A monoclinic mineral, Na₂Ca(MgFe₊₂)₅Si₈O₂₂(OH)₂; amphibole group with Mg/(Mg+Fe₂₊) = 0 to 0.49 and Fe₃₊/(Fe₃₊+Al) = 0.7 to 1.0; forms a series with magnesioriebeckite; fibrous; in soda-rich rhyolites, granites, and pegmatites; crocidolite variety is blue asbestos, tiger eye is crocidolite replaced by quartz. Compare: glaucophane