What is chert?

[Rick Brinkman]

I was recently asked this question. Not only could I not give a SIMPLE explanation, I really had no idea.

Can anyone help?

[Squirrel Girl]

Chert is cryptocrystalline quartz. That means it's formed of tinsy little crystals that are barely big enough to see in under a microscope.

It commonly forms in limestone when there is a source of dissolved silica and the proper geochemical conditions present. It forms very early after sedimentation, generally before too much compaction or lithification.

It often forms in nodules or in layers.

It is thought that sponge spicules (little needles that form an unarticulated skeleton for the beasties) are the source. Alternatively, it could be diatoms or radiolarians.

No one really understands the formation of chert very well.

[Scott McCrea]

Cryptocrystalline lithification nodules or unarticulated spicules with diatoms or radiolarians.

Rick, is that simple enough for ya?

[hank moon]

Well, this'll get ya started. Penned not too many years ago by a friend of mine name-a Roger Ling. Wonder if he minds me postin' this up here. Seeing as he's floating around in the ocean somewhere, guess it doesn't matter. Enjoy (and enjoy his site sometime: http://www.stationr.org)

The Chert Song
(To the tune of "The Beverly Hillbillies")

Come and listen to a story
'bout a rock named Chert
You trust that stuff
you'll end up in the dirt.

'Cause Chert is a rock
God put here as a joke
And not as a place
to rig a caver's rope.

Chert, that is.
Black gold! Texas turd!

Well he went to the pit
And he rigged it real good
He backed up his knots
like every caver should.

But he made one mistake
And that's how he got hurt
He rigged his rope
To a whole bunch of Chert.

That's Chert, I say,
Broken bones, lots o'scars.


Lyrics copyright 1993 by the
W Road Cave Fan Club, Inc.
Used without permission.

[Cheryl Jones]

Geologists keep pointing out that it is a rock band.

(Maybe that belonged in the (bad) Science Jokes topic!)

[bigalpha]

Also, more than likely...if you find fossils [coral, brachiopods, etc] they are silicified (sp?). That's why you can go collect them. They are much more resistant to weathering than limestone.

Essentially, quartz, chert and glass all have the same chemical formula --> SiO2. However, quartz has a crystalline structure [means the atoms and molecules are arranged the same way every time you get a quartz point], which is why it's classified as a mineral.

Of course, different color/varieties of cherts/quartz are caused by imperfections. Usually due to rogue atoms that have taken the place of the Si or O in the chemical make-up. i.e. Lithium, Iron, etc

[ian mckenzie]

Chert comes in lots of pretty colors, and was mined and traded prehistorically (shallow pit mines in the Canadian Rockies are thought to be 3000 years old). See http://en.wikipedia.org/wiki/Chert

[Rick Brinkman]

Ok...that's pretty much what I found on the internet.

So I guess I'll tell the person that asked me:

"Chert is a nodule or layer of quartz that was formed from bits of skeletons from sponges."


How's that sound???

[Squirrel Girl]

"Chert is a nodule or layer of quartz that may have formed from bits of skeletons from sponges."

[bigalpha]

It can also be precipitated if there is enough concentration of SiO2, and the right conditions.

[rockprof]

Some chert apparently also forms from chemical alteration of volcanic ash. There are chert deposits ranging back in geologic time before the advent of siliceous plankton or sponges.

By the way, flint is another name for chert.

[bigalpha]

Some chert apparently also forms from chemical alteration of volcanic ash. There are chert deposits ranging back in geologic time before the advent of siliceous plankton or sponges.

Very possible. Volcanic ash is just bits and shards of glass. So, you could get alteration between crystal forms. i.e. microcrystalline vs. qtz points

[Teresa]

No one really understands the formation of chert very well.

Actually, they do understand the formation of chert pretty well-- its just that it is a complex process of acidic dissolution/agglomeration or biogenic uptake/ deposition and chemical alteration over time from some glop which is basically dissolved silica gel opal to the nice hard shiny rock we all love to crawl on. I did an approximately zillion page geochemical report on the three major paths of chert formation (alkali pH driven (lacustrine), biogenic driven (spicules and radiolarians) and low solubility driven (perhaps bacteria altered, but basically abiotic bulk silica cycle in the typical oceanic pH conditions) some time ago, and both the conditions required for chert formation and the chemical equations cannot be described in less than several pages of tiny type with Greek letters which I don't think go on the discussion board very well.

Suggest that if someone is really interested in chert, they get a college level sedimentary petrology book, which will give a nice overview. And if you are really a nutcase on chert (as I am) you can spend a month surfing in the primary literature at a university geology library.

Just remember. Chert hurts--if you crawl on it, if it hits you as an arrow, if you get a pebble in your shoe, if you try to bite it) and make you fire, keep you company with its popping noises in a bonfire, or it can become shrapnel in a sweat lodge. Be it chalcedony or a waterpolished gem or brown stream gravel...it is my very favorite rock.

[Squirrel Girl]

if someone is really interested in chert, they get a college level sedimentary petrology book, which will give a nice overview.

Doodette, I *taught* sedimentary petrology. It depends on what you consider a really good understanding.

[Teresa]

Just because there are multiple formation pathways doesn't mean it isn't understood. It just takes more chemistry/biogeochemistry than is generally comfortable to endure.

The trouble is: there is no money in researching chert as there is in other mineral species. Most of the actual chert experts I have corresponded with have been anthropologists, not geologists. Now, those folks can look at a chert rock and tell you where it is from, why it is there, and what it is generally used for. It's fairly amazing compared with the 'duhs' I've gotten from many geos.

Nothing in this world is *completely* understood, IMO.

But disregarding the ultimate unknowability of the universe, it sounds like they've got a pretty good handle on what's going on.

You don't need to be so condescending, Barbara, with your 'doodette' remark. I can assure you I am no such thing.