Feldspar

Feldspars are a group of rock-forming aluminium tectosilicate minerals, containing sodium, calcium, potassium, or barium.[3] The most common members of the feldspar group are the plagioclase (sodium-calcium) feldspars and the alkali (potassium-sodium) feldspars.[4] Feldspars make up about 60% of the Earth's crust,[3] and 41% of the Earth's continental crust by weight.[5][6]

Feldspar
Feldspar crystal (18×21×8.5 cm) from Jequitinhonha valley, Minas Gerais, southeastern Brazil
General
CategoryTectosilicate
Formula
(repeating unit)
KAlSi
3
O
8
 NaAlSi
3
O
8
 CaAl
2
Si
2
O
8
IMA symbolFsp[1]
Crystal systemTriclinic or monoclinic
Identification
Colorpink, white, gray, brown, blue
Cleavagetwo or three
Fracturealong cleavage planes
Mohs scale hardness6.0–6.5
LusterVitreous
Streakwhite
Diaphaneityopaque
Specific gravity2.55–2.76
Density2.56
Refractive index1.518–1.526
Birefringencefirst order
Pleochroismnone
Other characteristicsexsolution lamellae common
References[2]
Compositional phase diagram of the different minerals that constitute the feldspar solid solution.

Feldspars crystallize from magma as both intrusive and extrusive igneous rocks[7] and are also present in many types of metamorphic rock.[8] Rock formed almost entirely of calcic plagioclase feldspar is known as anorthosite.[9] Feldspars are also found in many types of sedimentary rocks.[10]


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