The crystal structure of a germanate mica KMg_2.5+xGe_4−2xAl_2xO₁₀F₂ (x≈0.14) and distortion of (Ge, Al)O₄ tetrahedra

Abstract

The system KMg_2.5Ge₄O₁₀F₂–KMg₃Ge₃AlO₁₀F₂ forms a complete solid solution. The analyzed specimen has a structure of the 1 M mica with cell dimensions of a=5.413(1), b=9.368(2), c=10.531(6)Å and β=99.88(5)° and the experimental composition of KMg_2.64Ge_3.72Al_0.28O₁₀F₂. The structure was refined with the full-matrix least-squares procedure to the final R value of 0.042 for 1568 observed reflections. The structural features are the intermediate between the two opposite end members. The Mg²⁺ ions and vacancies are partially ordered in two independent octahedral sites. The distortion of the (Ge, Al)O₄ tetrahedra has been discussed in comparison with that of the (Si, Al)O₄ in silicate micas.