[Fibrous gypsum] from Terlingua area, Brewster County, Texas.
A [massive], fine-grained, and [translucent] variety of [gypsum], known as [alabaster], is used for articles such as lamp bases, statuettes, vases, and book-ends.
A loose, earthy, crumbly variety of [gypsum], called [gypsite], is ordinarily found mixed with other materials, such as [clay], [sand], and soil. It occurs either at or near the surface of the ground. Gypsite is found in Culberson, Reeves, and other counties in west Texas.
A [massive], [granular] variety of [gypsum], called [rock gypsum], may occur in large deposits. This is the gypsum that is used for making products such as plaster, wallboard, and some cements.
Deposits of [rock gypsum] are found both underground and at the surface in Texas. Surface deposits occur in [Permian] rocks in several counties to the east of the Texas [High Plains]. They also occur in the area between the Pecos River and the Delaware and Apache Mountains in Culberson and Reeves counties. Some of the other surface deposits are found near the Malone Mountains in Hudspeth County and in Lower [Cretaceous] rocks in Gillespie and Menard counties. Rock gypsum has been mined from the deposits in Fisher, Gillespie, Hardeman, Hudspeth, and Nolan counties. It also has been produced from the cap-rock at Hockley [salt] dome in Harris County.
[Gypsum] and another mineral, [anhydrite], have very nearly the same composition. Both are calcium sulfates. Gypsum, however, contains water of crystallization, and anhydrite does not. It is likely that most of the rock-gypsum deposits of Texas originally were beds of anhydrite. By absorbing water that seeped through it, the anhydrite changed into gypsum.
Halite
[Halite], sodium chloride, is the table [salt] you sprinkle on food for seasoning. This mineral ordinarily is white or colorless, but other materials cause it to be tinted red, blue, gray, brown, or green. When you rub halite across a [streak] plate, it leaves a white streak.
Because [halite] cleaves in three directions, all at right angles to each other, the [cleavage fragments] are shaped like [cubes]. You can see some of them by looking at a few grains of table [salt] through a magnifying glass.