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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.

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