A monoclinic mineral, (Fe,Mg)₇Si₈O₂₂(OH)₂; amphibole group; has Mg/(Mg + Fe₂₊) = 0.30 to 0.69 ; prismatic cleavage; may be asbestiform; in amphibolites and dacites; fibrous varieties (amosite, magnesium rich, and montasite, iron rich) are used as asbestos.