Description:
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 patterened 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 seperating 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 refered 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 prominant 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. |