Tetrahedral order in phengite 2M1 upon heating, from powder neutron diffraction, and thermodynamic consequences

Fe-rich phengite 2M1 from Valle Passiria (Italian Eastern Alps,), with chemical composition (K0.94Na0.03)(Al1.67Mg0.04Fe0.34)(Si3.26Al0.74)O10 (OH0.94, F0.06)2 has been investigated using neutron powder diffraction on the high-resolution diffractometer (HRPD) at the ISIS Facility, UK. Data were recorded at 293, 473, 673 and 873 K and again at 293 K, after the high-temperature measurements, in order to investigate hysteresis effects. Occupancy refinements show that the tetrahedral cation partitioning is disordered at room temperature, whereas Al fully orders into the T1 site at 873 K. This result, in keeping with that from mean T-O bond length analysis, is interpreted on a thermodynamic basis, and suggests that the energetic contributions are dominant with respect to the configurational ones. A qualitative interpretation of the causes favouring the formation of the polytype 2M1 rather than 3T is presented. The thermal expansion coefficients [α(a) = 8.9(2) α(b) = 8.0(2) α(c) = 16.6(1) α(V) = 34.3(2) 10-6 K-1] are discussed in the light of bonding, and compared with those obtained for phengite 3T and muscovite. The role of structural and symmetry differences in polytypes are pointed out. After the heating cycle, the occurrence of hysteresis effects relating to the b lattice parameter was observed. These are ascribed to residual tetrahedral rotation through which stresses due to the misfit of octahedral/tetrahedral sheets can be relieved, and/or due to a different T-partitioning with respect to that determined prior to the thermal treatment.