The NSS Bulletin - ISSN 1090-6924
Volume 43 Number 4: 127-132 - October 1981
A publication of the National Speleological Society

Mineralogy of Cave Nitrates
Carol A. Hill

Abstract

Nitrate minerals occur in dry southwestern United States caves, but do not crystallize in humid southeastern caves. At 12°C, the humidity at which nitrate minerals crystallize is 54% for nitrocalcite (Ca(NO3)2•4H2O) and nitromagnesite (Mg(NO3)2•6H2O), 74% for ammonia-niter (NH4NO3), 79% for soda-niter (NaNO3), and 92% for niter (KNO3).

The nitrate minerals niter, soda-niter, and darapskite (Na3(NO3)(SO4)•H2O) have been identified in non-saltpeter, southwestern caves. Niter (crystal size = 1.5 mm) occurs as a tranparent, colorless to light brown, bitter-tasting wall crust in a lava tube near Socorro, New Mexico. Soda-niter also occurs as a crystalline wall crust, as two small stalactites, and as a one small stalagmite with the Socorro niter. The Socorro soda-niter crystals are rhombohedral, 2 to 3 mm long, transparent, and colorless. Darapskite occurs in Flower Cave, Big Bend National Park, Texas, as cave "flowers", crust, "hair", flowstone and stalactites. The darapskite is colorless with 2V(-) = 20-30°, alpha = 1.30 ±0.005, beta = 1.480 ±0.003, y = 1.489 ±0.003, a = 10.558(3), b = 6.870(2), c = 5.186(1) and beta = 101.46.1(5).

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