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

Abstract

The crystal structure and M-site populations of a series of micas-1 M 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 ≤ Li ≤ 2.82, 0.90 ≤ Fe total ≤ 5.00, 0.26 ≤ [6] Al ≤ 2.23 apfu. Ordering of trivalent cations (mainly Al 3+ ) is revealed in a cis-octahedral site (M2 or M3), which leads to a lowering of the layer symmetry from C 12 /m (1) (siderophyllite and annite crystals) to C 12(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 ≠ M2, M2 = M3 ≠ M1 and M1 ≠ M2 ≠ M3. As the siderophyllite content increases, so do the a , b, and c unit-cell parameters, as well as the refractive indices, primarily n β . The tetrahedral rotation angle, α, is generally small (1.51 ≤ α ≤ 5.04°) and roughly increases with polylithionite content, whereas the basal oxygen out-of-plane tilting, Δz, is sensitive both to octahedral composition and degree of order (0.0 ≤ Δz ≤ 0.009 A for siderophyllite and annite, 0.058 ≤ Δz ≤0.144 A for lithian siderophyllite and ferroan polylithionite crystals).