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.