Cookeite - Encyclopedia
Class : Silicates
Subclass : Phyllosilicates
Crystal system : Triclinic
Chemistry : LiAl4(Si3Al)O10(OH)8
Rarity : Quite common
Cookeite is one of the four main species of the chlorite group. It is the lithiniferous term of the group, the rarest. It is found in lithiniferous pegmatites, associated with other lithium minerals. Its name is dedicated to the American chemist and mineralogist Josiah B. Cooke, a member of Harvard University. Like other chlorites, cookeite can appear in chunky, pseudohexagonal lamellar crystals, flattened on {001}, sometimes looking like barrels. However, it generally occurs in spherolites and aggregates with a radiate texture, or in scaly aggregates measuring up to 4 mm. Its color is pale green to yellowish green, white, pink or brown, exceptionally intense purple.
Main photo : Cookeite and quartz from Green's Crystal Mine, Arkansas, USA © Jamison K. Brizendine
Cookeite in the World
Twinning
No twin known for this mineral species.
Fakes and treatments
No fake identified for this mineral species.
Hardness : 2.5 to 3.5
Density : 2.58
Fracture : Micaceous
Streak : White
TP : Translucent
RI : 1.572 to 1.600
Birefringence : 0.014
Optical character : Biaxial +
Pleochroism : Visible
Fluorescence : Yellow
Solubility : Hydrofluoric acids
Magnetism : Paramagnetic
Radioactivity : None