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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
Environments:
Plutonic igneous environments
Volcanic 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 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.
Localities
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).
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
(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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