Marble may be of uniform color, banded, spotted, or streaked. If it is made up only of pure calcite or dolomite, the marble is white. If, however, it contains carbonaceous material, such as graphite, it is grayish or black. Limonite impurities cause the marble to be yellowish brown, and manganese oxides and hematite give it a brownish, pinkish, or reddish color. Marble is a rather soft rock, and you can scratch it easily with a pocket knife. A few drops of dilute hydrochloric acid will bubble and fizz readily on calcite marble; on dolomite marble, it may fizz slightly. 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.) [Illustration: 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.) [Illustration: Mica minerals have perfect cleavage in one direction, resulting in thin, sheet-like cleavage fragments.] Basal Cleavage