Refinement of the crystal structure of zinnwaldite 2M1

Abstract

Crystal structure of zinnwaldite 2M(1) was refined in the Cc space group with R equal to 5.8% and R(w) 7.5%. Its octahedral sheet is hetero-octahedral, with octahedra around M1 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 M1 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 1M (Guggenheim & Bailey, 1977), the 2M(1) polytype is hetero-octahedral. This is the first two-layer hetero-octahedral 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 2M(1) biotite and 1M and 2M(1) zinnwaldite gave no indication that a geometrical or other advantage is associated with either 2M(1) or 1M stacking in zinnwaldite.