Petalite

Petalite
Petalite from Minas Gerais State, Brazil (size: 3x4 cm)
General
Category	Tectosilicate
Formula (repeating unit)	LiAlSi4O10
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
Fusibility	5
Solubility	Insoluble
References

Petalite, also known as castorite, is a lithium aluminium tectosilicate mineral LiAlSi₄O₁₀, crystallizing in the monoclinic system. Petalite is a member of the feldspathoid group. It occurs as colorless, grey, yellow, yellow grey, to white tabular crystals and columnar masses. 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. The colorless varieties are often used as gemstones.

Discovered in 1800, by Brazilian naturalist Jose Bonifacio de Andrada e Silva. Type locality: Utö Island, Haninge, . The name is derived from the Greek word petalon, which means leaf (perfect cleavage).

Economic deposits of petalite are found near Kalgoorlie, Western Australia; Aracuai, Minas Gerais, Brazil; Karibib, Namibia; Manitoba, Canada; and Bikita, Zimbabwe.