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Formula: K(AlSi3O8)
Tectosilicate (framework silicate), feldspar group
Varieties
Adularia is a more ordered low-temperature variety of
orthoclase or partially
disordered microcline
Amazonite is a green to blue-green variety of K-feldspar, usually microcline, but sometimes
orthoclase
Hyalophane is a barium-rich variety of microcline or
orthoclase.
Properties
Crystal System: Triclinic
Specific gravity: 2.54 to 2.57 measured, 2.56 calculated
Hardness: 6 to 6½
Streak: White
Colour: White, blue, green, pink, yellow
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Common impurities: Fe,Ca,Na,Li,Cs,Rb,H2O,Pb
Environments:
Pegmatites
Sedimentary environments
Metamorphic environments
Microcline occurs mainly as a primary mineral in
schist and gneiss. K-feldspars
are essential constituents of
granite, and microcline is the common
K-feldspar of
pegmatites. In sedimentary rocks microcline is present in
feldspar-rich sandstone.
Microcline is characteristic of the
granulite facies and it is also a mineral of the
albite-epidote-hornfels,
hornblende-hornfels and
pyroxene-hornfels facies.
Localities
At Broken Hill, New South Wales, Australia, microcline variety hyalophane has been described from lenses and streaks in
acid gneiss
(DHZ 4 p177).
The Needle Hill Mine, Needle Hill, Sha Tin District, New Territories, Hong Kong, China, is a tungsten mine, abandoned
in 1967. The principal ore is wolframite, and the principal gangue mineral is
quartz. Molybdenum also occurs. The
mineralisation consists of a series of parallel fissure veins that cut through
granite. Wolframite and
quartz are the main minerals, but galena,
sphalerite, pyrite,
molybdenite and fluorite have also
been found here
(Geological Society of Hong Kong Newsletter 9.3.29-40).
The quartz-wolframite veins are of
high-temperature hydrothermal formation,
and grade into wolframite-bearing
pegmatites.
Microcline has been mined from the pegmatites, and it is also
a common constituent of the granitic rocks, where it occurs as small grains
(Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)
At the Kaso mine, Japan, microcline variety hyalophane occurs in veins with manganese-rich
tremolite, rhodonite,
rhodochrosite and spessartine
(DHZ 4 p176).
In the manganese ores of Otjosondu, Namibia, microcline variety hyalophane is found in a rock consisting mainly of
calderite, and it also occurs as small veinlets in the
calderite
(DHZ 4 p176-177).
At Slyudyanka, Siberia, Russia, microcline variety hyalophane occurs in
phlogopite-calcite veins in a
pyroxene-amphibole
gneiss
(DHZ 4 p177).
At the Emmons pegmatite, Greenwood, Oxford county, Maine, USA, microcline forms masses to 1 metre across in the interior of
the pegmatite, and crystals to 25 cm in the pockets. The Emmons pegmatite is an example of a highly evolved
boron-lithium-cesium-tantalum
enriched pegmatite
(R&M 94.6.511).
The Cliff Mine, Phoenix, Keweenaw county, Michigan, USA, is situated at the base of a roughly 70-metre
basalt cliff. A curious feature of the impressive thickness of the
greenstone flow here is that it contains zones of “pegmatoid”: areas
where
slow cooling in the core of the lava flow allowed for large feldspar crystals
exceeding 1 cm to grow. Such features are normally only observed in intrusive igneous rocks and are almost unheard of
in basalt flows.
The Cliff mine primarily exploited rich copper mineralisation in the Cliff
fissure (vein). Although mineralised with copper to some extent along its
entire length, the part of the vein just below the greenstone flow
carried the richest copper mineralisation by far. A significant amount of the
copper recovered at the Cliff mine came from amygdaloids in the tops of 13
basalt flows which were cut by the Cliff vein. The discovery and mining
of this vein proved that the veins were the source of the large masses of float
copper that were already well known, and proved that the
primary ore mineral in the district was native
copper, not sulphides, as had been suspected earlier.
Microcline is a common alteration mineral at the Cliff mine, occurring as curved pinkish crystals to 2 mm or so
that are often twinned. It is typically associated with other silicate alteration species including
prehnite, epidote and
zeolites
(MinRec 54.1.25-49).
At the Little Gem amethyst mine, Jefferson county, Montana, USA, microcline is ubiquitous, intergrown with other
feldspars and quartz. Many of the
microcline crystals are Baveno twins, and less commonly Manebach twins. Alteration products include muscovite variety
sericite and
kaolinite
(R&M 93.6.512).
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