Cummingtonite

Formula:  (Mg,Fe,Mn)₇Si₈O₂₂(OH)₂

Origin of name:  For locality; Cummington , Massachusetts

Crystal system:  Monoclinic

Crystal class:  2/m

Twinning:

Unit cell:

Colour:  Dark green, grayish-green, brown, white, light gray

Diaphaneity:  Translucent to opaque

Luster:  Silky

Habit:  Fibrous or fibro-laminar; radiated

Hardness:  5 to 6

Specific gravity:  3.10 to 3.56

Cleavage:  1; {110} good

Tenacity:

Streak:

Synonyms/varieties:

Comments:  Series with manganocummingtonite and grunerite