Mineralpedia Details for Petalite

Formula

LiAlSi₄O₁₀ 

Crystal System

Monoclinic 

Crystal Habit

Massive 

Cleavage

Perfect, Imperfect, None 

Luster

Vitreous - Pearly 

Color

colorless, gray, yellow, yellow gray, white 

Streak

colorless 

Class

Monoclinic - Prismatic 

Fracture

Brittle - Conchoidal 

Hardness

6-6.5 

WebMineral

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Mindat

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A water clear crystal fragment of this rare lithium aluminosilicate found exclusively in granitic (LCT) pegmatites. Lithium in pegmatites is usually characterized by one of the primary lithium minerals: Spodumene or Petalite and to a lesser extent Amblygonite-Montebrasite. Lithium pegmatites are rare and constitute about 1-2% of all pegmatites. Most lithium pegmatites are zoned and usually occur at the furthest extents from the parent source and usually intrude metamorphic rock. Rarely do the minerals Petalite and Spodumene occur in the same pegmatite since Spodumene is stable at higher pressures and moderate temperature unlike Petalite (high T and moderate P)...then there is the secondary lithium mineral Eucryptite which is stable at low P and T. See Cerny and/or London...

Dr, David London of University of Oklahoma has shown that pegmatite formation under conditions of silica (quartz) saturated conditions, the lithium aluminum silicates petalite, spodumene and eucryptite are good indicators of the temperature and pressure of formation. Spodumene at equilibrium occurs at high pressure, while petalite at high temperatures, But upon cooling, spodumene is sometimes seen as replacing petalite.

In this specimen thin white laminations can be seen within the otherwise colorless matrix. This is possibly the sort of replacement London has described on pages 55-58 of his "Pegmatites" - a highly recommended resource. Optical work would be required to determine if this replacement occured in this particular specimen.

A pinkish mass of crystalline Petalite.