Feld"spar` (?), Feld"spath` (?), n. [G.
feldspath; feld field + spath
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
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,
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.
© Webster 1913.