Note: The group includes the monoclinic (orthoclastic)
species orthoclase or common potash feldspar, and the
rare hyalophane or baryta feldspar; also the triclinic
species (called in general plagioclase) microcline,
like orthoclase a potash feldspar; anorthite or lime
feldspar; albite or soda feldspar; also intermediate
between the last two species, labradorite, andesine,
oligoclase, containing both lime and soda in varying
amounts. The feldspars are essential constituents of
nearly all crystalline rocks, as granite, gneiss, mica,
slate, most kinds of basalt and trachyte, etc. The
decomposition of feldspar has yielded a large part of
the clay of the soil, also the mineral kaolin, an
essential material in the making of fine pottery.
Common feldspar is itself largely used for the same
purpose.