A trioctahedral member of the true mica group. The ideal end-member formula is KFe2+3AlSi3O10(OH)2. The ideal end-member composition is unlikely to occur because of tetrahedral- octahedral misfit (the inability of the tetrahedral sheet to link with the octahedral sheet at certain compositions), with at least 10 % Fe3+ required to minimize misfit. Annite occurs in granites, granitic pegmatites, greisens, and some alkalic (syenite) rocks. Annite forms most commonly in the 1M polytype and belongs to the collective name “dark” mica.
Cf., biotite