Amazomite
(Microcline)
Microcline is a common, but not a well known mineral and
has been used as a semi-precious stone under the names of Amazonite and Perthite.
Amazonite is a variety that is deep green and is suitable for carving and
polishing. The perthite variety is a stripped, veined or almost zebra patterned
stone, that is produced from lamellar intergrowths inside the crystal. These
intergrowths result from compatible chemistries at high temperatures becoming
incompatible at lower temperatures and thus a separating and layering of these
two phases. The colored stripes are microcline and the white or clear stripes
are plagioclase feldspars. If there is more plagioclase than microcline it is
called "antiperthite".
Microcline is a polymorph of other minerals that share the
same chemistry but have different crystal structures. If positive identification
can not be made by field methods then the specimen may be referred to as a
potassium feldspar or K-spar. The other k-spar minerals are sanidine,
orthoclase, and anorthoclase
. The differences between these minerals are minor in hand samples but
microcline tends to be deeper colored, is usually striated on cleavage planes
and is the only one that can be, but is not always, a deep green (amazonite).
Twinning is common in all feldspars and follow certain twin
laws such as the Albite Law, the Pericline Law, the Carlsbad Law, the Manebach
Law and the Baveno Law. Albite and pericline Law type twinning produce stacks of
twin layers that are typically only fractions of millimeters to several
millimeters thick. These twinned layers can be seen as striation like grooves on
the surface of the crystal and unlike true striations these also appear on the
cleavage surfaces. Albite and pericline twinning is more common in plagioclase
feldspars than with microcline. The Carlsbad Law twin produces what appears to
be two intergrown crystals growing in opposite directions. Two different twin
laws, the Manebach and Baveno laws, produce crystals with one prominent mirror
plane and penetrant angles or notches into the crystal. Single crystals showing
a perfect twin are rare and are often collected by twin fanciers.
Well formed crystals can be quite striking in appearance. At
some localities fine amazonite clusters are intermixed with smoky quartz
crystals and this results in an outstanding classic mineral specimen.
PHYSICAL CHARACTERISTICS: