2M1-phlogopite from Black Hills (South Australia):the first case of configurational polytype in micas

The 2M1-phlogopite from the potassic gabbronorite (Black Hill, Australia) has been studied by electron microprobe and single-crystal X‑ray diffraction analyses. The crystal-chemical formula was (K0.95Na0.01)(Al0.15Mg1.27Fe2+1.16Fe3+0.04Ti4+0.38)(Si2.85Al1.15)O10.76F0.11Cl0.03OH1.10. The structural analysis has shown that the crystal has the cell parameters a = 5.352(1), b = 9.268(1), c = 20.168(1) Å, and β = 95.10(1)° and exhibits symmetry lowering from the ideal space group C2/c to C1. An octahedral cation ordering pattern was revealed from the refined site-scattering powers. Specifically, using the scattering species Mg vs. Fe, it was found that the M1 site at z = 0 was occupied principally by Mg (~77%) and subordinately by Fe (~23%), whereas that at z = 0.5 was completely occupied by Fe; the M2 sites at z = 0 displayed ~88% Mg and ~12% Fe, whereas those at z = 0.5 were occupied by ~86% Fe and ~14% Mg. The analysis of geometrical features shows that the Ti uptake in the structure via the Ti-oxy mechanism induces structural distortions of different extents on the z = 0 and z = 0.5 layers, with stronger effects for the layer at z = 0. Minor chemical and structural differences, instead, affect the T sheets at z = 0 and z = 0.5.