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Download hereGENERAL DESCRIPTION
The microcline is a potassic feldspar: it is thus a potassium aluminosilicate although it can also contain noticeable quantities of sodium and calcium (to replace potassium) and of iron (to replace aluminium).
It is one of minerals constitutive of many plutonic rocks (granites, syenites), it is also present in certain metamorphic rocks. But the microcline is the typical feldspar of pegmatites.
It is generally of color white, pink clearly, gray or green-bluish (amazonite variety whose color is possibly due to defects probably related to traces of irradiated lead).
The crystals of microcline are prismatic, flat and often present twins (primarily polysynthetic like albite; but also Manebach and Carlsbad, Baveno twinnings; these two last being rare for this species). They can reach impressive dimensions - a sample of 10.5 m x 4.5 m x 1.8 m was found at Keystone in South Dakota in the United States, another of 9 m x 3.5 m x 2 m was found in Tveit in Norway.
The locality of Pikes Peak in Colorado is well known for its amazonite samples that are associated smoked quartz (but often intensely cleaned oxides iron and repaired with glue). Giant crystals of amazonite (weighing several hundred kilos) that one can observe in the Giant Crystals Hall of the Muséum national d’Histoire naturelle come from the Valley of Rio Doce in Brazil. Norway and Madagascar also produced a remarquable series of crystals.
In France, microcline is common but the high-quality crystals are of (relative) small dimensions. Among the localities: the Armorican massif, the Marvejol area in Lozere or Saint-Pierre-la-Bourlhonne in Puy-de-Dôme. One finds amazonite at Vizzavona in Corsica.
The location of conservation of the type of this species is un known (sample describes in 1830 coming from Stavern (Fredriksvärn), close to Larvik, area of Vestfold, Norway).
HISTORY : Name inspired from the Greek word [mikros] meaning small and "kλνειν" [klinein] meaning "lean". Named from the angle of cleavage plans is different from 90°
Species described in 1830 par J.F.A. Breithaupt (1791-1873), Saxon mineralogist
Type-locality : Frediksvarn, Norway
CHEMICAL FORMULA : K Al Si3O8
CRYSTAL SYSTEM : Triclinic
COLOR : White, yellow pale, red, blue
DIAPHANIETY : Transparent to translucent
LUSTER : Vitreous
STREAK : White
MORPHOLOGIE : Prismatic crystals
HARDNESS : 6,0-6,5
CHEMICAL CLASS: 2,56
DENSITY : VIII - Silicates
GROUP : Alkaline feldspath
STRUNZ CLASS BEFORE 2001 : 8/J.06-30
STRUNZ CLASS AFTER 2001 : 9.FA.20
