With over 4,000 mineral species, you could overflow this calendar with beautiful pictures and words about minerals, but most minerals don’t have a lot of specific connection to particular time periods in earth history. Some mineral deposits do, and we’ll talk about them. Today’s mineral, glauconite, does have a connection to the Cambrian, at least to some degree.
Glauconite is a complex potassium-iron alumino-silicate, K2(Mg,Fe)2Al6(Si4O10)3(OH)12. It can be found in many kinds of sedimentary rocks, and in many ages right up to the present, but it’s pretty common in the Cambrian. It occurs as little green pellets, often intermixed with good quartz sand, or interbedded with limestone. What made these pellets?
Cambrian Lion Mountain Sandstone (green in lower portion from abundant glauconite), central Texas. |
Some rocks contain enough glauconite to be called greensands, but more often, the sand-sized glauconite grains are scattered through the rock and aren’t obvious until you look at it under magnification. Then they practically pop out at you. The Lion Mountain Sandstone, in the Llano Region of central Texas, is a Cambrian formation rich in glauconite – and no real surprise, some parts of the rock are mostly broken up trilobite skeletons. It’s a cool rock, and it was probably laid down in a wide sandy tidal flat. With trilobites crawling all over the place and pooping left and right. Occasional storms must have broken up the trilobite shells and dumped them into the piles in which they are found today.
Glauconite is green because the iron in it is in its reduced state, rather than oxidized which would lead to a rusty red color. That means relatively anoxic, low oxygen, conditions, such as might be found on a sea floor below wave base or in a stagnant mud, or the gut of a trilobite. The presence of glauconite pellets is taken to mean that the rock they are in was formed in marine conditions, and that’s a useful conclusion to draw from the presence of little green grains in a rock.
—Richard I. Gibson
Cambrian Lion Mountain Sandstone (green in lower portion from abundant glauconite), central Texas. Photo by Erimus via Wikipedia, public domain.
Do the pellets show up in a higher concentration in rocks that formed from seas that had a higher population of marine animals or is it more about the conditions that existed when the rocks were being formed?
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Great question. Glauconite can form diagenetically (meaning, as a result of chemical and physical conditions of lithification of rocks) but I've had the impression that at least in these older rocks it is probably proportional to the concentration of animals. But I really don't know that for sure, and it's beyond my expertise. I will try to find someone who knows more about it. Thanks for the question!
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