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Formula: (Al,Li)3Al2(Si,Al)4O10(OH)8
Phyllosilicate (sheet silicate), chlorite group
Crystal System: Monoclinic
Specific gravity: 2.58 to 2.69 measured, 2.968 calculated
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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