Microcline

microcline

hyalophane

calderite

spessartine

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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

Specific gravity: 2.54 to 2.57
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).

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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