Turritella agate
From Wyoming
Life in the USA is not normal. It feels pointless and trivial to be talking about small looks at the fascinating natural world when the country is being dismantled. But these posts will continue, as a statement of resistance. I hope you continue to enjoy and learn from them. Stand Up For Science!
Turritella agate isn’t made of marine turritella snails (gastropods), but the erroneous name is thoroughly entrenched and continues to be used. These snails are actually Elimia tenera, extinct fresh-water gastropods that lived in Lakes Gosiute and Uinta about 45 to 53 million years ago (but spanning from about 58 to 38 million years ago in places; Grande, 1984, Paleontology of the Green River Formation, with a Review of the Fish Fauna: Geological Survey of Wyoming Bulletin 63, p. 333) in what is now Wyoming, Utah, and Colorado.
It’s also not really agate, at least not in the usual sense of agate being translucent to transparent banded cryptocrystalline quartz. It IS extremely fine-grained quartz, a brown variety of chalcedony, and if you want to call it agate I won't fight. The silica probably percolated through the snail-rich rock after it was lithified (or at least near the end of lithification, the process that turns soft, loose sediment into rock).
The sediments in Lakes Gosiute and Uinta solidified to form the Green River Formation, famous for its well-preserved fish fossils, and even sting rays and bats. At the other end of the animal scale, many of the gastropod shells in this rock contain huge numbers of ostracod fossils, jam-packed both inside the shells and outside, in the cement. They had to enter the shells after the gastropod animal had died and the space was vacant.
Ostracods (sometimes called seed shrimp) are a class of Crustaceans, a subphylum of arthropods which also includes shrimp, crabs, lobsters, and barnacles. The word is from Greek óstrakon meaning shell or tile, and as a group, ostracods have been around at least since the early Ordovician Period, 480 million years ago, making them some of the best survivors in the history of life. There are about 13,000 living species of ostracods, and 20,000 known fossil species. Like my specimens, most ostracods are about a millimeter or less in size, but some are known that grew to 32 mm. They live in marine and in fresh water.
Some or even much of the brown color of the chalcedony comes from the ostracod shells, perhaps also with some plant matter incorporated in it.
Although the lakes supported huge populations of fish and snails at times, the water periodically evaporated. The Green River Formation in southwestern Wyoming hosts the world’s largest deposit of trona, Na3H(CO3)2 · 2H2O, a sodium carbonate mineral that precipitated in those evaporative situations. It’s called soda ash in the glass business. Trona is critical in making clear window glass because it prevents undesirable scums from forming in the silica melt. Half the world’s trona comes from Wyoming, part of a $2.5-billion industry in the U.S. The United States also holds 92% of the world’s trona reserves (Turkey is a distant #2, and Ethiopia is #3) and natural trona or soda ash is one of the few commodities for which the US is a net exporter, but US production is about 50% of the world total (with Turkey a close second with 46%). Turkey has come to the fore as a leader in soda ash production relatively recently; in 2010, the United States produced 93% of all the soda ash in the world, and Turkey’s production was virtually zero.
Trona is a Swedish term, deriving from Arabic natrum, native salt, ultimately from Ancient Egyptian ntry, godlike, referring to the mineral natron, Na2CO3 · 10H2O, used in mummification.
Gosiute is a variation on the name, Goshute, of a tribe of Western Shoshone Native Americans. Uinta is from a Ute Native American word meaning pine forest or pine tree.
This is wonderful
Fascinating.