Petalite

Petalite, also known as castorite, is a lithium aluminum phyllosilicate^[7] mineral LiAlSi₄O₁₀, crystallizing in the monoclinic system. Petalite occurs as colorless, pink, grey, yellow, yellow grey, to white tabular crystals and columnar masses. It occurs in lithium-bearing pegmatites with spodumene, lepidolite, and tourmaline. Petalite is an important ore of lithium, and is converted to spodumene and quartz by heating to ~500 °C and under 3 kbar of pressure in the presence of a dense hydrous alkali borosilicate fluid with a minor carbonate component.^[8] Petalite (and secondary spodumene formed from it) is lower in iron than primary spodumene, making it a more useful source of lithium in, e.g., the production of glass. The colorless varieties are often used as gemstones. ^{[citation needed]}

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Petalite
Petalite from Minas Gerais State, Brazil (size: 3x4 cm)
General
Category	Phyllosilicate
Formula	LiAlSi4O10
IMA symbol	Ptl[1]
Strunz classification	9.EF.05
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P2/a
Unit cell	a = 11.737 Å, b = 5.171 Å, c = 7.63 Å; β = 112.54°; Z = 2
Identification
Color	Colorless, grey, yellow, pink, to white
Crystal habit	Tabular prismatic crystals and columnar masses
Twinning	Common on {001}, lamellar
Cleavage	Perfect on {001}, poor on {201} with 38.5° angle between the two
Fracture	Subconchoidal
Tenacity	Brittle
Mohs scale hardness	6–6.5
Luster	Vitreous, pearly on cleavages
Streak	Colorless
Diaphaneity	Transparent to translucent
Specific gravity	2.4
Optical properties	Biaxial (+)
Refractive index	nα = 1.504, nβ = 1.510, nγ = 1.516
Birefringence	δ = 0.012
2V angle	82–84° measured
Melting point	1350 °C[2]
Fusibility	5
Solubility	Insoluble
References	[3][4][5][6]

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