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 Others changed into dolomite much later after the sea had withdrawn and the limestones had become a part of the land; underground waters containing magnesium seeped through these limestones and altered them into dolomite.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].
Feldspar
[Feldspar] is the name given to a group of nonmetallic minerals that are much alike. Several of them are so similar that a petrographic microscope must be used to tell them apart. Each of the feldspar minerals is an aluminum silicate. Each of them contains, in addition, at least one of the following [elements]: potassium, sodium, calcium, and barium. The feldspar minerals that are found in Texas include [albite], a sodium-aluminum silicate, and [orthoclase] and [microcline], which are both potassium-aluminum silicates.
The [feldspar] minerals are [transparent] to [translucent] and have either glassy or pearly lusters. They can be white, cream, or a shade of red, brown, yellow, blue, gray, or green. When you rub a feldspar across a [streak] plate, it leaves a white streak. The feldspars are rather hard—a pocket knife will not scratch them, although a piece of [quartz] or a steel file will. These minerals have good [cleavage] in two directions. The cleavages meet at an angle of about 90°, so that the [cleavage fragments] have square corners.