Tetrahedral order in phengite 2M (sub 1) upon heating, from powder neutron diffraction, and thermodynamic consequences

A neutron diffraction study of Fe-rich phengite from Valle Passiria in the E Italian Alps, with composition (K (sub 0.94) Na (sub 0.03) )(Al (sub 1.67) Mg (sub 0.04) Fe (sub 0.34) )(Si (sub 3.26) Al (sub 0.74) )O (sub 10) (OH (sub 0.94) Fe (sub 0.06) ), is reported at 293, 473, 673 and 873 K, and also at 293 K after the high-T measurements, to investigate hysteris effects. Occupancy refinements show that the tetrahedral cation partitioning is disordered at room T, whereas Al fully orders into the T1 site at 873 K, suggesting that the energetic contributions are dominant with respect to the configurational ones. A qualitative interpretation of the causes favouring the formation of the 2M (sub 1) polytype rather than 3T is presented. The thermal expansion coefficients 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 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 to a different T-partitioning with respect to that determined before the thermal treatment.