petalite

{{Infobox mineral | name = Petalite | category = Phyllosilicate | boxwidth = | boxbgcolor = | image = Petalite.jpg | imagesize = 260px | caption = Petalite from Minas Gerais State, Brazil (size: 3x4 cm) | formula = LiAlSi4O10 | IMAsymbol = Ptl | molweight = | strunz = 9.EF.05 | system = Monoclinic | class = Prismatic (2/m) (same H-M symbol) | symmetry = P2/a | unit cell = a = 11.737 Å, b = 5.171 Å, c = 7.63 Å; β = 112.54°; Z = 2 | color = Colorless, grey, yellow, pink, to white | 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 = 6–6.5 | luster = Vitreous, pearly on cleavages | refractive = nα = 1.504, nβ = 1.510, nγ = 1.516 | opticalprop = Biaxial (+) | birefringence = δ = 0.012 | 2V = 82–84° measured | pleochroism = | streak = Colorless | gravity = 2.4 | density = | melt = 1350 °C | fusibility = 5 | diagnostic = | solubility = Insoluble | diaphaneity = Transparent to translucent | other = | references = }}

Petalite, also known as castorite, is a lithium aluminum phyllosilicate 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. 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.