A monoclinic mineral, (Fe,Mg)₇Si₈O₂₂(OH)₂; amphibole group, with Mg/(Mg+Fe)=0- 0.3 ; forms series with cummingtonite and with magnesiocummingtonite; fibrous or needlelike, commonly in radial aggregates; characteristic of iron formations in the Lake Superior and Labrador Trough regions. Also spelled gruenerite.