A phyllosilicate (sheet silicate) similar to biotite with the formula (K,Na,Ca)_0–1.4 (Fe²⁺,Fe³⁺,Mg,Al,Mn)_5.9–8.2 [Si₈O₂₀] (OH)₄(OH,F)_3.6–8.5; sp. gr. 2.59–2.96; hardness 3.0–4.0; properties similar to those of biotite; found in metamorphosed iron- and manganese-rich sedimentary deposits and in some glaucophane schists.