CUMMINGTONITE

    Class : Silicates
    Subclass : Inosilicates
    Crystal system : Monoclinic
    Chemistry : (Mg,Fe)7Si8O22(OH)2
    Rarity : Common

Cummingtonite belongs to the group of monoclinic amphiboles and forms two series, with magnesium-cummingtonite and grunerite. It is a magnesium amphibole that is found mainly in amphibolites deriving from the metamorphism of basic rocks, sometimes in gabbros and norites and less frequently in metamorphosed iron deposits. Its name derives from its locality of discovery : Cummington (Massachussetts). It appears in fibrous to acicular crystals, more rarely prismatic, reaching 20 cm, often united in fibrous aggregates. Its color is dark green to greenish gray, sometimes brown. The term "amosite" partly covers the asbestiform variety of cummingtonite. This amphibole is therefore occasionally used as asbestos.

Main photo : Cummingtonite from Morro da Mina Mine, Minas Gerais, Brazil © John Sobolewski

Cummingtonite in the World

The well-known samples come from Cummington (Massachussetts), the type locality of this mineral, where grunerite is however more abundant than cummingtonite. Abukuma (Japan) also provided good samples, as did Delta Marble (South Africa), where cummingtonite forms large cleavable masses.

Photo on the right : Cummingtonite & spessartine from Morro da Mina Mine, Minas Gerais, Brazil © Izaac Da Silva Leite

Cummingtonite in France

In France, cummingtonite is reported in the Alps and the Pyrenees as well as in the gabbros of St-Brieuc (Côte d'Armor).

Twinning

Twins are known and parallel to {100}.

Fakes and treatments

No fakes listed for this mineral species.


Hardness : 5 to 6
Density : 3.1 to 3.6
Fracture : Fibrous
Streak : White

TP : Translucent to transparent
RI : 1.639 to 1.708
Birefringence : 0.025 to 0.037
Optical character : Biaxial +
Pleochroism : Visible
Fluorescence : Red

Solubility : Insoluble


Magnetism : None
Radioactivity : None