Refinement of the crystal structure of zinnwaldite 2M1
Rieder, Milan; Hybler, Jiři; Smrčok, L'ubomir; Weiss, Zdeněk
European Journal of Mineralogy Volume 8 Number 6 (1997), p. 1241 - 1248
published: Jan 8, 1997
manuscript accepted: Jul 16, 1996
manuscript received: Jun 3, 1996
ArtNo. ESP147050806002, Price: 29.00 €
Abstract Crystal structure of zinnwaldite 2M1 was refined in the Cc space group with R equal to 5.8% and Rw 7.5%. Its octahedral sheet is hetero-octahedral, with octahedra around Ml and M3 having the same mean cation-anion bond length, whereas the third (around M2) has the mean bond significantly shorter. In terms of electrons, the cation ordering between Ml and M3 is less pronounced (16.2 e- vs 17.4 e-) than that between M1 (M3) and M2 (14.3 e-). Li and Fe2+ prefer M1 (M3) to M2, Al does the opposite. Thus, like zinnwaldite \M (Guggenheim & Bailey, 1977), the 2M1 polytype is hetero-octahedral. This is the first two-layer heterooctahedral mica polytype known to date. The degree of tetrahedral cation ordering is very low in both zinnwaldites. Also, distortion characteristics of individual octahedra and tetrahedra in these zinnwaldites are very close to each other. A comparison of the details of structures of 1M and 2M1, biotite and 1M and 2M1 zinnwaldite gave no indication that a geometrical or other advantage is associated with either 2M1 or 1M stacking in zinnwaldite.