Cookeite

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
Environments:

Pegmatites
Hydrothermal environments

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, diaspore, böhmite, illite, sandstone and bauxite.

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.

Common impurities: Fe,Mn,Mg,Ca,Na,K