Fun Fossil and Mineral Investigations

Pictured below is a nice Eucalyptocrinites crinoid calyx I found in a Racine Formation spoil pile. The sedimentary rocks in this formation are from the Wenlockian Epoch, mid-Silurian Period (approx. 430 million years old). These are carbonate rocks formed when Illinois was submerged in an epicontinental tropical sea, not unlike the coral reefs of the Bahamas today. The fossils found in the Racine Formation are extinct reef faunas preserved through deep time, long before the dinosaurs or humans. Corals, crinoids, trilobites, cephalopods and brachiopods are some of the common faunas. The fossils are considered "steinkerns," which means the original skeletal material of the organisms have dissolved away during diagenetic processes leaving perfect internal molds. Some taphonomic features can be easily observed in some fossil assemblages of the Racine Formation, but others are not so obvious as they seem to be floating in a sort of void with seemingly little context. Here, with this crinoid, we cannot readily observe much in the realm of taphonomy, except that it was probably a fairly rapid burial, but if we look at the mineralogy, we can at least infer a few other intriguing aspects.

First, there is a light sprinkling of tiny dark-colored iron oxide crystals, many of which are only visible under magnification. These are commonly indicative of anoxic waters and may have precipitated as a result of postmortem microbial activity. Dolostone is formed from limestone on the ancient seafloor and, over time, magnesium-rich waters flushed through the limestone recrystallizing into dolostone in a process called neomorphism, which often retains original depositional textures, bedding and fossils. In general, dolostone is composed of the minerals dolomite, calcium, aragonite and magnesium. Sometimes impurities such as iron, may stain the crystals brown to red. In the case of this crinoid specimen, the dolomite seems to be quite pure and fine grained, so it is difficult to distinguish between crystal types (e.g., calcite, dolomite, etc.).

In order to identify what the white crust-like mineral near the top right of the crinoid calyx was, I decided to do an acid test, aka fizz test. An acid test is used by geologists to see what minerals react with acid by observing carbon dioxide gas bubbling. The test was performed by breaking off a small piece and crushing into a powder using a mortar and pestle. The powder was then submerged in 5% vinegar and also in a separate vial containing 10% hydrochloric acid, at room temperature, then carefully observed for reactive fizzing. There was no reaction. Because calcite, dolomite and aragonite react to acids, those minerals could be ruled out. After a look at the morphology of the mysterious white mineral under the microscope, I came to the conclusion that it is gypsum efflorescence, since gypsum is a white salt commonly associated with dolomite. Now, I am just an amateur, so please take this analysis with a grain of salt (no pun intended)!

Back to the iron oxide crystals. How to distinguish between marcasite and pyrite? Both minerals are considered polymorphs composed of iron sulfide. Pyrite occurs as pale brass-yellow cubes, pyritohedra and octohedra lattices, often twinned, radiating or globular. Marcasite has the same chemical formula (FeS2) as pyrite but crystallizes in orthorhombic prismatic crystal structure, spearpoint twins and radiating forms. To determine whether these were pyrite or marcasite, it was necessary to examine under the microscope. I found a crystal in the upper center of the calyx that looked particularly beautiful and carefully plucked it off the fossil using a scalpel. Then it was placed on a concave glass slide using tweezers under reflected light. It was about the size of a grain of sand. 

Magnified 20X

Magnified 40X. 

Magnified 250X. 

It appeared to have an orthorhombic prismatic crystal structure with a lush purplish-red iridescence. The crystal's 3-dimensionality proved difficult to photograph despite its minuscule size, but after careful observation I felt quite certain these are not pyrite, but oxidized marcasite crystals. 

This article was originally written for the Chicago Rocks & Minerals Society. 

March 2019

Volume 78, Issue3, Edition724