Crystal structure and chemistry of lithium-bearing trioctahedral micas-3T

Authors: Brigatti, Maria Franca; Kile, Daniel E.; Poppi, Luciano

Source: European Journal of Mineralogy, Volume 15, Number 2, 1 April 2003 , pp. 349-355(7)

Publisher: E. Schweizerbart'sche Verlagsbuchhandlung

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Abstract:

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 P3₁12 (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 orderedcation distribution with M1 and M3 positions substantially larger than M2. In the sample with the largest iron content, the M2 mean electron count increases aswell 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 Å and from 1.663 to 1.678 Å for T1 and T2 sites, respectively, being consistent with Al³⁺ enrichment in the T2 sites. The tetrahedral rotation angle, α, is generally small (3.1 ≤ α ≤ 4.6°) and decreases with siderophyllite content. As Fe increases, the T1 tetrahedron becomes flatter (112.4 ≤ τ_T1 ≤ 110.5°), whereas T2 tetrahedron distortion appears unchanged (110.7 ≤ τ_T2 ≤ 110.9).

Keywords: LITHIAN SIDEROPHYLLITE; CRYSTAL STRUCTURE; CHEMISTRY; OCTAHEDRAL OCCUPANCY

Document Type: Research article

DOI: http://dx.doi.org/10.1127/0935-1221/2003/0015-0349

Publication date: 2003-04-01

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