The refinement of zinnwaldite-1M in subgroup symmetry

The structure of a zinnwaldite-1М from the Sadisdorf Mine, D.D.R., has been refined in C2/m, C2, and Cl symmetries. The composition is (K_0.90Na_0.05) (Al_1.05Fe_0.16³⁺Ti_0.01Fe_0.77²⁺Mn_0.05Mg_0.01Li_0.67)(Si_3.09Al_0.91)O₁₀(OH)_0.79F_1.21. The cell parameters are a = 5.296, b = 9.140, с = 10.096 A, and β = 100.83°. Refinement in triclinic symmetry Cl using the ordered-model approach gave atomic coordinates consistent with monoclinic C2 symmetry. In C2 symmetry, octahedra M(2) and M(3) related by the pseudo-mirror plane are significantly different in size (mean M–O,F = 1.882,2.131 A) and, to a lesser extent, in electron count (11.5 and 13.5). Octahedral Al completely occupies M(2), and the remaining Fe, Li, other cations, and vacancies are nearly randomly distributed over M(3) and the trans octahedron M(1). The (F,OH) atom has moved off the pseudo-mirror plane in order to coordinate more closely with Al in M(2). It is anticipated that all fluorine-rich zinnwaldites and lepidolites will have a similar ordering pattern. The cis-orientation of fluorine along the M(2):M(3) shared edge allows a closer F–F approach around a small cation in either site than is possible along any M(1) shared edge involving larger oxygen atoms. The larger M(1) and M(3) octahedra are flattened considerably (ψ = 60.8°) in order to fit onto the smaller, more regular (ψ = 56.5°) M(2) octahedron. The two nonequivalent tetrahedra differ slightly in size (mean T–О = 1.646,1.639 A), are elongate, and rotated by 5.8°.