A trioctahedral member of the true mica group. The ideal end-member formula is KFe2+2Al(Al2Si2)O10(OH)2, although such a chemical composition has not been reported. The siderophyllite composition is useful to describe solid solution series where there are Fe2+ + Al substitutions, whereas eastonite components involve Mg + Al substitutions. A Tschermak type substitution, which is common in some biotites, involves Aloct + Altet = R2+ + Si.
Cf., eastonite