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Andrew Alden

Texas Gypsum

By July 18, 2013

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rock gypsumWith only 20 percent of his phone charge remaining, Doug snapped this mysterious sugary crust in the rocks of Palo Duro Canyon, in the Texas Panhandle, and sent the image to My Sedimentary Rock. Now that's dedication, and I hope he got to a charging station in time.

It's rock gypsum, also called satin spar, part of the Quartermaster Formation, of Permian age. It points to a geographic setting at that point in time in which seawater was prone to evaporation, like a natural salt pan. In that setting, gypsum (calcium sulfate) is the first mineral to come out of solution—not because its concentration is especially great but because its solubility is much lower than salt (halite) or the other evaporite minerals yielded by seawater.

I get a steady trickle of photos to My Sedimentary Rock, My Igneous Rock and My Metamorphic Rock, especially at this time of year when people are out on the land. Unfortunately a lot of them come to the wrong category. So I'm glad that Doug could tell that much about this specimen. If you have a mystery rock, the best place to post a photo is in the Geology Forum, where I and several other regulars can put it through its paces.

More:
How to look at a rock
Basic rock identification
Basic mineral identification
Geology Forum, "What Is This Rock?" section
More Texas geology
Rock gypsum — photo by About.com reader Doug

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