Cookeite

Cookeite is a mineral species of the silicate group and the phyllosilicate subgroup, part of the chlorite family, with the formula LiAl₄(Si₃Al)O₁₀(OH)₈. This soft, low-density mineral of variable color has a crystalline structure made up of alternating layers LiAl₂(OH)₆ and Al₂O₄(OH)₂Si₈O₁₂ having several polytypes. Cookeite is often found as a product of hydrothermal alteration of silicates in pegmatites. It forms at relatively low temperatures (below 200°C) and variable pressures.