Cookeite

minerals

tourmaline

chlorite

spodumene

Formula: (Al,Li)3Al2(Si,Al)4O10(OH)8 phyllosilicate (sheet silicate) chlorite group
Specific gravity: 2.58 to 2.69
Hardness: 2½ to 3½
Streak: White
Colour: White, yellowish green, pink, brown. Colourless when pure.
Solubility: Insoluble in common acids; soluble in HF
Common impurities: Fe,Mn,Mg,Ca,Na,K
Environments:

Pegmatites
Hydrothermal environments

Cookeite occurs as late stage mineralisation in gem-pocket-bearing granite pegmatites associated with albite variety cleavelandite, elbaite, lepidolite and spodumene. It has also been found as pseudomorphs of spodumene, petalite and elbaite. Cookeite also occurs in some simple quartz-crystal lined veins, and in hydrothermally altered sedimentary rocks with kaolinite, diaspore, böhmite, muscovite variety illite, sandstone and bauxite (Dana).

Localities

At the Golconda mine, Minas Gerais, Brazil, cookeite pseudomorphs after tourmaline have been found (KL p238).

At the Emmons pegmatite, Greenwood, Oxford county, Maine, USA, cookeite has been observed replacing spodumene. The Emmons pegmatite is an example of a highly evolved boron-lithium-cesium-tantalum enriched pegmatite (R&M 94.6.506).

Alteration

Cookeite has been synthesised at 260 to 480oC, 1-14 kbar pressure.
It can be formed by the alteration of kaolinite and diaspore by lithium-bearing solutions, releasing quartz (Dana).

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