chert?

[plainasty]

ok, question? what exactly is chert?

[L Roebuck]

Az can give you a great description and more information than I about chert but you might know chert by its other name which is flint.

[jonsdigs]

Hi Mark,
I'm a mine geologist down here at Gordonsville, TN.

Chert is a rock made of quartz. It is found in limestone. It's fine grain makes it very tough and difficult to break. Since it's quartz, it is very hard as well as insoluble so it is left over when the limestone it occurs in dissolves away.


Photo by Ryan Fraley

Here a picture of chert in a cave ceiling.

[plainasty]

well.. thats really interesting. i saw the term and had no idea what to relate it to. thanks for help on that one

[Azurerana]

What jonsdigs says is about 80% correct. Chert is what people in the US Midwest call flint, or cryptocrystalline (meaning crystals are only visible under a microscope)quartz or silicon dioxide. It is a 7 of 10 on the Mohs hardness scale, and is one of the harder common rocks. The name chert usually refers to the lighter colored varieties of the rock. It is usually a sedimentary rock, (sometimes can form in igneous rock) formed in one of three ways.
1) Dissolved silica globules much like silica gel agglomerate in the ocean, form soft nodules, and then turn to stone within the sedimentary rock sequence of limestone, sandstones and shales. These nodules form around a speck of something and form like pearls.
2) Silica is incorporated into diatoms, radiolarians and algal mats (all living things in bodies of water) and form stromatolites (colonies) or reef structures. These reef structures get inundated, and turned to rock, inside limestone (the photo jonsdigs posted is one such cryptozoan reef structure.)

3) Chert can be laid down as nodules or layers in volcanic lava (thundereggs) during the normal process of volcanic cooling. It also can form as thick continuous layers on the deep seafloor, below the depth at which limestone will form.

Chert can be nearly all quartz, or a mix of quartz and calcite on a microscopic level. Many stone implements such as arrowheads, needles, knives, spearpoints and beads are made of chert. Another common occurence of chert is as brown river gravel. Lots and lots of common Paleozoic fossils are replaced by chert.

Actually, chert is not as insoluble in water as many people believe. When chert erodes, it first develops an oxidation rind-- turning a glassy rock to a punky, light colored, dull-surfaced/pitted stone. The brown in brown river gravel is such an oxidation rind. In the final stages, weathered chert is very brittle and lightweight.

And chert is murder on your knees if you crawl on it like cavers do. By the way, the black coloring on the photo isn't chert: chert, for some reason, has an affinity for manganese, and picks up manganese dioxide as a hard varnish if immersed in manganese containing water.

[plainasty]

so would a diamond rate 10 out of 10 on the mohs hardness scale? i believe i've identified some before, is it possible to have a few small fossils imbeded in it? thanks AZ, what do you do?

[Azurerana]

plainasty said:

so would a diamond rate 10 out of 10 on the mohs hardness scale? i believe i've identified some before, is it possible to have a few small fossils imbeded in it? thanks AZ, what do you do?

Yes, diamond is 10. The complete list, in order, are talc, gypsum, calcite, fluorite, apatite, orthoclase feldspar (there are a wide variety of feldspars with hardnesses from 5.5 to 6.5, some different in different directions in the same specimen) quartz (rock crystal, chert, chalcedony-- same mineral, different sizes and shapes of crystals), topaz, corundum (ruby and sapphire) and diamond.

Sure, chert can have fossils in it, especially radiolarians, fossil sponge spicules, and diatoms. Most are microscopic, but some are naked-eye size. There are oodles of Cambrian and Ordovician stromatolites which have become chertified, and double oodles of Mississippian age (@ 350 million year old) crinoid stem pieces, bryzoan fans and stems, brachiopods and gastroods (both shell looking fossils) which have turned to chert. Some fossils like the radiolarians, spicules and diatoms have silica shells. Most of the rest were calcite or aragonite at one time, and were silicified later, just like petrified wood-- most of the 'pretty' petrified wood was silica soaked, and became agate, chalcedony (both also microcrystalline quartz) or chert.

Don't know where you are, but there are chert nodules, reefs and fossils, and layers over 3 billion years old (think the upper peninsula of Michigan and the Gunflint chert and banded iron formations (chert and iron). In Missouri, there are chert nodules, layers and fossils in the Cambrian, Ordovician and Mississippian aged rock. As a matter of fact, our state rock (mozarkite) is a rather colorful variety of pink, blue, yellow, white and tan chert suitable for use in jewelry. On
Link
both the mozarkite and all the crinoids except the head on the sawn gray square at the lower center are made of chert. (We don't have a lot of volcanic chert around here that I know of-- but there is a lot out west, and some in Brazil. Near San Francisco, there are rumpled chert layers which used to be ocean floor, but which have now been scraped up and pasted to North America. Google San Franciscan complex or accretionary wedge,and you'll find out about some of these younger seafloor cherts.

Since chert can form 'after limestone' you can get nodules which still show the limestone structure, instances where only the fossils are certified, or entire layers where the former limestone has turned to chert.

What do I do? I'm a freelance writer who also does computer graphics, geology and hydrology fee speaking, websites, office work and bindery work in my self-employment as Geo Communications Services. I've got degrees in writing and geology, and have been collecting chert since I was 6 years old (a long time ago). The reason I am a fount of chert knowledge is because it is pretty, has very little economic value (used in grinding a little, but mostly if you're past the stone point or flintknapper stage, no one buys or trades chert) and I had to do a major term paper/ Powerpoint investigating some sedimentary rock, and I jumped at it. Was told no one ever studies chert...what a challenge!

best
Azurerana

[brianc]

Sometime back I was enlightened after watching a special on Mamoth Cave! Chert nodules in lower sections of the cave appear to have been formed as worm type animals excreted in their trails and travels through the sand bed that at that time was the floor of an ocean! The excrements then solidified into silicanised chert as the sand and other debris formed the limestone around it. The limestone dissolves leaving the less soluble chert! As I look at chert now, I can see what the program was describing! Of course as AZ describes there are many formations of Chert!

[Azurerana]

Ah. One of my profs called these the trails of mud-sucking vermin. (Well, vermin means worms, so he was right.)

Actually the mud sucking vermin can make either positive casts as brianc describes, or negative casts (holes) in limestone. The Kimmswick limestone has these mud filled worm cast burrow marks, which are less consolidated than the matrix limestone. To start with, the stone looks marbled but if it's left out in the rain for a number of years it turns into Swiss cheese, with holes entirely through the rock. A Girl Scout camp near here has exposed Kimmswick-- I first was introduced to them because they were called friendship rocks or fairy stones. (Naw, don't go there-- we're talking 9 and 10 year old girls!)

When I found out they were worm tracks....ICK!

[kenredux]

Yes Azurerana, the geo in you and the definition of chert in the Dictionary of Geologic Terms are in perfect accord.

But the writer in you should also note that the popular term "chert' refers to any discontinuous rock that has value only in fill or as road metal especially in Alabama and other enlighten states.

[Azurerana]

Never heard that. Around here it's called chat or chat gravel, and refers to any crushed stone, including road metal and mine tailings.

[kenredux]

Say Azurerana, is chat a corruption of chert or is chad a corruption of chat? Inquiring minds..etc.

"Douglas G. Wilson"
Subject: "chad"-- a possible origin


The word 'chat' for the gravel that is used in resurfacing country
roads in that area. The chat comes from steel mills in northeast Texas that process locally mined iron ore. ..."chat" as gravel is well known and the Scottish term (chad) is not known at all.

This is what Hendrickson referred to, more or less: mining refuse used on roads. I think in etymological speculation, an exact match generally should be preferred: score 1 for "chad" = "gravel". But an English word should be
preferred over a Scots word: score 1 for "chat". I tend to trust MW more than I do Hendrickson (who dates the word "chad" to the late 1960's!): 1 for "chad". But after all, I'm not sure that these two words for gravel are really distinct!

DARE gives
"chat" = "gravelly tailings from mines", and some of the quotations indicate that "chat" is a corruption of "chert" -- but I'm not sure this is substantiated, and "chert" itself is of mysterious origin.

The "English Dialect Dictionary" (Oxford, 1961) shows
"chad" = "gravel"/"riverbed stones" (Scotland) and also =
"chaff" (in food) (Norfolk).

The EDD also gives
"chat" = (1) "catkin" [hazel, maple],
(2) "key" [ash,sycamore]
(3) "fir-cone",
(5) "chip of wood",
(7) "small inferior potato",
(8) "small piece of coal",
(9) "piece of stone blended with lead ore" [like
the "chat" in DARE], etc. -- but notes that this word "chat" also occurs as"chad" (Yorkshire, Derbyshire)!

All of these refer to small objects occurring in heaps ... just like
punch-card or punch-tape chad(s).

Several of the above would give credible origins for "chad" = "punched paper chip(s)".

["Chat" refers to a bunch of other things, including lice, birds (DARE gives "chat" and "chad" as bird names), kittens, etc., etc., and -- of course -- like about 100,000 other words -- the female sex organ (cf.French "chat").]

Note that "chad" is a very common word, perhaps virtually a recognition signal, among teletype hobbyists as evidenced in -- e.g. -- the "Greenkeys"
archive at a ham radio site on the Web, with maybe 200 instances of "chad": one page includes the imaginary (?) fountain statue of a "nude maiden pouring chads", no doubt a symbol of the brotherhood:

----------------------------------------------------------------- Doug Wilson

[Azurerana]

I haven't a clue. I wouldn't think that chat and chert are related except perhaps in Boston, where 'r" is rarely pronounced. You may not have noticed, but "chat gravel" is the preferred term. I wouldn't be surprised if chat is actually a gravel size somewhere, but around here it always refers to composition.

Now chad and chat refering to river gravel-- with a Scots Gaelic reference, that I could believe considering the number of Scots-Irish who settled Appalachia, and at one remove, the Ozarks, as well as the number of British miners who ended up in the lead mines here.

Chad is also a country in Africa. I don't doubt there are numerous unrelated meanings for that word, too.

[kenredux]

Sure, Azure, "chert" is a word like all words; indefinite, with fuzzy parameters. But here in Northern Aladamnbama chert is largely characterized by the sharp angularized silica of the Fort Payne Formation, a basal Mississippian deposit that outcrops in folds in the Valley and Ridge Province and in incised river cuts of the Tennessee Valley.

And, to a lesser extent, in a Cambrian deposit known as the Copper Ridge Dolomite, alternately known as the Copper Ridge Chert.

Most dirt roads in Northern Alabama are not dirt but chert.
You cannot walk or run barefooted on chert unless you are Dave Howell. Dave Howell goes to weddings and tea parties barefooted. His feet are tough. Back when he was President of the Birmingham Grotto he conducted the meetings wearing barefeet. Some guest speakers thought that we were rednecks. Dave has this theory that mankind's demise came about because we began wearing shoes. Besides, he says, wearing shoes is unnatural and unhealthy.

I don't know about all that but Dave sure does have tough feet.

[plainasty]

ok another question about it az.. what makes it black like that? Just over time?

[Brian Roebuck]

Perhaps magnesium or iron in the surrounding rock? I'm not sure though. The geologists will tell us for sure.

plainasty said:

ok another question about it az.. what makes it black like that? Just over time?

[jonsdigs]

The manganese oxide minerals pyrolusite, manganite, and psilomelane are usually responsible for the black staining of rocks underground. By the way, in their crystallized form they make dendrites (Below.)

[Azurerana]

Chert rock river gravel which is that familiar soft brown color is caused by oxidation of iron in the chert. If you crack these rocks open, you will find this color is just a thin layer. Iron is also the coloring agent when you burn chert rocks, and they turn bright red. This is also oxidation, but I think it is the difference in moisture state (fire vs water) which causes the different colors. Iron is cool stuff. It can turn a rock any color in the rainbow plus many of the neutral colors out there depending upon the conditions. Iron plus manganese, and you get a purple color.

What fascinates me about this is: why manganese oxides generally have a preference for silicates (including chert) over carbonates. I've seen dendrites on chert and limestone and dolomite-- it seems the smoother the rock surface the more likely it has dendrites-- but have never seen manganese thoroughly coat a carbonate rock the way they do chert.

If you ever get to Wind or Jewel Caves (esp. Jewel) you will find them filled with a fine black powder. More manganese, under semi-arid conditions.