Dolomite is the name given both to a rock and to a mineral. The mineral
is a calcium-magnesium carbonate and has a glassy or a pearly luster. It
is any of a number of colors, such as white, pink, brown, or gray, or it
can be colorless. Dolomite leaves a white streak on a streak plate and
is transparent to translucent. It is not particularly hard and can be
scratched with a pocket knife, although not with a copper penny.
Dolomite cleaves perfectly in three directions, and some of the cleavage
fragments are rhombohedrons. However, the cleavages of the individual
mineral grains in specimens of fine-grained massive dolomite are not
readily distinguishable.
[Illustration: Dolomite rock from the vicinity of Fairland, Burnet
County, Texas.]
Most Texas dolomite occurs as coarse-, medium-, and fine-grained
crystalline masses as the chief mineral in dolomite rock and in
dolomitic marble. It is also found as 6-sided crystals that are
rhomb-shaped; when the faces are curved, they have a saddle-like
appearance.
Crystals of the mineral dolomite commonly occur in cavities in the
dolomite rocks. It is believed that they were deposited there by seeping
underground waters. The waters dissolved some of the dolomite in the
rocks and then re-deposited it as crystals.
Dolomite rock is made up mostly of crystalline grains of the mineral
dolomite. In addition, quartz grains, calcite, and other minerals may be
present. Dolomite rock is almost any color—white, buff, pink brown,
gray. It resembles some limestone, and these two rocks actually are
closely related.
To help tell them apart, dilute hydrochloric acid often is used. A few
drops of this acid will readily fizz and bubble if the rock you put them
on is a limestone. If the rock is dolomite, the acid will effervesce
only very little or not at all. (If, however, the acid is put on
powdered dolomite, it then will fizz readily.) Dolomite is slightly
harder than limestone, and it also is slightly heavier.
Some dolomite rocks formed directly from materials that were dissolved
in sea water, and others are altered limestone rocks. Some limestones
altered into dolomite on the sea floor by the addition of magnesium from
the sea water. Others changed into dolomite much later after the sea had
withdrawn and the limestones had become a part of the land; underground
waters containing magnesium seeped through these limestones and altered
them into dolomite.
Many of the dolomite rocks are found with limestones. In Texas they
occur mostly in Cambrian, Ordovician, Mississippian, Pennsylvanian,
Permian, and Cretaceous formations. The geologic map (pp. 4-5) indicates
where these strata appear at the surface in Texas.
Dolomite is abundant in the Llano uplift area of central
Texas—particularly in the Cambrian and Ordovician rocks. A number of
these central Texas dolomites have been quarried for use as building
stones. Some of them also have been crushed and used as a road-building
material and as a stone aggregate that is mixed with cement to make
concrete. This dolomite is also used as terrazzo chips (terrazzo floors
are described with serpentine on p. 88). In addition, Ellenburger
(Ordovician) dolomite from Burnet County was used during World War II as
a source of the lightweight metal magnesium.
Dravite. _See_ Tourmaline.