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.