Orange Patina on Tiger Chert

[Ron Kelley]

My question is: "What creates the orange patina on Tiger Chert?" I searched the internet but didn't find an answer to that question. I really like this material: I like the way the flake scars show and how it knaps. I'm wondering if the orange patina is a result of the material in the rock or perhaps a result of the environment where the spalls were stored for a very long time. Here is what I can tell you about the rock: 1.This Tiger Chert is from Wyoming. 2. This Tiger Chert was spalled in ancient times and transported to a Native American encampment.

[sneakygroundbuzzard]

Ron, im no expert but if i had to guess i would say its a combination of the minerals in the area it was stored and minerals that make up the rock it self
as you can see in the pics, that some of the material where the different colored lines are doesnt turn that orangish color.at least not as easily or readily as the rest of the material

but what do i know,cause like i said i aint no expert on such things

[Hoss]

I would say Iron

[chase]

Nature of the chert. I have not been to the quarry. and Iron rich soil is likely.

[Hoss]

Ron I think Tiger chert is just a nickname on paleo planet there is a thread about tiger chert and it says the stuff patinates fast Only a month out in the sun. To me I feel that is because of moisture accumulation either rain or dew is condensing over night and causes the iron to oxidize rather quickly and change the outward appearance of the stne. It is also called oil chert. others said MiKinnon chert. Sorry I thought I had the link but it did not take for some reason. I hate Paleo Planet something about it makes it load so slow. It is like my PC goes into pencil nd paper mode when I visit there. Dont get me wrong they have some great info but the site just lags every time I visit.

[Ron Kelley]

Thanks Matt, After doing a little looking I learned that this lithic material has many names which include: Oil Chert, Tiger Chert, Zebra Flint, Bridger Basin Chert and Green River Chert. Most of the rock is a chocolate brown but some is darker and glassier.

[painshill]

The locality for this material, which is part of the Bridger Formation, is beneath what was a large freshwater lake system in the Eocene. The former clay sediments of the lakes are generally red and iron-rich. As the lakes dried out and became infilled with younger sediments, everything beneath lithified and much of it became chert, sandwiched between limestone beds within the clay. There are also cobbles of it from glacial erosion surfaces. The cherts are sometimes referred to as “oil cherts” since they often emanate an earthy hydrocarbon-like smell during knapping. The hydrocarbons arise from the high content of organic matter, which was principally blue-green algae (cyanobacteria).

It’s those cyanobacteria which are responsible for the banding seen in the “Tiger Chert” variant since it’s what’s known as “stromatolitic”. The banding in a stromatolite formation represents the seasonal growth of the cyanobacteria in warm shallow water; a dark coloured layer during each annual growth season and a lighter coloured layer in between. The lake system had a complex chemistry, but cyanobacteria love water which is rich in unusual materials and habitually concentrate various metals as part of their metabolism, resulting in an array of exotic colours after diagenesis. The Parachute Creek member of the associated Green River formation contains the world’s only known deposits of the purplish nickel-containing mineral abelsonite for example, which derives from diagenesis of chlorophyll a.

It’s this complex chemistry which gives Tiger Chert its unusual properties. I’ve never seen a full chemical analysis, but it has just about the highest magnetic susceptibility (MS) of any North American chert at 4.41 x 10^-8 m³/kg [Thacker & Ellwood 2002]. Generally, American cherts fall between 1.5-3.2 and are typically two-point-something. Materials with a high MS (ie ferrimagnetic materials) are most usually rich in oxides of iron complexed with one or more of the transition metals such as nickel (or manganese or zinc), so that ties in with the unusual chemistry. Thacker & Ellwood also checked the MS values from the exterior and interior independently and found that the exterior was significantly lower.

They concluded that “consistently lower MS of cortical surfaces and weathering rinds is probably caused by the leaching of mineral inclusions from weathered surfaces. Paramagnetic and ferrimagnetic minerals can easily be lost during exchange of pore fluids upon exposure during chemical weathering (Bush and Sieveking, 1986), and leaching of carbonates from cherts has been documented by Hurst and Kelly (1961)”. Some of the material contains red splodges in addition to the banding, and these are likely weathered iron oxide-rich inclusions.

These are the kinds of changes that result in the red-brown you are seeing. The iron oxide complex in the darker organic-rich bands is likely losing its nickel component. All of the oxides are then progressively moving to the highest oxidation state they can achieve (probably with a bit of hydration to oxide-hydroxide too) and obscuring the difference in chemistry that created the banding.

As already stated, the material is also vulnerable to colour change from photo-oxidation, promoted by UV which, again, is a function of the complex chemistry derived from the organic components. Freshly broken material may not show any banding at all until there has been at least some weathering or exposure to light.

“The banded pattern of the chert [generally interpreted to be a preservation of stromatolitic structure by silica solutions] only becomes apparent on weathered pieces, so prehistorically the material was selected for practical reasons, not aesthetic ones" [Love 1977].

I note also that your piece has a major component of fossilised fenestrate bryozoan (the cellular portion, most apparent in the final picture).

[Ron Kelley]

Hey Roger, Thanks for the great info on Tiger Chert. I have about 135 pounds of this material and many pieces are totally covered with the fossilised fenestrate bryozoan. The fossils are obvious on the surface but once I knap the surface away I don't see the pattern of the fossils.

[painshill]

Bryozoans like that frequently grow as an upright fan-like structure or as a creeping horizontal mat on the bottom of a body of water, growing over anything in its path. In both of those cases, the overall growth is a thin layer that usually flattens and preserves in a bedding plane.

[Hoss]

learn something new every day around here

[Hoss]

This sure is a beautiful piece of flint. Love the "fossilised fenestrate bryozoan" appearance.

[Ron Kelley]

Hey Matt, There's a lot going on with that rock. Did you notice the freeze pop or fire pop? I'll post another with more obvious pops.

[Ron Kelley]

Here's another Tiger Chert with freeze pops or fire pops:

[clambellies]

I'd love to see an artifact made of that material.