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 miarolitic pegmatites that formed within host granitic rocks of the Precambrian, 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. Ordering of trivalent cations (mainly Al3+) is revealed in a cis-octahedral site (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 siderophyllite and ferroan polylithionite crystals). On the basis of mean bond length the ordering scheme of octahedral cations is mostly meso-octahedral, whereas the mean electron count at each M site suggests both meso- and hetero-octahedral ordering, the calculated mean atomic numbers being M1 = M3 not equal M2, M2 = M3 not equal M1 and M1 not equal M2 not equal M3. As the siderophyllite content increases, so do the a, b, and c unit-cell parameters, as well as the refractive indices, primarily np. The tetrahedral rotation angle, alpha, is generally small (1.51 less than or equal to alpha less than or equal to 5.04 degrees) and roughly increases with polylithionite content, whereas the basal oxygen out-of-plane tilting, Delta z, is sensitive both to octahedral composition and degree of order (0.0 less than or equal to Delta z less than or equal to 0.009 Angstrom for siderophyllite and annite, 0.058 less than or equal to Delta z less than or equal to 0.144 Angstrom for lithian siderophyllite and ferroan polylithionite crystals).