[Marble] forms from [limestone] or from [dolomite] rock. Heat and pressure below the earth’s surface cause the [calcite] and dolomite mineral grains in these rocks to recrystallize. A fine-grained limestone can be changed into a coarse-grained calcite marble. The marble is not made up of new and different minerals, but it has a new texture unlike that of the limestone. (To a builder, the word marble has another meaning. He considers rocks such as unaltered limestone, unaltered dolomite, or even [serpentine] to be marble, if they will take a high polish.)
Polished section of [Precambrian] [metamorphic] [marble] from Llano County, Texas.
[Metamorphic] marbles occur at the surface in central Texas and in west Texas. Some of the west Texas occurrences are in the Van Horn area of Culberson and Hudspeth counties and in the Big Bend area of Brewster County. In central Texas, [Precambrian] marbles are found in Burnet, Gillespie, Llano, and Mason counties of the [Llano uplift] area. Many of them are suitable for use as monument and building stones. Some of the Llano County [marble] is quarried and used as [granules] for roofs and as terrazzo chips for making colorful floors (described with [serpentine] on [p. 88]).
Martite. See [Hematite].
Mica
[Mica] is not just one mineral but is the name given to a group of similar minerals. The mica minerals are easy to recognize. Because they have perfect [cleavage] in one direction, they split into thin, flat sheets. You can see through some mica sheets, and they are elastic enough to be bent back and forth. (Another mineral, [selenite] [gypsum], also will split into thin, flat, [transparent] sheets, but selenite sheets break when you bend them.)
[Mica] minerals have perfect [cleavage] in one direction, resulting in thin, sheet-like [cleavage fragments].
Basal [Cleavage]