Crystal structure and chemistry of lithium-containing trioctahedral micas-3T.

Chemical analyses and crystal structure refinements were performed on lithian siderophyllite-3T crystals from granitic pegmatites of the anorogenic Pikes Peak batholith (Colorado) to characterize the crystal chemistry and relations with trioctahedral lithium-bearing micas showing different stacking sequences. Chemical data show that the studied samples fall on the siderophyllite-polylithionite join, closer to the siderophyllite end-member. Single-crystal X-ray refinements were carried out on three samples (two of which were taken from core and rim of the same crystal) in space-group P3(1)12 (the agreement factor, R-obs, varies between 0.034 and 0.036). Mean bond distances and mean electron counts of M1, M2 and M3 octahedral sites indicate an ordered cation distribution with M1 and M3 positions substantially larger than M2. In the sample with the largest iron content, the M2 mean electron count increases as well as the <M2-O> mean distance, whereas <M2-O> remains smaller than <M1-O> or <M3-O>. The tetrahedral cation-oxygen atom mean distances range from 1.614 to 1.638 Angstrom and from 1.663 to 1.678 Angstrom for T1 and T2 sites, respectively, being consistent with Al3+ enrichment in the T2 sites. The tetrahedral rotation angle, a, is generally small (3.1 less than or equal to alpha less than or equal to 4.6) and decreases with siderophyllite content. As Fe increases, the T1 tetrahedron becomes flatter (112.4 less than or equal to tau(T1) less than or equal to 110.5), whereas T2 tetrahedron distortion appears unchanged (110.7 less than or equal to tau(T2) less than or equal to 110.9).