Feldspar
is the family name of several minerals closely related, and indeed
grading into each other, but distinguished by mineralogists by separate
specific terms. These minerals are all silicates of aluminum, with some
alkali or alkali earth, having a hardness of about 6 and a specific
gravity varying from 2.5 to 2.7. They are fusible with difficulty
before the blowpipe, crystallize in the monoclinic or triclinic system,
and cleave in two well-marked directions nearly or quite at right
angles to each other. It is this latter property, probably, which led
to the grouping of these minerals as spar, since this term is applied
in common language to any minerals which break with bright crystalline
surfaces. The term field spar, of which feldspar is probably a
corruption, was perhaps given the minerals of this group because of
their widespread occurrence. The English spelling of the word is
felspar. The feldspars form an essential part of nearly all eruptive
rocks, and by their decomposition produce clays and other soils which
may harden into great areas of sedimentary rocks. They are thus of
great geological importance and interest. Usually the white crystals to
be seen in an eruptive rock in contrast to the dark green or black of
the pyroxene or hornblende, or the glassy, nearly colorless quartz, are
feldspar. The feldspar may, however, contain more or less iron, and
then take on a flesh color or become even darker. Feldspar crystals
can best be recognized by their prominent cleavage, which appears as
numerous bright flat surfaces extending in any given crystal in the
same direction. The crystals, while they may be of so minute dimensions
as to be visible only with the microscope, may, on the other hand,
reach in veins in coarse-grained granites a length of a foot or more.
As
ornamental stones only certain varieties of feldspar are valued, and
their value depends on accidents of color or structure. The first of
the feldspars which may be mentioned as being prized as an ornamental
stone is amazonstone, or green feldspar. This in composition is what is
called a potash feldspar, potash being the alkali which in combination
with alumina and silica goes to make up the mineral. The percentages of
each in a pure amazonstone are, silica 64.7, alumina 18.4, and potash
16.9. The mineralogical name of the species is microcline, meaning
small inclination, and refers to the fact that the angle between the two
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