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.