Schist
[Schist] is a [metamorphic rock] that splits easily along thin, generally parallel layers, called folia. These layers may be either straight or curved, and they are made up of [crystalline] grains of one or more than one mineral. This structure is called schistosity or foliation. When you examine schist, you will see that many of the mineral grains are flat or long, and that they are lined up in one direction to form the layers. Some schists have fairly large crystals (many with perfect shapes) scattered through them. For example, [mica] schists may contain beautiful crystals of [garnet].
Each kind of [schist] is named for an outstanding mineral that it contains. [Mica] schist contains a large amount of mica. We also find hornblende schist, actinolite schist, chlorite schist, [talc] schist, and [graphite] schist. (Graphite schist is discussed with graphite on [p. 63].)
Schists form from other rocks, such as [granite], gabbro, or [shale]. The rocks are changed into schists by [fluids] and by heat and pressure below the earth’s surface.
Extremely ancient schists that formed during [Precambrian] time are exposed at the surface in the Allamoore—Van Horn area of west Texas and in the [Llano uplift] area of central Texas. Geologists believe that the Packsaddle [Schist] of the Llano uplift area was once [shale]. Good exposures of this schist are seen in the Honey Creek area near Packsaddle Mountain in Llano County.
Schorl. See [Tourmaline].
Sedimentary Quartzite. See [Sand and Sandstone].
Selenite. See [Gypsum].
Serpentine
[Serpentine] is the name given both to a rock and to a mineral. The mineral serpentine (a hydrous magnesium silicate) is found in two different forms. If it is fibrous, it is called [chrysotile]; if it is layered and platy, it is known as [antigorite]. Antigorite is brownish green and smooth and waxy looking. Some of it can be split into thin sheets. Chrysotile is made up of greenish, silky fibers, which may be brittle and break apart in large pieces. If, however, the fibers can be pulled apart into soft flexible, little threads, the mineral is called chrysotile [asbestos].