Mineralogy of Mammoth Cave

Abstract

Mammoth Cave displays a wide variety of depositional environments that permit the deposition of some unusual minerals, although the cave overall is rather devoid of speleothems. Calcite in the form of stalactites, stalagmites, and flowstone is the common carbonate mineral and occurs where passages extend beyond the protective caprock. Aragonite also occurs but more rarely. Gypsum is the common sulfate minerals and occurs widely in dry passages beneath the caprock. Gypsum takes the form of crusts and curving crystals known as gypsum flowers. In the exceptionally dry portions of the cave are found other sulfate minerals including mirabilite, Na₂SO₄·10H₂O, epsomite, MgSO₄·7H₂O, hexahydrite, MgSO₄·6H₂O, celestine, SrSO₄, and blodite, Na₂SO₄·MgSO₄·4H₂O. In addition, there is evidence for the rare sulfate minerals eugsterite, 2Na₂SO₄·CaSO₄·2H₂O, and wattevilleite, Na₂SO₄·CaSO₄·4H₂O. The source of sulfate is likely the oxidation of pyrite, FeS₂ that occurs in the limestone and overlying sediments. Saltpetre of uncertain mineralogy occurs in the cave sediments and was mined in the nineteenth Century. Other minerals include black manganese oxides that occur on stream cobbles and on chert ledges.