Light will pass through both these varieties of [serpentine], and both are soft enough to be scratched by a pocket knife. When rubbed across a [streak] plate, they leave white streaks. [Antigorite] and [chrysotile] have no crystal shapes of their own, but several other minerals can alter to form these two varieties of serpentine. Thus antigorite and chrysotile may be found as pseudomorphs in a crystal shape that originally belonged to another mineral.
[Antigorite] and [chrysotile] are commonly found closely mixed with [dolomite], [talc], [magnetite], [calcite], [pyrite], and several other minerals. These minerals make up [serpentine] rock (also called [serpentinite]). This rock ordinarily is some shade of green (such as whitish, yellowish, brownish, bluish, or dark blackish green), and it may be mottled. It is brittle or tough and generally is [massive]. Serpentine rock, like the serpentine minerals, is fairly soft—you can scratch it with a pocket knife.
In the [Llano uplift] area of central Texas, [serpentine] rock is found among [Precambrian] [metamorphic rocks], such as [gneiss] and [schist]. An especially large deposit in this area is known as the Coal Creek serpentine mass. It is over 3½ miles long, and at one place, it is almost 1½ miles wide. This mass of serpentine extends across the Blanco-Gillespie County line in the extreme northern parts of these two counties. (A little fibrous [chrysotile] is found here, but it will not break into flexible enough threads to be called chrysotile [asbestos].) Several other deposits of serpentine occur in northeastern Gillespie County and in southern Llano County.
It is believed that the Coal Creek [serpentine] was formed from an [igneous] rock such as peridotite, which is made up chiefly of grains of the mineral olivine. The peridotite may have been altered into serpentine by underground waters that seeped through it. It is possible, however, that other serpentines in the area were formed when rocks were altered by hot [fluids] and great pressures far below the earth’s surface.
The Llano area [serpentine] has been widely used in terrazzo floors. To make these floors, small pieces of serpentine and other colored rocks are put into cement that is spread over a concrete slab. Then, after the cement has hardened, it is ground to a flat, smooth surface and polished. The resulting terrazzo floor is both colorful and durable.
[Serpentine] rock also is cut into slabs, polished, and used as indoor building stones. Verde antique, a variety often seen in the lobbies of office buildings, consists of green serpentine rock with streaks of white [calcite] or [dolomite] in it.
In the [Balcones fault zone] area (shown on the Texas [physiographic outline map], [p. 42]) from Uvalde County to Williamson County, [serpentine] occurs with Upper [Cretaceous] rocks. The serpentine rock is seen at the surface in a few places (such as in Travis and Uvalde counties), but much of it is underground. In several oil fields of this area (as at Thrall field in Williamson County and at Lytton Springs field in Caldwell County), the serpentine rocks contain oil.
Serpentinite. See [Serpentine].
Shale
[Shale] is a [sedimentary] rock made up of tightly packed [clay] and mud particles. It has a smooth appearance because it is so fine grained. In fact, most of the particles in it are too small to be distinguished with a magnifying glass. These particles are the weathered remains of earlier rocks. They were carried by creeks and rivers to other parts of the land or to the sea, where they formed layers of clay and mud. Later, other [sediments] were deposited on top of them. The weight of these new sediments squeezed the clays and muds together to form firm, compact shale.