Marble is a metamorphic rock made up chiefly of sparkling grains of
calcite or dolomite, but other minerals may be present. The marble may
be fine grained, medium grained, or coarse grained; commonly, all the
mineral grains are about the same size.
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.