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
Cookeite occurs as late stage mineralisation in gem-pocket-bearing granite
pegmatites associated with 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,
illite, sandstone and
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.
Common impurities: Fe,Mn,Mg,Ca,Na,K