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 from Pulsifer Quarry, Maine, USA © Vandall King
Cookeite from Bennett Quarry, Maine, USA © Henry Minot
Pink cookeite from Hewitt Gem Quarry, Connecticut, USA © Harold Moritz
Cookeite from Paris, Maine, USA © Peter Cristofono

Cookeite in the World

The most beautiful samples of cookeite come from Brazilian pegmatites from Minas Gerais (Galiléia in particular), and American pegmatites from Maine (Paris, Buckfield), Arkansas (Stand on Your Head) and California (Pala, Rincon, etc...). Beautiful specimens are also known from the pegmatite of Muiâne (Mozambique).

Cookeite in France

In France, cookeite was encountered in Castelnau-de-Brassas (Tarn) and Vanoise (Savoie) but also in Riou Bourdoux near St-Pons (Alpes-de-Haute-Provence), in Chaillac (Indre) and in La Mure (Isère).

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

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