Orthoclase, or orthoclase feldspar (endmember formula KAlSi3O8), 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. The gem known as moonstone (see below) is largely composed of orthoclase.
Orthoclase
Adularia (KAlSi3O8) with pyrite (FeS2) incrustations
Feldspar is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the plagioclase (sodium-calcium) feldspars and the alkali (potassium-sodium) feldspars. Feldspars make up about 60% of the Earth's crust, and 41% of the Earth's continental crust by weight.
Feldspar crystal 18 cm × 21 cm × 8.5 cm (7.1 in × 8.3 in × 3.3 in) from Jequitinhonha valley, Minas Gerais, southeastern Brazil
Specimen of rare plumbian (lead-rich) feldspar
Crystallized white feldspar, with an upright 4 cm aquamarine crystal perched on it
Feldspar and moonstone, from Sonora, Mexico