has been suggested for microcrystalline (etc.) quartz, ranging from that in Dana’s Textbook, 1932 edition, to what is on various websites today. And, from 28 years in the university classroom I also know what terms can be misconstrued and what terms have unequivocal meaning. Alfredo is correct about the fuzziness. But after all, these are just varieties, and I am not sure that “mineral variety” has a uniform definition. So, let me try something to submit for approval, as per Alfredo’s suggestions. That is indeed what I had understood for my task. And since I do not know the best way for presenting it, you all will tell me. Finally, I had assumed the management board would discuss whether or not my suggestions are appropriate.

Third, relative to Thomasz’ comments (Thomasz, I understand you perfectly!), the change from limestone to chert is a complete change. In other words, there is no such thing as half changed. The changes usually occur along a “front,’ where it all changes over less than a millimeter, and this is what I have seen in chert. I do not remember ever seeing a place where a few “islands” of chert are found in limestone, and they increase in size and/or abundance until the rock gradually becomes 100% chert. It may occur, but after looking at thousands of thin sections that is not my impression. Thomasz, if you can cite published literature, especially with photos, of how you say it is, I will incorporate that into my discussion.

Individual fossil grains may be partly silicified, but the boundary is rather abrupt, occurring over perhaps a few tens of microns–it does not happen gradually. However, it is common for the fossils to change first and the matrix to change later, and that may produce the partial effects you cite. I do not know of any studies documenting that in general that chert has less silica than flint. It is true that European flint is often purer silica than European chert, but this has proven not to be the general case across the globe. If you know of any literature that claims otherwise, let me know and I will review it before continuing.

As I said in my original posting, there is secondary chert, of which you (Thomasz) speak, but there is also primary chert like the photo of bedded chert I posted yesterday (photo 355432; BTW, I have locality photos waiting to be uploaded). This is a Paleoproterozoic rock that formed well before there were abundant sponges or other organisms that secreted siliceous skeletons whose silica could be remobilized. If you know of published literature claiming otherwise, let me know, and I will review that also before proceeding. It is important to me that people know there is primary chert–chert deposited originally as silica, as well as secondary chert–chert that results from replacement of the calcite in limestone. This is not my idea. It is well known.

My motivation is educational, and that is the thrust of many of my specimen descriptions, as well as my comments here on the mindat treatment of chert/flint. After all, I am an educator (with nearly ten years as a petroleum geologist; I’ve got to make that personal page!).
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Rock Currier December 25, 2010 10:57PM
Norman,
Why don't you write an article about flint, chert, jasper, etc. Just click on the My Home Page button at the top right of the page and click on the My Articles button at the top of your home page, and I think you can figure it out from there. I think it would be good to talk about which groups of people use what definitions, geologist, mineralogists etc. I think that when the managers see what you turn out they will have no problem turning you loose on the pages for chert, jasper, flint etc.

Rock Currier
Crystals not pistols.
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Norman King December 26, 2010 05:23AM
Gentlemen (and Ladies, too),

These communications do not seem to be going the way I would like. It was never my intent to propose any system of chert nomenclature that is more narrow or complicated than what is already being used at mindat. I think the issues I referred to in my first posting are actually easy to clear up. To keep this from continuing to stray from the important points, and to get right away to what I would like to see as an improved discussion, I submit the text below. Of course, it is not in final format, and it assumes that the chert, flint, and jasper galleries would be combined. Two of these galleries are especially small. So chert would include the subvarieties flint and jasper. I think taking the latter step would solve a lot of problems with the current postings in those galleries. These seem to be almost chaotic, but unnecessarily so, and I see no reason to think things will change unless the mindat structure here is changed. (My apologies for being so bold.) I reference chalcedony and agate, but this would be the text for chert.

Suggested text:

Technically chert is a rock, not a mineral, because it is an aggregate of tiny crystals of quartz that are so small they can only be seen with a microscope. This texture has been called microcrystalline or cryptocrystalline. Chert has a smooth surface, and breaks with conchoidal fracture, which are the characters that allow it to be shaped by chipping to make stone tools. Chert is found in nodules and irregular layers in limestone, forming as a secondary replacement of calcite that made up the limestone. Also, evenly-bedded chert forms where pure siliceous sediment accumulated directly on the sea floor as a primary deposit. Chert occurs in virtually all colors, including white, tan, yellow, red, brown, green, blue, and gray to black. Yellow, red, and brown chert may be referred to as jasper, and dark gray to black chert may be referred to as flint. Bedded chert is usually designated by that name. Chert and chalcedony are similar, but the microscopic crystals of chalcedony are fibrous. The distinction can only be made using polarized light and a petrographic microscope. With the unaided eye, chalcedony tends to be lighter colored and more translucent than chert, ranging to semi-transparent. The term agate has been applied to a seemingly unlimited variety of variously-colored chalcedonies, but properly refers only to chalcedony in thin curving or zig-zag bands. Chalcedony has a separate page and gallery, and is discussed in more detail there.


I guess this is more wordy than the usual introduction, but there were misconceptions and complicated issues here. I think anybody can follow it, there are no errors or omissions, and it avoids the controversies over terminology. It could be divided into two paragraphs to make it seem less dense; the comment on stone tools could be omitted (but I think it makes the discussion seem more friendly). The comments about distinguishing chert from chalcedony should be repeated on the chalcedony page.
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Ralph Bottrill December 26, 2010 05:51AM
Norman
I think what you are doing looks good, although I would agree chert and flint are rocks, not minerals, but we have a lot of grey areas.

Regards,
Ralph
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Amir C. Akhavan December 26, 2010 09:00AM
Norman, I'm glad that someone is willing to "take the heat" and triggered this discussion.
I don't have definitive answers to all these questions, and also suffer quite a bit from the incredible mess in the nomenclature, even in textbooks or scientific articles.
I've got my opinions, of course, which are not entirely in line with yours :D

To begin with, I've added a flint to the database.
http://www.mindat.org/photo-355602.html
And no, it is a flint. :D

My point is, that I'd at least replace all that is said about the colors of chert, flint, and jasper with "any color".
Otherwise people will (understandably) be confused when they pick up white flint in Denmark.
No, they are not all bleached, some are genuinely white (unfortunately I don't have a sample of it) . Bleaching is common for flints, too, of course, and turns them into white, light-brown, yellow pieces. Color as a way of distinguishing them from something else is just a bad idea. The "color issue" is my main complaint about the current Mindat description.

Also, I'd leave out all that is said about agate or other micro/cryptocrystalline quartz and microquartz varieties from the description (btw, the text contains a few ambiguities and errors). To my own surprise I've seen (in my petrographic microscope) that jasper can be length-fast chalcedony, too.
Jasper is even less well defined than chert and flint, I'd say the term is only useful in lapidary context, people call all sorts of things jasper without flushing. Basically, I could do without mentioning jasper in the text.

Also keep in mind that flint may contain small amounts of moganite. If this is also true for radiolarites and the like (chert), I don't know.

I'm not so sure about calling (all) flint nodules secondary either, as there are nodules around which the bedding of the rock is distorted in much the same way that is found around other concretions that form during diagenesis.
Calling flint secondary chert also makes it difficult to draw a line between flint and "replacement chert" (for example http://www.mindat.org/loc-216195.html).

I'd rather try to shorten the definition as much as possible, like:

"Chert: Any rock largely composed of irregularly intergrown microscopic quartz grains."

That would be a deviation from the common nomenclature, because it would include certain jaspers, but it would be a consistent and non-bloated definition. It would also automatically include flint, but the term flint could then be specified by adding other properties. It would also allow the presence of minor amounts of length-fast chalcedony or quartzine in the rock.
Of course, there are still difficult cases, for example the silicified siderite concretions in "Toneisensteingeoden" ("clay iron stone geodes"), which are indeed the result of a secondary transformation of an already present concretion.

I'm not at home right now and will post the literature references when I'm back home.

Amir
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Norman King December 26, 2010 03:58PM
Amir,

Thank you so much for your very interesting comments. I agree completely that the confusion stems largely from the terms used by lapidarists. As a result, it is almost entirely a hand specimen terminology. You are also correct in noting the inconsistency in usage in dictionaries and other supposedly authoritarian sources. I often find myself reading some such descriptions, and saying to myself, "That's not right. Where did they get that?" Yes, cryptocrystalline is a more restrictive term than microcrystalline, so they are not the same. But in view of what people can read all over the “web” I am simply conceding that it is hopeless to hold out for one term or the other. I'm glad you came along to say that, because I was beginning to fear people would start thinking I am full of hot air. Petrographic use of the term "granular" is also confusing, so I avoided the term altogether in my suggested new version. As YOU know, the petrographic term granular is used by petrographers to distinguish the texture of chert/jasper/flint from "fibrous" chalcedony, but these materials are not granular to someone simply handling a piece.

I am not surprised that you have found length-fast chalcedony (forgive me if I say “fibrous” on occasion) in jasper. It can be found intermixed in chert also. And I have seen places where the initial replacement of calcite by silica is chalcedony, and there are places within that where it has gone to chert. I GUESS that the transformation might be reversible, even if unlikely (since boundary energy must be much higher in elongated crystals than in equidimensional crystals). In any case, we may be left having only color as a distinguishing feature for non-petrographers. The question then becomes, can the public continue using these well-known words that have immediate meaning for people? I HOPE SO! So, rather than say that color is not valid for making the distinctions, I would like to say that color is how people distinguish these subvarieties in every-day practice, even if the crystallinity varies on a microscopic level.

I like your red flint! I can see that it is likely flint even from the photograph. After looking at the flint gallery I have added to MY list of valid flint two light gray specimens (145142 and 288159). After reading the specifics about the piece shown on the main flint page, I think that may also be legitimate flint, but it is not a good example to have used to give people an idea as to what flint should look like. I believe that originally the term flint referred specifically to silica nodules in chalk. If I am right about that, then by definition any chert/flint/jasper in chalk would be “true” flint. In this regard, I have these questions about your differentiation of these materials (because I don’t know the answers): How was jasper originally defined? How was chert originally defined? How was flint originally defined? I ask because if the crystalline features you refer to seem to be found across these varieties that have long had these different names, and the public cannot ever hope to see what these distinctions are, then perhaps your distinctions are not useful for mindat--nobody would pay any attention and no one coiuld assess if the labels used are correct. The problem with dictionaries and definitions is that they reflect the actual usage of words. I believe that chert, flint, and jasper have never really been technical terms. Correct me if I am wrong, but there are no type specimens, chemical or structural analyses, etc. that are standards in original or type locality materials such that everything else can be judged as to whether they "make the grade." Another question is, how SHOULD these varieties be defined? Isn't color used for tourmalines and many other gemstones?

With respect the bending of sedimentary layers around the nodules, this is actually what I would expect to see for any flint. It shows that the silica crystallized before much compaction took place. This shows that replacement of calcite was very early, with the silica probably derived from dissolution of near-by sponge spicules and/or radiolarians and diatoms. Where there is no such bending, the replacement occurred after compaction. Both cases are secondary. In chalk, this effect would be greatest because chalk is very porous before compaction–that’s why its bulk density is so low, and it compacts to a fraction of its original thickness as it converts to denser, hard limestone.

I would like to see any published material that discusses the criteria you use in your own work. Please do send the references.

Again, I am delighted that you came to the discussion! (tu)



Edited 1 time(s). Last edit at 12/26/2010 04:35PM by Norman King.
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Amir C. Akhavan December 27, 2010 09:57PM
Norman,
you give me too much credit.
I'm just cautious because I am really confused by the little I've read and the little that I've seen myself so far. I still got a long way to go until something of a big picture emerges and I feel more comfortable to talk about the topic. So don't be disappointed that I can't answer most of your questions.
Actually, I haven't done any "work" on this topic and I am a complete layman and newbie. I've just started to look closer at microcrystalline quartz varieties with the aid of a petrographic microscope - mostly because I want to understand what people talk about - and the first thin sections were a revelation. I'm sure you've seen much more than I have.

I wouldn't say that the lapidarist are the one to blame - they usually don't care too much about chert ;-)
The scientists are at least just as bad. To give you an example: ever heard of "nanocrystalline quartz"? A term used in the title of a paper in a respected scientific journal (BTW, I'm not generally opposed to making up new names).

I think it's o.k. to call an agate or chalcedony fibrous, it's just that individual quartz crystals, however small they are, are not fibrous. They are also not elongated to the extent of the entire "fiber". Dimensions given for grains and crystals in flint and chert are between 5 to 20 micrometers (Knauth, 1994), whereas in agate the ranges given are about 50-200 nanometers (Moxon & Carpenter, 2010) and 8-100 nanometers (Taijing & Sunagawa, 1994), the second value is for crystallites within a "fiber" (visualized with a TEM), which are in fact elongated to about 3:1. These are values for length-fast chalcedony, of course.

I don't know the original definition of jasper, chert and flint, and I doubt that there ever was a canonical definition. Max Bauer (1904) characterized jasper as "massive quartz" made of microscopic grains with conchoidal fracture, dull luster, perfect opacity, deep color. He did that from a somewhat lapidary perspective, though.

There are different approaches in explaining a term.
One can talk about:
1. What X is.
2. What properties X has.
3. How X forms.
4. How people call X.
5. What the things are that people call X (that's very different from 4.)

If one wants to start a real mess, just start the explanation with 3, 4 or 5.
I would only do that if there's really no consensus or simply no understanding of the essence of something.
Ideally: Start with 1., define the essence of X (structure, composition).
Then it is usually easy to say which properties are related to the structure or derived from it (texture, hardness, etc.) and which are accidential (for chert and flint that would be color, for example). There's a big difference between "flint is darker than chert" and "any color, typically dark", the latter shows that the color is not essential or indicative.
Now that we know what something is, we can ask how it forms. And we can answer questions 4. and 5. (the etymological perspective) without causing any confusion at the reader.
That's just an idealized approach, of course. (sorry for "doing the teacher" )

Finally a few references:

White flint, diagenesis, texture, composition
H.B. Madsen, L. Stemmerik
Diagenesis of flint and procellanite in the Maastrichtian chalk at Stevns Klint, Denmark
Journal of Sedimentary Research, Vol.80, 578-588, 2010

Flint and chert in general
L.P.Knauth
Petrogenesis of chert
in: Reviews in Mineralogy, Vol.29
Silica - Physical behavior, geochemistry and materials applications
Mineralogical Society of America, Washington, D.C., 1994

Helgoland flint

F.J. Krüger
Untersuchungen über die roten Flinte von Helgoland und eine Deutung möglicher Färbungsursachen
Meyniana, Vol.32, 105-112, 1980


Overview and nomenclature of microcrystalline quartz varieties (no, it is not consistent)

A. Michel-Lévy, C.P.E. Munier-Chalmas
Mémoire sur diverses formes affectées par le réseau élémentaire du quartz
Bull. Soc. Franç. Min., 15, 159-190, 1892

O. Braitsch
Über die natürlichen Faser- und Aggregationstypen beim SiO2, ihre Verwachsungsformen, Richtungsstatistik und Doppelbrechung
Heidelb. Beitr. Mineral. Pertogr., Vol.5, 331-372, 1957

O.W. Flörke, H. Graetsch, B. Martin, K. Röller, R. Wirth
Nomenclature of micro- and non-crystalline silica minerals based on structure and microstructure
Neues Jahrbuch Miner. Abh., Vol 163, 19-42, 1991

H. Graetsch
Structural characteristics of opaline and microcrystalline silica minerals
in: Reviews in Mineralogy, Vol.29
Silica - Physical behavior, geochemistry and materials applications
Mineralogical Society of America, Washington, D.C., 1994

S.L. Cady, H.R. Wenk, M. Sintubin
Microfibrous quartz varieties: characterization by quantitative X-ray texture analysis and transmission electron microscopy
Contrib. Mineral. Petrol., 130, 320-335, 1998


Grain size in agates
L. Taijing, I. Sunagawa
Texture formation of agate in geode
Mineralogical Journal, Vol.17, 53-76, 1994

T. Moxon, M.A. Carpenter
Crystallite growth kinetics in nanocrystalline quartz (agate and chalcedony)
Mineralogical Magazine, Vol.73, 551-568, 2009


BTW, the Danish flint paper is confusing to me, because they say they have identified "lutecite" by the extinction pattern being off by 30 degrees. "Lutecite" is a discredited term for other authors who would call it moganite ;-) and the angle would be larger than 30 degrees. The pattern could also be indicative of "pseudochalcedony", stacked along {20-21}.
Just to show it is confusing, and that terms are used by different authors with a different meaning.

I'm sorry that I can't contribute much more to the topic.
Maybe in a year or so, when I see things more clearly.
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Norman King December 28, 2010 07:50PM
Amir and others,

Amir. That is the easiest literature search I’ve ever done! :D Thank you very much. I’m going to send off for some of those right away.

I think you are right about the origins of these terms. They go back centuries, come from different places, and have never had universal or unequivocal definitions.

Taking Amir’s comments and those by Thomasz, my own ideas, seeing what has been put in mindat galleries, and what I have seen in other sources, I have rewritten a suggested text on chert for the mindat page. I took out everything that does not seem to be essential, and it is still longer than for most minerals. As before, flint and jasper are grouped with chert. Being completely realistic, this is probably the best way to bring order out of chaos. These terms are always going to be subjective and imprecise due to the weight of historical usage. The current situation suggests that, if the galleries aren’t reorganized, it will always be time-consuming and subjective for mindat editors to evaluate the uploaded photos. The terminology is not only poorly defined, imprecise, and subjective for users, but is also poorly defined, imprecise, and subjective for mindat editors. Sorting specimens according to the most rigorous scientific/technical criteria is important, and someone “out there” has to do it, but that is simply not going to get done for photos uploaded by amateur collectors and even by most professional geologists. Thus, I hope this write-up accommodates those realities and will provide a framework for moving ahead that will be practical for everyone, and also not incorrect in any way.

The revised version is below. Feedback is certain to be useful. Thanks.



Chert is actually a rock consisting of microcrystalline quartz. Most chert is secondary, forming nodules in limestone where silica replaces calcite. Primary chert forms in thin beds composed of microscopic recrystallized skeletons of siliceous marine organisms.

Chalcedony and chert have the same chemical composition, but chalcedony is very light colored and nearly transparent, consisting of fibrous microcrystals (see chalcedony page).

All non-fibrous microcrystalline quartz may be called chert. Flint and jasper are subvarieties, differing from chert proper as shown below. Distinctions are often subjective, and may not reflect critical underlying mineralogical differences.

Chert (narrower usage)–translucent; cream, light gray, tan, or pastel colors, often banded or patchy.

Flint–translucent; typically gray to nearly black.

Jasper–opaque; variable coloration, commonly vivid and in intricate patterns.
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Amir C. Akhavan December 29, 2010 10:59PM
If I were asked to change the text to something I like, the result would look like this (with comments in italic font). You see that our views of the matter are quite different.

= = = = = = = = = = = = = = = =

Chert is actually a rock consisting mostly of microcrystalline quartz.
Chert may contain a fair amount of impurities and moganite, hence the term "mostly"

Most chert is primary :P , and forms large bodies of radiolarite rock as well as thin beds within other rocks by recystallization of microscopic opaline skeletons of marine organisms. Radiolarite with interspersed thin layers of mudstone or slate and prominent bedding is called "ribbon chert" or "banded chert". Chert is also formed around submarine volcanic hydrothermal vents (black and white smokers).
Radiolarites (and sufficiently old diatomites) qualify as chert by the definition given in the first sentence. "Ribbon chert" is a well-known term.

Secondary chert nodules in limestone probably have formed when silica derived from opaline skeletons replaced calcite.
Just cautious

Chalcedony and chert have the same chemical composition, but chalcedony is very light colored and nearly transparent, consisting of fibrous microcrystals (see chalcedony page). Chert may contain significant amounts of impurities other than moganite, chalcedony can do so to a much lesser degree. Chalcedony is not necessarily light-colored or translucent.

All rocks that consist mostly of non-fibrous microcrystalline quartz may be called chert.
Non-fibrous, microcrystalline quartz is called microquartz. Chert may contain chalcedony, hence "mostly".

Nodular chert is often called flint. Some people distinguish between light-colored chert and dark-colored flint. Others call primary chert "chert" and all secondary chert "flint". There's no generally accepted definition of both terms.
Jasper does not generally qualify as chert, as it may be mostly composed of fibrous microcrystalline quartz. The properties of jasper should be dealt with on the jasper page. I don't feel comfortable calling flint a "subvariety". I also don't know if I would distinguish between chert and "chert in the narrow sense".

Chert is a hard, tough rock with little porosity. Chert and flint are colored by embedded impurities and can be of various colors, often gray, brown, red, or black. The coloration may be even, patchy or banded. Depending on the amount and type of embedded impurities chert and flint are more or less translucent.

= = = = = = = = = = = = = = = =

Of course this text is much too long.
And I still have a problem with epithermal silica veins and sinters that could be microquartz/chalcedony, but are often banded because individual silica layers have successively been deposited. Porcelanites (or porcellanites, depending on the author) and banded iron formations are other "problems".

More to read (pretty random selection, though).

S.L. Cady, H.R. Wenk, K.H. Downing
HRTEM of microcrystalline opal in chert and porcelanite from the Monterey Formation, California
American Mineralogist, Vol.81, 1380-1395, 1996

S.E. Calvert
Composition and origin of North Atlantic deep sea cherts
Contributions to Mineralogy and Petrology, Vol.33, 273-288, 1971

A. Goldstein, Jr.
Cherts and novaculites of Ouchita facies
in: H.A. Ireland (editor)
Silica in Sediments
A symposium sponsored by the Society of Economic Paleontologists and Mineralogists
Society of Economic Paleontologists and Mineralogists, Special Puplication No.7, 135-149, 1959

Y. Kolodny, M. Chaussidon, A. Katz
Geochemistry of chert breccia
Geochimica et Cosmochimica Acta, Vol.69, 427-439, 2005

L. Hopkinson, S. Roberts, R. Herrington, J. Wilkinson
The nature of crystalline silica from the TAG submarine hydrothermal mound, 26°N Mid Atlantic Ridge
Contributions to Mineralogy and Petrology, Vol. 137, 342-350, 1999

T. Grenne, J.F. Slack
Bedded jaspers of the Ordovician Løkken ophiolite, Norway: seafloor deposition and diagenetic maturation of hydrothermal plume-derived silica-iron gels
Mineralium Deposita, Vol.38, 625-639, 2003

K.L. Rai, S.N. Sarkar, P.R. Paul
Primary depositional and diagenetic features in the banded iron formation and associated iron-ore deposits of Noamundi, Singhbhum District, Bihar, India
Mineralium Deposita, Vol.15, 189-200, 1980
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Rock Currier December 30, 2010 01:13AM
Amir,
I would not be too worried about you definition being too long. You might want to arrive at a bare bones definition and then after that put something like:

However there is more to it that that and here is a more in depth definition: xxxxxxx

I know that there are a lot of people like myself who would welcome a more in-depth definition of the origin and differences between flint, chert, agate etc and the differences between them and that would be primarily what I would hope the Mindat pages on these minerals might provide. You might consider writing an article for Mindat about these things that would include simple and more complex definitions of them as well as summaries of the the more up to date characterizations of them, their origins, history etc and then use portions of that article to place in the Mindat pages for those "minerals". The article I wrote on cleaning quartz, which I have edited, updated and added to from time to time has to date has to date been viewed 24K times. I don't think that anything I have ever written for any of the amateur magazines has ever had that kind of attention. A good article on chert, flint, jasper, etc might well get that kind of circulation.

I just bought about 150 neolithic arrowheads from Gao, Mali that appear to be made from flint? chert? quartzite?, well, I really don't know what. These things were just brought to me by an exploration geologist/mine finder friend of mine in his suitcase directly from Bamako from a source that used to supply him with meteorites with that fall name. These are supposed to be 4 to 6 thousand years old and under the scope they certainly don't appear to be "freshly" made. I would like to know what they are made of. If you think you would like to take a look to see if you could put a more definite name to the type of quartz family mineral they are, I would be glad to send you a couple of them. If so, shoot me a private message with your mail address.

Rock Currier
Crystals not pistols.
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Norman King December 30, 2010 02:53AM
Amir,

Thank you for your comments, and for the additional references.

As for the precautions you noted, I can easily insert the term “mostly” when distinguishing what chert is and how it differs from chalcedony. In fact, I have already done so on my revisions page.

We have to keep in mind that this write-up would be for mindat users, most of whom lack much background in technical aspects of geology and mineralogy. Regardless of the truth of the matter, they probably do not care whether primary or secondary chert are more abundant. In fact, they probably are not aware of the distinction (but, of course, we are taking care of that).

It may indeed be true that some people call all primary chert “chert” and all secondary chert “flint,” but that is an extreme position that can only stem from lack of familiarity by those authors with the terminology being used by practicing sedimentologists. I have never heard anyone call such things “ribbon chert.” Someone read about that expression, and repeated it, not knowing that the term is not really being used by people who currently study it.

That being said, I can assure you that secondary chert is what mindat users are posting photos for–entirely (except for me–I uploaded the first photo of primary bedded chert). Secondary chert forms on the craton, where people can collect it. Primary chert forms in deep ocean basins over oceanic crust, and almost never finds its way onto dry land. This is true whether the primary (bedded) chert is recrystallized radiolarite or diatomite, or whether the silica was derived from volcanic/hydrothermal emanations from oceanic crust at spreading centers. Moreover, primary chert that is derived from radiolarians/diatoms can only be Phanerozoic, whereas a very large portion of bedded chert in the ancient rock record is Precambrian in age. For example, the photo of bedded chert that I posted is Paleoproterozoic.

I don’t believe that anyone has attempted to tally up the volume of secondary chert versus the volume of primary chert, but if they would did, that would still not address the issue of how much primary chert on ocean basin sea floors will ever be accessible to us, so it doesn’t matter for mindat purposes. In fact, most primary chert ever deposited on the sea floors has been recycled back into the mantle. The oldest that still survives is only about 170 million years old.

Documented radiolarite is uncommon. Mostly, people say that they see strange objects (usually quite uncommon and always controversial) in bedded chert that may represent fossil remains of radiolarians, sponge spicules, and diatoms. Diatoms are an issue, anyway, because they did not appear in the fossil record until the Late Jurassic.

Novaculite is an enigmatic rock of uncertain origin. It may be true that some people think they know how it forms, but that is an easy way to start an argument among professionals. I’d just as soon leave it out of the discussion, along with sinters. I found that there are so many more important things to talk about, even in my advanced sedimentology classes, that there was no time to bring up the latter (I did talk about novaculites for perhaps 5 minutes, but left the question of their origin open).

Moganite is already mentioned on the chalcedony page in mindat. I recommend waiting to see better documentation of its degree of presence in chert, flint, and jasper in general before bringing it up here. I dare say that only a very small percentage of mindat users have ever heard of it or seen it. And, since we are talking about essentially invisible micromixtures, I think we can hold off on the chert/flint/jasper page until we have a more thorough understanding.

I do not mean to seem dismissive of anything you wrote. I think much of it is true. But, in a rush to be COMPLETELY correct, I think we will “lose” many people along the way. Sometimes “less is more.” Or, to use another common expression, KISS--“Keep It Simple, Stupid!” No offense meant, of course–this is just a humorous (and well-known) cautionary note that bears considerable wisdom. The mindat management board will have to think about what kind of presentation there should be, and how the subvarieties of chert are handled. The concern that inspired me to suggest changes was that, up to now, the users of mindat have had difficulty with the basics of chert/flint/jasper (to say nothing of advanced, cutting-edge material). It might be best to refer on the basic mindat page to more technical write-ups such as Rock has suggested.



Edited 1 time(s). Last edit at 12/30/2010 02:58AM by Norman King.
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Norman King December 30, 2010 04:47AM
Here are some chert photographs that I uploaded today (12-30-10). I have several more to go. I may have to wait to do more thin sections because the university is closed for the holidays, and I can't figure out where they put my old collection of thin sections.





These are Photo Nos. 356793 and 356798. There is some text for each one in the photo file (chert gallery).
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Rock Currier December 30, 2010 10:23AM
I think two levels of definitions and or discussions are needed for mindat, and I think there is plenty of room for them both.

One is a sort of bare bones definition and description for the casual user who doesn't have any or only a rudimentary knowledge of mineralogy and geology and also below that a more in depth definition and discussion for those of us who know something, but would like to learn more.

Rock Currier
Crystals not pistols.
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Alfredo Petrov January 01, 2011 03:54PM
I agree, Rock. Mindat is used by all levels, from professionals to beginners and kids, so we will need both types of definition.
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Jolyon & Katya Ralph January 01, 2011 04:20PM
With care, we can do one definition that is suitable for everyone.

I'm always wary about "dumbing down" things for kids. My experience is that they want to know everything, and information that challenges them and encourages them to ask "what does that mean?" can be a very healthy thing.

If anything it's the lazy, more casual, adults who want the "simplified" explanations.

As long as the first sentence or paragraph of a description gives the simplified explanation, immediately followed up by the more detailed one, then I think we're fine.
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Norman King January 02, 2011 04:47PM
After a bit more reading inspired by technical considerations voiced by Amir and Thomasz, and seeing the opinions by Rock, Alfredo, Ralph, and Jolyon about format and overall approach, I have revised my chert/flint write-up, presented below. I think this includes all the technical information that is needed, and accommodates the several concerns that Amir had, as well as Thomasz' understanding. The big new thing here is that I added the information about moganite. I think this is actually reaching "old hat" status now because it has been known to be a major constituent of chert and chalcedony at least since the 1980's, even if only recognized by IMA since 1999. Thus, my treatment of it is completely matter-of-fact. Any more detail for chalcedony/agate should be on the chalcedony page.

Discussion invited!


* * * * *

Chert is actually a rock consisting dominantly of microcrystalline quartz. In continental settings most chert is secondary, forming nodules in limestone where silica replaces calcite. Primary chert forms in thin beds over large areas of the sea floor, resulting from recrystallization of microscopic skeletons of siliceous marine organisms.

Chert and chalcedony have the same chemical composition, but chalcedony tends to be very light colored and nearly transparent. Chert consists of blocky microcrystals, whereas chalcedony is microfibrous. It is common for both varieties to contain small amounts of the other. All quartz that crystallizes from low-temperature solutions contains up to 14% moganite, or even more under hypersaline conditions. Moganite is a cryptocrystalline polymorph of quartz having monoclinic symmetry. It is metastable, inverting to quartz with time, so that chert and chalcedony older than about 100 million years have only trace amounts of moganite, ultimately approaching zero. Weathering also causes moganite inversion to quartz.

All non-fibrous microcrystalline quartz may be called chert. Many people recognize flint and jasper as subvarieties of chert, differing from typical chert as shown below. Distinctions are often subjective, and may not reflect critical underlying mineralogical differences.

Typical chert–translucent; cream, tan, or pastel colors, often banded or patchy.

Flint–translucent; usually gray to nearly black, but may have other colors; many people use this term specifically for chert occurring in chalk or for especially pure chert.

Jasper–essentially opaque; variable coloration, commonly vivid and in intricate patterns, produced by mineral impurities.
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Rock Currier January 02, 2011 09:30PM
Excellent! I don't know enough abut it to make suggestions for improvements in the text. Perhaps a few other here will. Do you think images of typical pieces of chert, flint etc would enhance the article? Do you know of any micro photographs that distinctly show the blocky microcrystalline nature of of chert and the micro fibrous nature of chalcedony? Would young chalcedony show up distinctly different in a micro-photograph under cross polars from old chalcedony where the moganite had morphed to regular quartz?

Rock Currier
Crystals not pistols.
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Norman King January 02, 2011 11:39PM
Rock,

Your first question is reminiscent of the on-going discussion of head photos. I think that is a good idea, although I don't know how easy it would be. If it can be done, I will look for what I think are good examples to illustrate important points. I can supply more thin section photos, but actually the thin section photo I already uploaded shows the blocky texture of chert . That's it: that's what "blocky" looks like--no preferred orientation or shape, but nevertheless irregular and without distinct crystal faces. This texture has also been called "granular," but that petrographic term was coined by petrographers for use with igneous and metamorphic rocks. When used for sedimentary rocks (which chert is) it suggests a clastic texture such as in sandstone, and that is the wrong idea altogether. So I called it blocky, which is also a valid petrographic term, and one that will not likely be misconstrued in this context. I have thin sections somewhere that show the fibrous texture of chalcedony, and quite distinctly. It looks kind of silky. It may take me a while to locate them in the collections at the university, though.

Moganite is completely invisible macroscopically and in thin section. That's why it remained hidden for so long. But it's part of what makes most microcrystalline quartz, so it should be mentioned even if we can't point it out to anyone.
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Amir C. Akhavan January 03, 2011 01:55AM
Sorry for the late response, I've been busy with other things.
I will first make my own suggestion of a text, "before it is too late". ;)
The English is certainly not "elegant".
The text deals exclusively with chert, not with jasper. In my opinion, if one doesn't want to confuse people by a "hierarchy" that is not there, jasper should be dealt with on a "jasper page", an extra page on flint does not hurt either, even if it is mentioned here. The same goes for all the other rock types.
I will respond to the things said in earlier postings later, maybe in a few days.

Just one remark/questions:
I agree that from what you read in the literature and what can be seen in a thin section at usual magnifications, the grains in chert should show "blocky" extinction, much like what one sees in quartzite, just on a different scale. It's just that when I look at my flint thin section at high magnification ( 400x or 1000x ), I can see moving "grain boundaries" and extinction patterns, that is, I see a behavior that is like that in chalcedony. This puzzles me quite a bit, as I can't come up with an explanation except that the flint grains are actually chalcedony spherulites. At small magnifications the thin section looks like what I've seen in the literature. Unfortunately I don't have a sample of a chert from a different, ideally older rock. I would really like to know if an old chert of whatever type also shows this kind of extinction pattern.

Attached are 3 photos, showing a small flint nodule (Dänisch Nienhof, north of Kiel, Germany) and two photos of thin sections made from this rock, the first in plane polarized light, the second with crossed polarizers (field of view 1.2 mm). One can see small fossils and a few elliptic cavities (probably fossil casts) filled with typical length-fast chalcedony.








========================================

Chert is a name used for a variety of rocks that consist dominantly of microcrystalline quartz. Cherts are massive, tough rocks with conchoidal fracture, little porosity and a density close to that of quartz. They are colored by embedded impurities and can be of various colors, often gray, brown, red, or black. The coloration may be even, patchy or banded. Depending on the amount and type of embedded impurities cherts are more or less translucent to almost opaque.

Cherts are composed of microscopic quartz grains, mostly as so-called microquartz, irregularly intergrown quartz crystals. The size of the crystals is typically around 5-20 micrometers, but cherts with smaller and larger grains are also found. They may contain varying amounts of different types of chalcedony with fibrous texture, and small amounts of other silica polymorphs, like moganite. Depending on the geological environment, small amounts of organic matter, calcite, sulfides, iron oxides and other substances are embedded between the quartz grains. Most cherts contain water bound in SiOH groups and as molecular water.

Rocks that resemble cherts but are made mostly of silica polymorphs other than quartz (cristobalite, for example, or opal-A), are occasionally called "chert", too, usually with a prefix designating their mineral composition, like "cristobalite chert" or "opaline chert".

Cherts form in a variety of geological settings. Bedded cherts, for example radiolarites, form by recystallization of microscopic opaline skeletons of marine organisms (radiolaria, diatomes, siliceous sponges). Nodular chert and irregular, discontinuous chert beds form during the diagenesis of limestones and marls in shallow marine environents; the silica source of these cherts are also opaline skeletons. Nodular chert in limestones is also called flint. Magadi-type cherts, named after Lake Magadi in Kenya, are formed from silicates as magadiite in evaporite deposits by the removal of alkaline ions by meteoric waters. The interpretation of Archean cherts is difficult because the recrystallization into larger quartz grains can destroy much of the initial texture and possible fossil records; the Archean cherts of the Onverwacht group in South Africa are thought to have been formed by silicification of volcaniclastic sediments. Cherts occur in banded iron formations, then often colored brightly by iron minerals. Nodular chert in limestones can accumulate in chert breccias or bedded chert deposits in karst environments. Chert is also formed around submarine volcanic hydrothermal vents (black smokers). Rhynie chert, famous among paleontologists for its well-preserved fossil record, is a Devonian siliceous sinter formed at a hot spring at Rhynie, Scotland.

Literature:
Harvey Blatt, Robert J. Tracy, Brent E. Owens
Petrology - Igneous, Sedimentary, and Metamorphic
W.H. Freeman and Co., New York, 2006

Roland Vinx
Gesteinsbestimmung im Gelände
Elsevier, Spektrum Akademischer Verlag, Heidelberg, 2005

L.P. Knauth
Petrogenesis of chert
in: Reviews in Mineralogy, Vol.29
Silica - Physical behavior, geochemistry and materials applications
Mineralogical Society of America, Washington, D.C., 1994

H. Graetsch
Structural characteristics of opaline and microcrystalline silica minerals
in: Reviews in Mineralogy, Vol.29
Silica - Physical behavior, geochemistry and materials applications
Mineralogical Society of America, Washington, D.C., 1994



Edited 1 time(s). Last edit at 01/03/2011 07:07AM by Rock Currier.
open | download - fl_de_daenisch_nienhof_Q231_1_scl.jpg (75.6 KB)
open | download - fl_de_daenisch_nienhof_T000_01_scl.jpg (352.9 KB)
open | download - fl_de_daenisch_nienhof_T000_02_scl.jpg (715.3 KB)
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Rock Currier January 03, 2011 07:09AM
Amir,
If you will upload those images to the mindat gallery they can be embedded in these Bulletin Board forum postings and more easily be more flexibly arranged (sized) and captions placed below them.

Rock Currier
Crystals not pistols.
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Ralph Bottrill January 03, 2011 09:05AM
Norman
I know what you mean about the chert textures. I have never been able to find a beter term than "rice grain " texture. "Blocky" infers to me angular, oblong shapes, but the quartz grains usually have a rice-grain shaped, rounded prism habit.

Regards,
Ralph
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Amir C. Akhavan January 04, 2011 12:45AM
@Rock
Uploaded.

Two corrections to my text:

========================================
1. Bedded cherts, for example radiolarites, form by recystallization of sediments made of microscopic small opaline skeletons of marine organisms (radiolaria, diatomes, siliceous sponges).

2. Nodular chert and irregular, discontinuous chert beds form during the diagenesis of limestones and marls in shallow marine environents; the silica source of these cherts are also opaline skeletons embedded in the limestones and marls.
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Norman King January 04, 2011 09:39PM
Amir and Ralph,

Amir wrote:

“. . . . . when I look at my flint thin section at high magnification ( 400x or 1000x ), I can see moving "grain boundaries" and extinction patterns, that is, I see a behavior that is like that in chalcedony. This puzzles me quite a bit, as I can't come up with an explanation except that the flint grains are actually chalcedony spherulites. At small magnifications the thin section looks like what I've seen in the literature. Unfortunately I don't have a sample of a chert from a different, ideally older rock. I would really like to know if an old chert of whatever type also shows this kind of extinction pattern.”

I have often seen the effects you describe. I think moving grain boundaries result from limited depth of field in a transparent material, and the focal point varying with respect to the position and orientation of the grain boundaries. In places you may also be looking across a grain boundary and through two different crystals that have different extinction positions. Rotating the microscope stage will make those situations “come alive.” I have also seen undulatory extinction within individual chert grains. That is common in stressed quartz crystals of any size. I don’t know if anyone has taken a census to know how if these effects vary by age, but they occur in cherts at least as old as Ordovician. I haven’t really studied older ones so I can’t say much about them. I agree that your thin section shows scattered areas of chalcedony in cherty matrix, and the chalcedony seems to occur within skeletal grains.

Ralph commented on terms to describe chert texture. If you look closely at my thin section and Amir’s, you can see quite variety of chert grain sizes and shapes. I have seen that virtually “everywhere” in chert samples. In the same field of view domains of very small grains grade abruptly into domains having much larger grains, or the grains seem to vary almost randomly, with tiny grains next to larger ones and elongate grains next to equidimensional grains. Again, I don’t know if anyone has done a spatial or morphological census of grain size distributions or shapes in chert.

Like in all research, as soon as you get an answer to a question, several more questions come up!
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Ralph Bottrill January 04, 2011 11:59PM
Norman
I agree with your comments on grain boundaries - you get similar textures back in Precambrian cherts, if you can find some not too metamorphosed.
The textures and grain sizes in cherts can be complex - you can get very fine grained patches along side relatively coarse crystals. I think this has several causes: eg. zones with more impurities (eg iron oxides) can be slower to recrystallise than purer zones; you can have pre-existing quartz grains, pore fillings and replacement with different generations of silica (usually initially opal and/or chalcedony), etc. I have not done any statistics on the grain sizes and shapes but it could be interesting. You can spend a lot of time looking at some thin sections scratching your head as to whats happening. All good fun!

Regards,
Ralph
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Alfredo Petrov January 05, 2011 02:28AM
A bit off-topic, but for anyone who likes chert, I recommend a trip to the Caribbean beaches on the southwestern part of the Dominican Republic: several tens of km of beaches composed almost entirely of smooth chert shingles, eroded out of the adjacent limestone cliffs - perhaps a million tons of clean chert? Colour and translucency are highly variable, with many different shades of white, grey, brown, pink, reddish and toffee-coloured tones. Some of the pinker more translucent ones would make good lapidary material. Some are faintly banded like agate, with the banding only visible when wet. The transition between chert and limestone is very variable too - some very sharp boundaries perhaps only 0.1mm thick, and others where the gradation covers 5mm or more.
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Rock Currier January 06, 2011 07:20AM
Alfredo,
You observations about chert in the Dominican Republic should be placed under a locality for chert along with a couple of pictures of some of the beaches and a few close up images. This should not get lost in the Message Board. Have you a smart phone you can take and transmit images with? I think some smart phones even have a GPS ap that will give you your lat long if you are in range of a cell tower.

Rock Currier
Crystals not pistols.
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daniel schick November 05, 2012 06:47AM
Hi-

New here.
Need to find chemical compoditions of french and danish flint pebbles used in grinding media.
Need to find European or American as well as supplier sources.
Are ther American sources avaiable? In Tennesse or Kentucky?

Daniel
daniels@elconprecision.com
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