Formula: K(AlSi3O8)
Tectosilicate (framework silicate), feldspar group.
Adularia is a more ordered low-temperature variety of orthoclase or partially disordered microcline.
Crystal System: Monoclinic
Specific gravity: 2.55 to 2.63 measured, 2.563 calculated
Hardness: 6
Streak: White
Colour: White, green, yellow, pink. Red feldspars such as orthoclase are coloured by micro or nano sized particles of hematite as inclusions. These particles formed over geological time as water percolated very slowly through the orthoclase and reacted with Fe3+ that had replaced Al in the structure and caused it to precipitate out as hematite (https://www.youtube.com/watch?v=ejucgGmeJMA).
Solubility: Insoluble in hydrochloric acid, sulphuric and nitric acids
Common impurities: Na,Fe,Ba,Rb,Ca

Plutonic igneous environments
Volcanic igneous environments
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 K-feldspar variety adularia over muscovite variety illite.

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.


At the Spain mine, Griffith township, Renfrew county, Ontario, Canada, orthoclase pseudomorphs after scapolite have been found (KL p263).

At Oberwiesenthal, Erzgebirge, Germany, orthoclase pseudomorphs after leucite have been found (KL p262).

At Schneckenstein, Vogtland, Saxony, Germany, a topaz pseudomorph after orthoclase with Carlsbad twinning has been found (KL p221).

At the Keyes Mica Quarries, Orange, Grafton County, New Hampshire, USA, the pegmatites are beryl-type rare-element (RE) pegmatites.
The Number 1 mine exposed a pegmatite that shows the most complex zonation and diverse mineralogy of any of the Keyes pegmatites. Six zones are distinguished, as follows, proceeding inward from the margins of the pegmatite:
(1) quartz-muscovite-plagioclase border zone, 2.5 to 30.5 cm thick
(2) plagioclase-quartz-muscovite wall zone, 0.3 to 2.4 metres thick
(3) plagioclase-quartz-perthite-biotite outer intermediate zone, 0.3 to 5.2 metres thick, with lesser muscovite
(4) quartz-plagioclase-muscovite middle intermediate zone, 15.2 to 61.0 cm thick
(5) perthite-quartz inner intermediate zone, 0.9 to 4.6 meters thick
(6) quartz core, 1.5 to 3.0 metres across
The inner and outer intermediate zones contained perthite crystals up to 1.2 meters in size that were altered to vuggy albite-muscovite with fluorapatite crystals. This unit presumably was the source of the albite, muscovite, fluorapatite, quartz and other crystallised minerals found in pieces of vuggy albite rock on the dumps next to the mine.
The middle intermediate zone produced sheet mica with accessory minerals including tourmaline, graftonite, triphylite, vivianite, pyrite, pyrrhotite, and beryl crystals to 30.5 cm long and 12.7 cm across.
Orthoclase has been found as white crystals to 2 mm in coarse-grained, vuggy albite. The crystals show the adularia habit and lack exsolutions or significant sodium content. They are associated with microsized crystals of blue-green fluorapatite and an unidentified beige mineral (R&M 97.4.322).


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 (DHZ 2A p134).

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 (DHZ 3 p72).

phlogopite, muscovite and SiO2 to orthoclase, pyrope and H2O
KMg3(AlSi3O10)(OH)2 + KAl2(AlSi3O10)(OH)2 + 3SiO2 ⇌ 2K(AlSi3O8) + Mg3Al2(SiO4)3 + 2H2O
(DHZ 3 p72).

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