The crystal structure of a germanate mica, KMg₃Ge₃AlO₁₀F₂

Abstract

Crystals of KMg₃Ge₃AlO₁₀F₂ were synthesized from a mixture of KF, MgF₂, MgO, Al₂O₃ and GeO₂ by heating at 133°C and subsequent slow cooling. They have a structure of the 1M type mica with cell dimensions of a=5.417(6), b=9.345(5), c=10.468(1)Å and β=100.03(3)°. The structure was refined with the full-matrix least-squares procedure to the final R value of 0.037 for 1365 observed reflections. The (Ge, Al)–O_apical distance is 1.741 Å and the mean (Ge, Al)–O_basal distance is 1.744Å. Both of M(1)O₄F₂ and M(2)O₄F₂ octahedra are considerably flattened, having the octahedral flattening angle of 60.2°. The ditrigonal distortion of the tetrahedral sheet is much larger than that of the corresponding aluminosilicate mica, giving the tetrahedral rotation angle of 15.9°.