A mineral occurring typically as brownish fibrous crystals in some metamorphic rocks. It is a magnesium-rich iron silicate of the amphibole group.
- Accessory chlorite, magnetite or ilmenite, titanite, apatite, partly chloritized biotite, epidote, pyrite and sometimes cummingtonite, are present.
- This rock is a dark gray schistose rock containing radiating aggregates of cummingtonite and occasionally pyrrhotite.
- The two minerals can have similar distinctive brown colors and are hard to differentiate from each other, although cummingtonite is typically darker and slightly denser.
Early 19th century: named after Cummington, a town in Massachusetts, + -ite1.
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