The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages, and a few new episodes were posted. Now, the blog/podcast is on an occasional schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Sunday, February 23, 2014

February 23. Trilobite poop



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.
To put it bluntly, glauconite pellets are trilobite poop. OK, not just trilobites, and that’s not the only way glauconite forms. But the little round grains in marine rocks are thought to be an alteration from the original fecal pellets excreted by marine organisms. It can also precipitate directly, and it can form when some iron-bearing minerals are weathered, but the pellets in sandstones are generally accepted to represent fecal material.

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

2 comments:

  1. 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?

    By the way love the podcast

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    1. 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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