By Steve Hathcock
Doug Olson sent me the following email:
Here is a picture of some of the things I found while beachcombing on South Padre Island this past February. I hope the pictures are good enough for you to see some of the details.
It looks like you had a fun day at the beach. Gypsum crystals and rosettes, like the one you found (sometimes called “Desert Rose”), are formed below the surface of the wind-tidal flats and become exposed after heavy rains or sustained winds. Calcium carbonate is deposited as
irregular sand grains in the quiet waters of the Laguna Madre. As the water evaporates it leaves behind transparent crystals or crystalline masses of selenite. “Selenite” is the colorless and transparent variety that shows a pearl-like-luster and has been described as having a moon like glow. The word selenite comes from the Greek for Moon and means moon rock. (According to Greek legend, the crystals form when the moon is on the increase and coincidentally, the moon is near full as I write this column.)
A good place to hunt them is along the mudflats that lie to the east of the swing bridge that connects Long Island Village (Outdoor Resorts) with the mainland. The largest local specimen I have seen is on display at the Sea Life Nature Center located in the Lighthouse Square in Port
Isabel. Another variety is a compact fibrous aggregate called “satin spar”. This variety has a very satin like look that gives a play of light up and down the fibrous crystals. A fine grained massive material is called “alabaster” and is an ornamental stone used in fine carvings for centuries.
Extremely large crystals of selenite are found in southern Utah in geodes, or hollow rocks. Big blocks of selenite can be split into extremely thin, transparent plates, which were used by the ancients in place of glass. When heated it converts into plaster of Paris. When moistened with water, plaster of Paris expands, filling every crevice or opening. It sets in a short time forming a hard mass of gypsum, Plaster of Paris is used extensively in making casts for statuary, ceramics, dental plates, fine metal parts for precision instruments and surgical splints.
Gypsum is also used as a fertilizer for arid, alkaline soil, as a bed for polishing plate glass and as a base for paint pigments. In addition to all of the above uses, large amounts of gypsum are used as a retarder in Portland cement which is still the most commonly used cement around the globe.