Schist
Schist is a type of metamorphic rock in which lamellar
minerals, such as muscovite, biotite, and chlorite, or
prismatic minerals, such as hornblende and tremolite, are
oriented parallel to a secondary platy or laminated
structure termed the schistosity. The mineral grains in
many examples are large enough to be recognized in hand
specimens. Schist breaks easily into thin layers parallel
to the schistosity. Schists are commonly rich in quartz
and contain some feldspars and carbonates. The specific
mineral composition of a schist is indicated by placing
the name or names of significant subordinate minerals in
front of the word schist; commonly occurring types include
biotite schist, muscovite-chlorite schist, garnet-mica
schist, staurolite kyanite schist, and hornblende schist.
A general composition or texture of the schist can be
indicated by prefixing; examples include calc-silicate
schist and spotted schist. The mineral assemblages and
textures of the schist change with the temperature and
pressure of recrystallization. With increasing
metamorphism, the grain size usually increases and,
depending on appropriate chemical availability, minerals
such as chloritoid, garnet, staurolite, cordierite,
andalusite, and kyanite crystallize as large crystals
(called porphyroblasts) in a foliated micaceous matrix.
Many porphyroblasts contain inclusions, indicating that
they crystallized by replacement of some other mineral or
rock.