Cookeite

Cookeite
Cookeite on quartz, Paris, Maine, topotype deposit
General
Category	IX
Formula (repeating unit)	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.^[1] 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.