Formula: (Al,Li)3Al2(Si,Al)4O10(OH)8 phyllosilicate (sheet silicate)
Specific gravity: 2.58 to 2.69
Hardness: 2½ to 3½
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
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
spodumene, petalite and
elbaite. Cookeite also occurs in some simple
quartz-crystal lined veins, and in hydrothermally altered
sedimentary rocks with kaolinite,
muscovite variety illite, sandstone and
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).
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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