A dioctahedral member of the true mica group. The ideal end-member formula is KCr2□AlSi3O10(OH,F)2, where □ = vacancy. Chromphyllite, like muscovite, occurs as the 2M1 polytype in C2/c symmetry, and the Cr forms a solid solution series with Al in muscovite. Chromphyllite is known from the Lake Baikal region, Russia, Outokumpu, Finland, and elsewhere, and is believed to occur in association with metasomatism.
Cf., muscovite