Cookeite
| Cookeite | |
|---|---|
Cookeite on quartz, Paris, Maine, topotype deposit | |
| General | |
| Category | IX |
| Formula | LiAl 4(Si 3Al)O 10(OH) 8 |
| Strunz classification | 09.EC.55 |
| Dana classification | 71.04.01.02 |
| Crystal system | monoclinic |
| Crystal class | prismatic; C2 or Cc |
| Identification | |
| Color | white, green, brown, golden, pink |
| Twinning | around [310] |
| Cleavage | perfect over {001} |
| Fracture | flexible |
| Tenacity | 2,5 |
| Luster | pearly; silky |
| Density | from 2.58 to 2.69 |
| Refractive index | α = 1,572–1,576, β = 1,579–1,584, γ = 1,589–1,6 |
| Birefringence | biaxial (+), 0,0170–0,0240 2V = 35 to 60° |
| Pleochroism | x = y: pale green to pink, z: colorless to pale yellow |
| Ultraviolet fluorescence | cream yellow (SW) |
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.