Cummingtonite - Encyclopedia
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
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