Cation partitioning and substitutional mechanism in 1 M phlogopite: a crystal chemical study

The crystal chemistry of IM phlogopite was studied by single-crystal X-ray diffraction and microprobe analysis. Structural refinement was carried out in space group C2/m (R values between 0.021 and 0.052) on 24 samples from basic lamproites, leucitites, kimberlite, and carbonatites. The geometric and chemical features of the octahedral sites show the preferential partitioning of high-charge cations (Ti4+, Al3+, Cr3+, Fe3+) in the cis M2 sites. Bond-length and edge-length distortion parameters show the remarkably different behavior of the two octahedral sites when the high-charge cation content increases. The external shape of the trans MI polyhedron is heavily constrained by the average field strength in the M2 sites. In Ti-rich phlogopite the central cations of M2 sites show a significant off-center shift toward the 04 position. The substitution mechanism [6](R2+) + 2(OH)- = [6](Ti4+) + 2O2- best explains the observed structural modifications. The loss of a proton linked to 04 is mainly recognized from the shortening of the c lattice dimension. The consequences of octahedral cation partitioning and structural adjustments related to the Ti substitution may be important when dealing with the thermal stability field of phlogopite and mica-based geothermometry or geobarometry and can help in understanding the dependence of Ti solubility on temperature, f(O2), and a(H2O)