GREENALITE

    Class : Silicates
    Subclass : Phyllosilicates
    Crystal system : Monoclinic
    Chemistry : Fe2-3Si2O5(OH)4
    Rarity : Rare

Greenalite is a rare mineral from the kaolinite group. It is a primary phyllosilicate that is mainly found in banded iron deposits (BIFs), notably accompanying stilpnomelane, siderite, pyrite and chamosite. Greenalite also forms by diagenesis in tropical sediments. It was named so in reference to its green color. Greenalite is usually massive or in the form of rounded fibrous granules of pale yellow-green to dark green, millimeter-sized, which frequently causes confusion with glauconite. It is a mineral which alters into iron oxides.

Main photo : Gratonite from Excelsior Mine, Cerro de Pasco, Pasco province, Pasco, Peru © Rob Lavinsky

Greenalite in the World

Greenalite is widespread in banded iron ores of the Mesabi Mountains in the United States (Biwabik and Gilbert in Arizona), Australia (Cue) and Canada (Knob Lake, Newfoundland). It is also a newly formed mineral from the sediments of the Ogooué estuaries (Gabon), Niger and Congo.

Greenaite in France

In France, greenalite is known in two original contexts : in decimetric bottle-green masses in a serpentinite on the island of Groix (Morbihan) and in the Salsigne gold mine (Aude).

Twinning

No twins known for this mineral species.

Fakes and treatments

No fakes recorded for this mineral species but possible confusion with glauconite.


Hardness : 2.5
Density : 2.85 to 3.15
Fracture : Micaceous
Streak : Gray-green

TP : Translucent
RI : 1.650 to 1.675
Birefringence : 0.024
Optical character : Biaxial +
Pleochroism : Visible
Fluorescence : None

Solubility : -

Magnetism : ParamagneticRadioactivity : None