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Formula: K(AlSi3O8)
Tectosilicate (framework silicate), feldspar group, fully disordered in its Al
and Si distribution,
paramorph of kokchetavite,
microcline (fully ordered) and
orthoclase (partially ordered). Sanidine forms a series with high
albite (disordered, stable above 800oC) (Mindat).
Crystal System: Monoclinic
Specific gravity: 2.56 to 2.62 measured, 2.56 calculated
Hardness: 6
Streak: White
Colour: Colourless, transparent to opaque grey, grey brownish. Can be yellow due to iron Fe3+ replacing some
of the aluminium in the tetrahedral framework (https://www.youtube.com/watch?v=ejucgGmeJMA)
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Common impurities: Fe,Ca,Na,H2O
Environments:
Sanidine is a high-temperature K-feldspar; it occurs as phenocrysts
(larger crystals embedded in
a finer-grained matrix in an igneous rock) in
volcanic igneous rocks such as
rhyolite and trachyte.
It is characteristic of
rocks that cooled quickly from an initial high temperature of eruption.
Sanidine may occur in
hornfels.
It is characteristic of the sanidinite facies.
Associated minerals include quartz, sodium-rich plagioclase,
muscovite, biotite,
hornblende and magnetite
(HOM).
Localities
The type locality is Drachenfels, Königswinter, Rhein-Sieg-Kreis, Cologne, North Rhine-Westphalia, Germany.
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