Grunerite from South Dakota
|Crystal class||Prismatic (2/m)
(same H-M symbol)
|Formula mass||1,001.61 g/mol|
|Color||Ashen, brown, brownish green, dark gray|
|Mohs scale hardness||5 - 6|
|Diaphaneity||Translucent to opaque|
|Density||3.4 - 3.5|
|Other characteristics||Not radioactive|
Grunerite is a mineral of the amphibole group of minerals with formula Fe7Si8O22(OH)2. It is the iron endmember of the grunerite-cummingtonite series. It forms as fibrous, columnar or massive aggregates of crystals. The crystals are monoclinic prismatic. The luster is glassy to pearly with colors ranging from green, brown to dark grey. The Mohs hardness is 5 to 6 and the specific gravity is 3.4 to 3.5.
It was discovered in 1853 and named after Emmanuel-Louis Gruner (1809–1883), a Swiss-French chemist who first analysed it.