Formula: K(AlSi3O8)
Tectosilicate (framework silicate), feldspar group


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.


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

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.


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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