Crystal chemical variations in Li- and Fe-rich micas from Pikes Peak batholith (central Colorado)

The crystal structure and M-site populations of a series of micas-1M from m iarolitic pegmatites that formed within host granitic rocks of the Precambr ian, anorogenic Pikes Peak batholith, central Colorado, were determined by single-crystal X-ray diffraction data. Crystals fall in the polylithionite- siderophyllite-annite field, being 0 less than or equal to Li less than or equal to 2.82, 0.90 less than or equal to Fe-total less than or equal to 5. 00, 0.26 less than or equal to([6])Al less than or equal to 2.23 apfu. Orde ring of trivalent cations (mainly Al3+) is revealed in a cis-octahedral sit e (M2 or M3), which leads to a lowering of the layer symmetry from C12/m(1) (siderophyllite and annite crystals) to C12(1) diperiodic group (lithian s iderophyllite and ferroan polylithionite crystals). On the basis of mean bo nd length the ordering scheme of octahedral cations is mostly meso-octahedr al, whereas the mean electron count at each M site suggests both meso- and hetero-octahedral ordering, the calculated mean atomic numbers being M1 = M 3 not equal M2, M2 = M3 not equal M1 and M1 not equal M2 not equal M3. As t he siderophyllite content increases, so do the a, b, and c unit-cell parame ters, as well as the refractive indices, primarily np. The tetrahedral rota tion angle, alpha, is generally small (1.51 less than or equal to alpha les s than or equal to 5.04 degrees) and roughly increases with polylithionite content, whereas the basal oxygen out-of-plane tilting, Delta z, is sensiti ve both to octahedral composition and degree of order (0.0 less than or equ al to Delta z less than or equal to 0.009 Angstrom for siderophyllite and a nnite, 0.058 less than or equal to Delta z less than or equal to 0.144 Angs trom for lithian siderophyllite and ferroan polylithionite crystals).