Refinement of the crystal structures of two "protolithionites"
Weiss, Zdeněk; Rieder, Milan; Smrčok, L'ubomír; Petřiček, Vaclav; Bailey, Sturges W.
European Journal of Mineralogy Volume 5 Number 3 (1993), p. 493 - 502
published: Jun 14, 1993
manuscript accepted: Dec 14, 1992
manuscript received: May 4, 1992
ArtNo. ESP147050503003, Price: 29.00 €
Abstract Crystal structures of "protolithionites" 1M and 3T were refined in the Cllm and P3112 space groups with R equal to 3.8 and 3.0 % and Rw 5.4 and 3.2 %, respectively. Refinement of the 1M structure in subgroup symmetry (C2) did not result in a significant improvement of fit and was not accepted. The 1M mica has a mesooctahedral cation ordering (two octahedra identical), similar to that of muscovite but with less pronounced differences in size between Ml and M2 ; Li and Fe2+ prefer Ml to M2, Al does the opposite. Like all 3T structures refined to date, the "protolithionite" 3T has a heterooctahedral ordering rather similar to that found in "protolithionite" 3T of Pavlishin et al. (1981) and zinnwaldite 1M of Guggenheim & Bailey (1977). The two large (though not identical) octahedra M3, Ml contain all Li and most Fe, the small (M2) octahedron is occupied with Al. This type of ordering is apparently suitable for a relatively wide range of compositions of Li-Fe micas; the lepidolites are not included, and neither are iron-rich compositions. "Protolithionite" 1M has no pronounced octahedral ordering and all octahedra are nearly the same in size.