Hydrothermal Selenite

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Selenite is the colorless and transparent variety of gypsum (calcium sulfate: CaSO₄ •H ₂O) that has a pearl-like luster and has been described as having a moon-like glow. The word selenite comes from the greek word for Moon, and means Moon rock. Gypsum is one of the more common minerals formed in sedimentary environments, such as tropical seas.

At the Haughton impact structure on Devon Island, Canada, selenite was formed by hydrothermal activity associated with the impact event. Hydrothermal systems active today are generally associated with volcanic regions (e.g., Yellowstone National Park), but it turns out that impact craters can also provide the two most important components of a hydrothermal system: heat and water. The heat source at Haughton were the pale gray impact melt breccias which were originally at temperatures of >1000 ^o C. As groundwater and rainwater came into contact with these hot rocks, the fluids were heated and circulated through the crater. Some of the target rocks at Haughton contained sedimentary gypsum, which was dissolved by these hot hydrothermal fluids. These fluids then migrated through the crater and re-deposited gypsum or selenite within cavities in the impact melt breccias as they cooled.

See selenite in the Haughton impact structure