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Orthoclase, or orthoclase feldspar (endmember formula K Al Si 3 O 8), is an important tectosilicate mineral which forms igneous rock. The name is from the Ancient Greek for "straight fracture", because its two cleavage planes are at right angles to each other. It is a type of potassium feldspar, also known as K-feldspar.
Feldspar (/ ˈ f ɛ l (d) ˌ s p ɑːr / FEL(D)-spar; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. [3] The most common members of the feldspar group are the plagioclase (sodium-calcium) feldspars and the alkali (potassium ...
According to the Goldich dissolution series, anorthite, a plagioclase feldspar, should weather quickly, with a lifetime of 10 5.62 years quantified by Kowalski and Rimstidt. [1] [5] Conversely, the lifetime of K-feldspar should be much longer, at 10 8.53 years based again on Kowalski and Rimstidt’s work. However, White and Brantley’s ...
Sanidine is the high temperature form of potassium feldspar with a general formula K(AlSi 3 O 8). [2] Sanidine is found most typically in felsic volcanic rocks such as obsidian, rhyolite and trachyte. Sanidine crystallizes in the monoclinic crystal system. Orthoclase is a monoclinic polymorph stable at lower temperatures.
Microcline may be chemically the same as monoclinic orthoclase, but because it belongs to the triclinic crystal system, the prism angle is slightly less than right angles; hence the name "microcline" from the Greek "small slope". It is a fully ordered triclinic modification of potassium feldspar and is dimorphous with orthoclase. Microcline is ...
cordierite + quartz + sillimanite + K-feldspar (orthoclase) ± biotite ± garnet (If the temperature is below 750 °C there will be andalusite instead of sillimanite) cordierite + orthopyroxene + plagioclase ± garnet, spinel; In carbonate rocks: calcite + forsterite ± diopside, periclase; diopside + grossular + wollastonite ± vesuvianite
If cooling is sufficiently slow, the alkali feldspar may exsolve to form separate grains with near-endmember albite and K-feldspar compositions. The largest documented single crystal of perthite was found in Hugo Mine in South Dakota and measured about 10.7 m (35 ft) × 4.6 m (15 ft) × 1.8 m (5 ft 11 in).
Potassium feldspar refers to a number of minerals in the feldspar group that contain large amounts of potassium in the crystal lattice. Orthoclase (endmember formula K Al Si 3 O 8), an important tectosilicate mineral that forms igneous rock; Microcline, chemically the same as orthoclase, but with a different crystalline structure