orthoclase one of the most widespread silicates in the mineral world


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a specimen of orthoclase one of the most widespread silicates in the mineral world

Mineral Group  silicate
Composition  KAlSi3O8
dimension (cm)  4×3.5×2
weight (g)  42
 Orthoclase takes its name from the Greek orthos (straight) and klasis (breaking), on account of its good cleavage. The thickly tabular and short columnar crystals are often intertwinned. The Carlsbad twins occur most frequently and are so named because they were originally discovered in the region of Karlovy Vary (for which Carlsbad is the German name) in the Czech Republic. Finely tabular crystallised orthoclase with an admixture of sodium is called sanidine (from the Greek sanis, ‘plate’). It occurs in younger igneous rocks, such as trachytes and phonolites. Pure orthoclase is completely clear, but mostly it is white, yellowish or pinkish. The variety adularia originates from hot solutions and is the purest of the orthoclase family. It was named after the deposits in the Adula mountains south-east of Andermatt (Switzerland), where it occurs in cracks of Alpine rocks, just as in hundreds of other deposits in the Mont Blanc massif. Adularia is transparent, often permeated with chlorites, and sometimes displays a delicate play of colours. Its crystals, which frequently fill cracks in schists. are often bounded by perfect faces. In contrast to the other feldspars, they have a pearly lustre.
Orthoclase is one of the most widespread silicates in the mineral world and one of the most abundant rock-forming minerals. Not only does it represent a substantial part of many igneous rocks, but it also forms part of a great number of crystalline schists and rocks of sedimentary origin, such as greywacke, and other sandstones. In such rocks it usually forms tiny, evenly distributed grains. In some granites. orthoclase occurs abundantly in the form of large crystals, superior to the minerals of uniform grain size found in the rock. Such so-called porphyritic phenocrystals are often perfectly bounded and have the shape of flattened simple columns. Basically they are phenocrystals several centimetres long, from granite magma, which are older than the adjacent basic substance and which crystallised from as yet unsolidified melt.
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Additional information

Weight 42 g
Dimensions 4 x 3.5 x 2 cm