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The Collaborative International Dictionary of English v.0.48 (gcide)

Feldspar \Feld"spar`\, Feldspath \Feld"spath`\, n. [G.
feldspath; feld field spath spar.] (Min.)
A name given to a group of minerals, closely related in
crystalline form, and all silicates of alumina with either
potash, soda, lime, or, in one case, baryta. 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.
[1913 Webster]

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. Feldspathic