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Thin white layers in red rock selenite gypsum
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Thin white layers in red rock selenite gypsum

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Selenite gypsum

A thin layer of white rock in this red sandstone may be selenite gypsum.

Thin lines of white stone layered through the red sandstone of a bluff southeast of Bridger prompted some questions.

The white stone varied in thickness, was flaky and crystal like in appearance, kind of like mica.

In asking around for an explanation BLM geologist Dorothy Van Oss led me to selenite gypsum. (If you know otherwise, please tell me.)

The cliff is in the Chugwater Formation, which as Van Oss noted was "deposited during the Triassic Period, along the tidal flats, river deltas, and floodplain lakes on the west edge of the North American continent. At that time, North America was close to the equator, and still part of Pangaea (a huge super continent), which was beginning to break apart. The characteristic red color is due to the mineral hematite, an iron oxide. Veins of limestone, gypsum (hydrated calcium sulfate), anhydrite (calcium sulfate) were deposited when shallow areas of water were restricted from flow and then evaporated."

In searching through those suggestions, I found photos of selenite gypsum that resembled the rock I had seen. There's a cool explanation of selenite gypsum on the University of Minnesota's website: "Gypsum, a soft non-metallic mineral, almost exclusively forms in sedimentary settings, as seawater is evaporated or as dissolved ions precipitate from groundwater.

"Gypsum also occurs in crystal form, with the two most common varieties being large clear crystals that are often termed selenite ... ‘Selenite’ is the Greek word for moon, and refers to the crystals well-developed pearl-like luster that can reflect a soft moon-like glow."

It's intriguing to think that the layer was created when an ancient sea evaporated again and again. Was the varying thicknesses related to how deep the sea was at the time, or maybe how mineral rich the water was?

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