Flint vs Chert?

[painshill]

Can you guys clear this one up for me? Where I come from (UK) flint is defined as coming from chalk deposits and chert is defined as coming from limestone deposits. On those definitions I had always understood that flint was a rare material in the States. Does it in fact even exist? When you use the term flint, do you really mean flint as we define it geologically or are you using the term in a looser sense?
I was in Georgia and South Carolina last year (had a GREAT time... thanks y'all for the hospitality!) and we were told that the flint cobbled streets in places like Savannah were made with European material brought in as ballast on empty ships coming to pick up cotton in the Colonial era, on account of the lack of any flint in the area.
Not being picky here, y'understand. Just want to be clear if we are both using the same terminology.
Thanks - Roger

[tomclark]

http://www.theaaca.com/Learning_Center/flintvs.htm

We use the terms interchangeably but should not.

[greywolf22]

Flint and chert are dense, cryptocrystalline varieties of quartz, slightly translucent to almost opaque. As a general rule, one should primarily categorize rocks according to what they are, and not to how they formed. It matters not that one was formed in Limestone and one in Chalk, it is the same. I for one would lump chert, flint (and possibly jasper) together.
Jack
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I pulled the below information from different articles on Chert:
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:
1. Flint is hard, and has no preferred direction of cleavage.
2. Flint has a tendency to split into pieces with a curved but even surface.
3. Flint has a homogeneous structure and its surface is smooth and not grainy as most rocks. It is thus much less susceptible to grinding.
4. Flint is quite tough and not as brittle as crystalline quartz. Although the structure is homogeneous, it is not crystalline and stress will dissipate in the material, whereas in a crystal a stress-induced small crack will often propagate through the entire crystal and split it.
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.

[JoeM]

Good question, Roger. And I guess the answer would be "No", since you are using the
geological definition and any rock that was knapped by the aboriginals can be, and is,
called flint here.
For example, the main source of knapping material here in North Carolina is the
Rhyolite of the Slate Belt, but since it was utilized by the Indians, it is our
"Flint rock".
You are correct in your definitions and there is very little, if any, true flint in
the United States. We have cherts, but for the most part folks just don't like
calling it chert and prefer to call it flint.
Even the article Tom linked us to above tries to justify this mis-use of the word;
Quote; " The reality of the flint versus chert debate is that in most cases it's something
like "splitting hairs", there is really very little difference, chemically speaking."
Chemically speaking, that may be true, but when you here the word "flint" used
'round hea, take it with a grain of salt.
Enjoy your posts. Keep them coming.
Thanks,  JoeM

[Lithophylic]

Think of flint as high quality chert. This is dependant on the purity of the host rock and the grain size of the host rock.
Chalks are esentially marine muds made up almost entirely of the skeletons of microscopic planktons.  during diagenesis the skeletons of the siliceous tests mobilize and replace the chalk to form flint-grade chert nodules or beds.  Chalks can be found in the Coastal Plain of the southeast but are generally not common throughout the US..
Similarly, most limestones originated from carbonate sands and may have abundant macroscopic fossil fragments.  These can form micrites (fine grained limestones) to sparites (coarse grained limestones) Most of the silica in these types of deposits come from both plankton skeletons and sponge spicules.  When these mobilize they tend to form a coarser version of chert which is more common here in the states.  The degree of luster of the chert is dependant on the grain size of the microcrystals and the purity of the silicified zone.  Often chert begins by replacement of a single fossil or carbonate grain.  As it continues, it forms concentric layers resulting the the "bulls eye pattern" seen in hornstone, fort payne chert etc.  The occurrence of impurities such as iron can lead to various colors (reds, yellows, browns) in the chert. carbon can give it a darker color.  Other times, the chert replacement of carbonate can occur along beds resulting in chert beds or bands.  In the fort payne chert, dark gray chert beds can be 10s of feet thick.
I can't say this with certainty but the chalks here are much younger than the limestones containing cherts.  Chalks within the coastal plain are generally cretaceous whereas many of the limestone units in the eastern US range from Mississippian to Cambrian in age.