Pópulo, Alijó, Vila Real, Portugal
KAlSi3O8
Crystal System: monoclinic
Colour: colorless to white, greenish white, grayish yellow or pale pink
Lustre: vitreous, sub-vitreous, resinous
Habitus: usually as cleavable, granular, massive masses. Crystals are common, as short prisms or tabular individuals. Twinning is common (e.g., according to Carlsbad, Baveno, or Manebach laws)
Hardness: 6
Fracture: irregular/uneven, conchoidal
Cleavage: perfect on {001}, good on {010}
Density: 2.563 g/cm3
Origin and geological occurrence: orthoclase is the common feldspar in granite, granite pegmatite, and syenite. It occurs in high-grade metamorphic rocks, as well as a result of potassic hydrothermal alteration. It could also be authigenic or detrital in sedimentary rocks.
Samples:
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9F.9 Orthoclase Twinned phenocryst in ignimbrite. Valfloriana, Trento, Trentino-Alto Adige #tectosilicates |