Oil on calcite

Rensselaer, Indiana

Cat. No. 1748, 8 cm wide. Given to me by a friend.

A quarry near Rensselaer, in northwest Indiana USA, is famous for interesting specimens of calcite, marcasite, and pyrite, often coated or impregnated with petroleum.

The host rocks that were quarried there for aggregate are dolomites of the Middle Devonian (about 387 million years old) Muscatatuck Group, which also serve as reservoirs for numerous oil fields in Indiana. At Rensselaer, cavities (many filled with oil) in the dolomite layers allowed large vitreous brown calcite crystals to form. Dolomite is a mineral (calcium magnesium carbonate) as well as a rock composed mostly of that mineral, analogous to limestone and sometimes called dolostone.

Some of the rocks of the Muscatatuck Group, which includes the Jeffersonville Limestone and other formations, contain interbedded gypsum and anhydrite, evaporites that suggest shallow and hypersaline conditions of deposition. Primary dolomite is sometimes associated with such restricted, evaporative environments, as for example in modern salt flats (sabkhas) in the Middle East. During the Devonian, northwestern Indiana was a relatively shallow sea on the flank of the Cincinnati Arch where at times the waters must have become restricted. Indiana at that time was probably only a few degrees south of the equator, so conditions were probably tropical, contributing to the evaporative conditions.

Oil from older (Ordovician and Silurian) source beds probably migrated into the Muscatatuck dolomite reservoirs. The quarry at Rensselaer is essentially in an oil reservoir that is almost at the surface. Sulfur in the petroleum may have contributed to the formation of the marcasite and pyrite, two iron sulfides common in the deposit.

Two additional Rensselaer specimens showing common habits for calcite. Gifts from a friend.

The specimen of calcite in the top photo shows a modified highly elongate scalenohedral habit, with crystals radiating away from a center. Many examples from Rensselaer are stubby transparent brown crystals with complex rhombohedral forms, so the one at top is relatively unusual. This piece was probably almost “floating” in an oil-filled cavity since there are minimal contact points on it. The brown portions are coated with oil; the white crystals were protected from oil by something, perhaps clay or some soluble mineral that did not inhibit the calcite crystal growth. Conceivably, some of the oil was naturally removed at some point, but the specimen has not been cleaned. It’s very difficult to remove the oil, and anyway, that’s the mineral’s natural state.