Microcline is a mineral with a hardness of 6 out of 10 on the Mohs scale of mineral hardness [?]. These Triclinicly structured gems are made of potassium aluminum silicate, their full chemical compound being KAlSi3O8.
Microcline is a silicate of potassium and aluminum, and an alkali member of the feldspar group. It is the phase of this compound that is stable at low temperature. The feldspars are major constituents of the rocks on the earth's crust and comprise some of the most important rock-forming minerals.
The four feldspars - orthoclase, sanidine, microcline and anorthoclase - comprise a group called the potassium or alkali feldspars. They all have the same chemical composition, potassium aluminium silicate, but different crystal structure and are therefore polymorphs.
The differences among these feldspars result from the different ways in which aluminium is distributed in the aluminosilicate atomic structures of each.
Microcline has a triclinic crystal system, occurs as prismatic crystals, frequently twinned. Polysynthetic twins are also common, giving a characteristic "Tartan twinning" structure.
Simple and polysynthetic twinning are ubiquitous and perthitic intergrowth with albite, extremely common. Reflections from incipient cleavages give polished surfaces a shimmering effect.
It is opaque to translucent, with a vitreous luster, and is rarely transparent. It is infusible and insoluble in acids, except hydrofluoric.
It is commonly white to pale yellow or salmon, also may be blue to green in the microcline variety amazonite. It should be noted that although nearly all green potassium feldspar is microcline, much microcline, as well as most orthoclase, is commonly white, flesh- or salmon-pink.
Microcline commonly occurs in plutonic igneous rocks such as granites and syenites that cooled at medium-to-low temperatures, and at considerable depth. It is the common potassium feldspar in pegmatites where it takes the place of orthoclase. It is often intimately associated with quartz (myrmekite) and albite (perthite).
It can form the largest known crystals of any mineral (in a pegmatite in Kareliya, Russia, a microcline mass weighing over 2000 tons showed the form of a single crystal!). Microcline is also common in metamorphic rocks in gneisses and in sedimentary rocks in arkosesand conglomerates.
Samples are found in Russia, Virginia, Pikes Peak in Colorado (USA), Norway, Sweden, Italy, South Africa, Japan, Canada (Canada), India, Australia, and other sources.
Notable deposits are found in Italy, Norway, Madagascar, Namibia, Zimbabwe, India, Russia, Brazil, Australia, Canada and the USA.
Amazonite localities include USA (Colorado, California, Montana, Pennsylvania, Virgina-Amelia County, deep, blue-green, translucent), Canada (Ontario-Renfrew and Parry Sound districts and Quebec-Kipawa), Brazil, India (Kashmir district and elsewhere), Kenya, Tanzania, Madagascar (Anjanabonoina, Andina, Immody and Mahabe), Namibia and South Africa.
Russia has the biggest gem amazonite deposits in the world (Ploskaya Gora and Parus mountain). Also in Russia are relatively large quantities of amazonite used as gemrock, come from pegmatite masses near Miass in the Ilmen range of the southern Ural mountains, Afghanistan at Madan Shar, Kabul Province in small deep blue-green, semi-translucent crystals and Australia (Broken Hill, New South Wales - also in semitranslucent masses).
Green microcline is reported from various localities in Brazil.
Synonyms for microcline are: Potash feldspar, alkali feldspar, K-feldspar. Microcline is sometimes called amazon stone.
Varieties of microcline are:
Perthite - named after the town of Perth, Ontario, Canada, where it was first found. It is the general variety name for alkali feldspar consisting of parallel or subparallel intergrowths
Graphic granite - an interpenetrating pattern when quartz and microcline crystallize simultaneously in pegmatites, as they often do. The appearance resembles cuneiform script. Graphic granite as a decorative stone has been used since antiquity and, more recently, is reported as being used by cooperatives and amateur craftsmen as a decorative material in the Chelyabinsk region of the southern Urals, Russia - also used to make carvings and cabochons. Also known as Hebraic pegmatite or Hebrew stone, corduroy rock, graphic pegmatite and runite.
Amazonite a variety of microcline that is a gem of ornamental quality. The name amazonite appears to have evolved from the term "Amazon stone" which, perhaps through confusion, was applied originally to another green stone once found in the grave of the Amazon River, as amazonite is not found anywhere in the Amazon basin. It is also known as mother of emerald, green feldspar, microcline, Colorado Jade, and Amazon Jade.
The name microcline comes from two Greek words mikro meaning small and kleinen meaning inclined, referring to the slight deviation from 90 degrees of intersecting cleavages.
The only variety gem of microcline that is of ornamental quality is amazonite. It is often cut into cabochons, beads, cut gems, or carving objects. Some microclines create moonstones effects. Microcline is of particular interest to petrologists.