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Mineralpedia Details for Petalite

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Petalite

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 
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Petalite from Taquaral, Itinga, Minas Gerais, Brazil

Petalite
            from Taquaral, Itinga, Minas Gerais, Brazil

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...

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Petalite from Londonderry Lithium deposit, Western Australia, Australia

Petalite
            from Londonderry Lithium deposit, Western Australia, Australia

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.

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Petalite from Rubicon mine, Karibib dist., Erongo, Namibia

Petalite
            from Rubicon mine, Karibib dist., Erongo, Namibia

A pinkish mass of crystalline Petalite.

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