Abstract
Structure variation with octahedral cation substitution in manganoan germanate micas KMg3−xMnxGe3AlO10F2 has been examined by refining the structures of two specimens with Mg/Mn ratios of about 1/0.3 and 1/2. The crystals are monoclinic with the space group C2⁄m. The cell dimensions are a=5.435(1), b=9.413(2), c=10.458(3) Å and β=100.03(3)° in the Mg-rich specimen and a=5.489(1), b=9.509(1), c=10.462(3) Å and β=100.12(2)° in the Mn-rich one. The stuctures were refined with the full-matrix least-squares procedure to the final R values of 0.039 for 1528 observed reflection data of the Mg-rich specimen and 0.050 for 1413 reflection data of the other. The mean Ge,Al-O distance in (Ge,Al)O4 tetrahedra is 1.746 Å in the former specimen, while 1.749 Å in the latter. Octahedral cations are randomly distributed over the two independent octahedral sites in each specimen. The tetrahedral rotation angles are 14.9° and 13.2° in the Mg-rich and Mn-rich ones, respectively. The thickness of the octahedral sheet increases approximately linearly with the degree of octahedral substitution and that of the tetrahedral sheet remains almost constant, while the interlayer thickness shows a discontinuous change.
