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

Abstract

The 2 M 1 -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 (K 0.95 Na 0.01 )(Al 0.15 Mg 1.27 Fe 2+ 1.16 Fe 3+ 0.04 Ti 4+ 0.38 )(Si 2.85 Al 1.15 )O 10.76 F 0.11 Cl 0.03 OH 1.10 . The structural analysis has shown that the crystal has the cell parameters a = 5.352(1), b = 9.268(1), c = 20.168(1) A, and β = 95.10(1)° and exhibits symmetry lowering from the ideal space group C 2/ c to C 1. 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.