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Feldspar

Feldspar crystal (18×21×8.5 cm) from Jequitinhonha valley, Minas Gerais, Southeastern Brazil.
Name	Feldspar
Category	tectosilicate
Caption	Feldspar crystal (18×21×8.5 cm) from Jequitinhonha valley, Minas Gerais, Southeastern Brazil.
Formula	KAlSi₃O₈ � NaAlSi₃O₈ � CaAl₂Si₂O₈
Color	pink, white, gray, brown
System	triclinic or monoclinic
Twinning	tartan, carlsbad, etc
Cleavage	two or three
Fracture	along cleavage planes
Mohs	6
Luster	vitreous
Birefringence	first order
Pleochroism	none
Diaphaneity	opaque
Other	exsolution lamellae common

Home | mineral | Feldspar

Feldspars (KAlSi₃O₈ � NaAlSi₃O₈ � Ca Al₂Si₂O₈) are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust.

Feldspars crystallize from magma in both intrusive and extrusive igneous rocks, as veins, and are also present in many types of metamorphic rock. Rock formed almost entirely of calcic plagioclase feldspar (see below) is known as anorthosite. Feldspars are also found in many types of sedimentary rock.

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