They
occur in crystals and crystalline masses, vitreous in luster, and
breaking rather easily in two directions at right angles to each
other, or nearly so. The colors are usually white or nearly white,
flesh-red, bluish, or greenish.
&fist; 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.
{ Feld*spath"ic (?), Feld*spath"ose (?) },
a. Pertaining to, or consisting of,
feldspar.
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