Orthoclase

tectosilicate

adularia

Formula: K(AlSi3O8) tectosilicate (framework silicate), K-feldspar.
Adularia is a more ordered low-temperature variety of orthoclase or partially disordered microcline.
Properties of orthoclase
Specific gravity: 2.55 to 2.63
Hardness: 6
Streak: White
Colour: White, green, yellow, pink
Solubility: Insoluble in hydrochloric acid, sulphuric and nitric acids
Environments:

Plutonic igneous environments
Pegmatites
Carbonatites
Metamorphic environments
Hydrothermal environments

In the Bowen reaction series orthoclase is the first major mineral to crystallise after the two branches, continuous and discontinuous, combine.
Orthoclase is a mineral of the zeolite facies.
Adularia is a low temperature form of either orthoclase or partially disordered microcline. It occurs mainly in low temperature veins in gneiss and schist, where it is associated with low sulphidation, low temperature mineralisation. Increased pH (lower acidity) promotes stability of adularia over illite (a variety of muscovite).

K-feldspars are essential constituents of granite and syenite, and major constituents of granodiorite. When these rocks have cooled at moderate depth and at reasonably fast rates orthoclase is the characteristic K-feldspar. In more slowly cooled granite and syenite microcline is the characteristic K-feldspar.

Alteration

biotite and quartz to enstatite-ferrosilite, orthoclase and H2O
K(Mg,Fe)3(AlSi3O10)(OH)2 + 3SiO2 → 3(Mg,Fe2+)SiO3 + KAlSi3O8 + H2O
Enstatite-ferrosilite may develop from the breakdown of biotite according to the above reaction.

muscovite, iron-rich biotite and SiO2 to orthoclase, almandine and H2O
KAl2(AlSi3O10)(OH)2 + KFe2+3(AlSi3O10)(OH)2 + 3SiO2 ⇌ 2KAlSi3O8 + Fe2+3Al2(SiO4)3 + 2H2O
Iron-rich biotite is likely to react at lower PT conditions than iron-poor biotite.

phlogopite, muscovite and SiO2 to orthoclase, pyrope and H2O
KMg3(AlSi3O10)(OH)2 + KAl2(AlSi3O10)(OH)2 + 3SiO2 ⇌ 2K(AlSi3O8) + Mg3Al2(SiO4)3 + 2H2O

Common impurities: Na,Fe,Ba,Rb,Ca

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