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Feldspars comprise a group of rock-forming tectosilicate minerals that make up as much as 60% of the Earth's crust.
They are divided into two groups, K-feldspars, which
are rich in potassium, and
plagioclase feldspars
that contain no potassium.
The major K-feldspars are
orthoclase,
sanidine,
microcline and
anorthoclase.
The major plagioclase feldspars are
albite and its varieties
oligoclase, and albite variety
andesine,
anorthite and its variety
labradorite.
Perthite is a variety of the feldspar group that occurs as an intergrowth of albite or
its variety oligoclase with a microcline host,
occasionally also with an orthoclase host
(Mindat).
Mesoperthite is a perthite with approximately equal amounts of alkali feldspar and
plagioclase
(Mindat).
Myrmekite is an intergrowth of plagioclase feldspar (commonly
oligoclase) and vermicular quartz,
generally replacing K-feldspar; formed during the later stages of
consolidation in an igneous rock or during a subsequent period of plutonic activity. The
quartz occurs as blobs
(Mindat).
Sunstone from Oregon is a labradorite with inclusions of
copper crystals that cause patches of red colour
(https://www.youtube.com/watch?v=ejucgGmeJMA).
Moonstone a trade name for any white feldspar with a blue schiller. Commonly
microcline rarely orthoclase,
and the plagioclase series
(Mindat).
Feldspars are primary minerals; they are essential
constituents of
granite,
syenite,
diorite,
gabbro,
rhyolite,
trachyte,
andesite,
basalt and ,
sandstone.
Feldspars are common constituents of
phyllite and
gneiss,
and medium constituents of
quartzolite.
K-feldspars are minerals of the
greenschist and
amphibolite facies.
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