Geo-pic of the week: Satin Spar

Satin spar1 edited

Satin spar is a variety of the mineral gypsum and, aside from it’s attractive fibrous appearance, it’s used for many practical purposes, including for making plaster, chalk, and drywall.  Some ideally suited varieties are carved by sculptors.  This piece was collected from a gypsum mine in Howard County, Arkansas, near the town of Nashville, where it’s mined and processed to make drywall.

Around 100 million years ago, the water of the Gulf of Mexico reached all the way to southern Arkansas, forming a huge marine bay.  Because that water was somewhat isolated from the ocean’s circulation, evaporation concentrated dissolved minerals there, to the point that the water became oversaturated and minerals, such as gypsum, began to crystallize out of it.  It’s the same process by which most of the world’s salt deposits formed.  In fact, gypsum is often found associated with salt.

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