Feldspar is a white to pink or red mineral having a glassy luster on its
flat broken surfaces (cleavage faces). It will scratch window glass.
It is the most abundant mineral in granite and usually controls the
color of that rock; for example, the red granite at Graniteville
contains red feldspar, and the pink-gray granite in the Knoblick region
has feldspar of those colors. Small bodies or bands of very coarse
feldspar, quartz, and mica (pegmatite dikes) which cut the granite may
contain crystals of feldspar large enough to be recovered as small, hand
specimens, but otherwise it does not occur in coarse fragments. The
recognizable crystals, or phenocrysts, in the porphyry are mainly
feldspar.
[Illustration: A large piece of feldspar showing cleavage surfaces.]
Feldspar is really a family name for a group of several minerals, all of
which are crystallized in the igneous rocks. The potassium
(potash)-containing varieties, named orthoclase and microcline, occur in
the granite and porphyry, whereas plagioclase, a calcium-sodium
(lime-soda) feldspar is in gabbro, diabase, and basalt.
Plagioclase commonly has a thin, lath shape, is a shade of gray, and
makes up the lighter colored part of the greenish to dark gray igneous
rocks. Further differences between it and orthoclase may interest the
mineralogist but are of little concern to the non-technical person.
Pulverized feldspars are used extensively in the ceramic industries, but
Missouri does not have any productive deposits. Under natural, long-time
weathering processes feldspar usually decomposes to clay which may be
used technically, but the usual fate of it is soil formation.