Mineralogy Meets Paleontology
Again
These fossil brachiopods and other concretions weathered out of the Mississippian Madison Lodgepole limestone near the Missouri River in southwestern Montana USA. They are compositionally goethite, Fe3+O(OH), iron oxide hydroxide. It’s not Fe3+HO2 because the oxygen ion (O) and the hydroxyl (OH) ion occupy different places crystallographically.
The goethite used to be pyrite, FeS2, and we know that because where crystals are visible, they retain the isometric forms of pyrite, mostly cubo-octahedrons, octahedrons, and a few pyritohedrons. Pseudomorphs (“false forms”) of goethite after pyrite are very common, a result of the reaction under near surface conditions that oxidizes the pyrite (adds the oxygen) and adds water (in the form of the hydroxyl, OH) to goethite, and also releases the sulfur. In liquid water, this produces sulfuric acid, the common component of acid mine drainage in mining situations.
In soils and other near-surface environments, the pyrite changes chemically to goethite, gradually enough that the original geometry of pyrite crystals is maintained.
But the original fossils were undoubtedly calcium carbonate, calcite or aragonite, in the original Lodgepole limestone laid down about 330 million years ago, in shallow, warm, near-equatorial marine waters in what is now Montana. I’d stay that most of the fossils in the Lodgepole remain calcium carbonate today; some are silicified to quartz, and it’s unusual for them to be replaced by pyrite.
There’s usually enough iron, even if uncommon, in natural systems that it would not be difficult to get it into the limestone, but the sulfur to make pyrite might be a little more difficult. In the Mississippian-age Lodgepole, it might come from organic matter in the underlying Devonian Jefferson and Three Forks formations, which serve as oil reservoirs in the Williston Basin, a few hundred kilometers from the location where I found these specimens. Even here, the Jefferson Formation often gives a rotten-egg (hydrogen sulfide) smell when you break it.
The sulfur might have fortuitously come into the porous limestone along with the iron, in groundwater or otherwise. Small amounts of pyrite are common in the Lodgepole limestone (Roberts, 1966, Stratigraphy of the Madison Group near Livingston, Montana: USGS Prof. Paper 526-B). The location where the specimens were collected is near a major thrust fault, the Lombard Thrust, which could have easily helped provide a conduit for moving waters.
It’s also possible that mineralizing fluids passed through the rock and left deposits behind. The location where my specimens are from, north of Three Forks, Montana USA, is only about a mile and a half (2.5 km) from Copper City, a small mining operation that despite its name produced only a little copper and traces of gold and silver. There is a small Cretaceous igneous body, the 10N Pluton, about 5 miles (8 km) to the west.
I mapped the 10N Pluton in 1969. It contains large, mappable bodies of Paleozoic sedimentary rocks including the Madison Mission Canyon limestone within it, probably as “roof pendants,” bodies of the country rock lying near the top of the granitic igneous intrusion. In some places, the limestone was altered to skarn by the contact with the igneous material, and pyrite was common.
The brachiopods in the top photo include a heavy-ribbed Rhynchonellid at lower right and a smooth shell (upper left) that I think is actually the cast of the internal shell, not the exterior. All the obvious brachiopods show pyrite crystal forms that are now goethite, or perhaps in some places, the mixture of iron oxides and hydroxides called limonite.
A few other concretions of goethite after pyrite from this location have obvious fossil precursors: crinoid columns and one little gastropod. Many concretions are elongate or globular and might have started with pyrite deposited on or replacing a fossil fragment. Plenty of non-pyritized fossils are present as well.
