The NSS Bulletin - ISSN 1090-6924
Volume 36 Number 1: 19-24 - January 1974

A publication of the National Speleological Society

Unusual Mineralogy of the Crystal Pit Spatter Cone, Craters of the Moon National Monument, Idaho
Stewart B. Peck


Crystal Pit Spatter Cone lies on the Idaho RIft System of the Snake River Basin Lava Pleateau in south-central Idaho. The former magma chamber under the spatter cone, about 80 feet long and 35 feet wide, is still open for examination. It is reached by a descent of 90 feet through the narrow spatter cone throat. The magma chamber contains large quanties of the secondary sulfate minerals gypsum, mirabilite, and jarosite, all of which seem to be scarce or absent in caves in other volcanic regions. The gypsum and mirabilite probably were deposited from mineralized capillary groudwater seeping into Crystal Pit, rather than by condensation from mineralized volcanic gasses. However, the basaltic rocks overlying Crystal Pit seem to be poor in calcium-, sodkum-, and sulfur-containing primary minerals that could be leached by the capillary groundwater. This extensive mineralization has occurred within the past 1000 to 2000 years.

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