The crystal structures of germanate micas, KMg2.5Ge4O10F2and KLiMg2Ge4O10F2

Abstract

The crystals of KMg2.5Ge4010F2 and KLiMg2Ge4O10F2 were synthesized from mixtures of KF, MgF2, MgO and Ge02, and of KF, LiF, MgO and Ge02, respectively. They have structures of the 1M type mica with cell dimensions of a = 5.421(2), b = 9.353(4), c= 10.533(2)Å and β= 100.14(4)° in KMg2.5Ge4O10F2 and a= 5.395(1), b=9.341(2), c = 10.547(1) Å and β = 99.87(2)° in KLiMg2Ge4O10F2. The structures were refined with full-matrix least-squares procedures (R = 0.055; 1413 hkl; KMg2.5Ge4O10F2 and R = 0.038; 1451 hkl; KLiMg2Ge4O10F2). The non-bridging Ge—O distance is 1.702 Å in the former and 1.704 Å in the latter crystals, whereas the mean bridging Ge—O distances are 1.757 Å in both crystals. In KMg2.5Ge4O10F2, the Mg2+ ions are distributed over the two independent octahedral sites with different populations, and the structure corresponds to that of dioctahedral micas. In KLiMg2Ge4O10F2, the Li+ and Mg2+ ions are distributed practically at random. The ditrigonal distortions of the tetrahedral layers are much larger than those of the corresponding silicates owing to the larger size of GeO4 tetrahedra having the tetrahedral rotation angles of 12.9° and 13.5° in KMg2.5Ge4O10F2 and KLiMg2Ge4O10F2, respectively.