[Gypsum] is a hydrous calcium sulfate. This mineral is normally colorless or white, but impurities cause it to appear gray, brownish, yellowish, or reddish. It is [transparent] or [translucent] and is not heavy. When you rub gypsum across a [streak] plate, it leaves a white streak. This mineral is so soft that a fingernail scratches it easily. Gypsum occurs in several varieties.
The colorless, glassy, and [transparent] variety of [gypsum] is called [selenite]. It is found as cleavable masses and as crystals that are prism-shaped or flat and diamond-shaped. It is not uncommon for two crystals to be joined together so that they have a swallow-tail shape—these crystals are twinned. Groups of flat selenite crystals arranged together so that they resemble flowers are called rosettes. Many of these have been found in Nolan County.
[Gypsum] has four directions of [cleavage]. One of these directions is so perfect that some [selenite] splits into thin, clear sheets that may be mistaken for [mica]; other selenite [cleavage fragments] may be mistaken for [calcite]. You can distinguish selenite sheets from calcite by testing their hardness (selenite is softer) and by putting a drop or two of dilute hydrochloric acid on them. The acid will fizz and bubble on calcite but not on the selenite gypsum. There is also a quick way to distinguish the thin selenite cleavage fragments from mica. After you carefully bend a thin sheet of mica, it will snap back to its original shape without breaking. Selenite gypsum, however, is not elastic. It will bend, but it will break if you try to straighten it again.
[Selenite] [gypsum] crystal from Bastrop County, Texas.
[Selenite] is found in cracks and cavities in rocks. Good crystals have been collected at Gyp Hill, a [salt] dome southeast of Falfurrias in Brooks County, and some selenite has been mined there. Selenite crystals also occur scattered through clays, particularly along creek banks, in Lee, Fayette, Bastrop, and several other counties.
Another variety of [gypsum] is known as [fibrous gypsum]. It is made up of slender, brittle, needle-like fibers that fill the cracks in some rocks. If fibrous gypsum has a silky or pearly luster, it is called [satin spar]. One of the places where satin spar occurs is in [Permian] rocks in Hardeman County.
[Selenite] [gypsum] rosettes from Nolan County, Texas.
Most of the [fibrous gypsum] and [selenite] is formed by solutions. Some of these solutions develop when underground waters, seeping through rocks, pick up and dissolve minerals that contain [sulfur] (such as [pyrite]). This dissolved material changes the water into very weak sulfuric acid. When the sulfuric acid meets calcium carbonate (as in [limestone] or [calcite]), it combines with the calcium to form the [gypsum].