Cation site ordering in phengite 3T from the Dora-Maira massif (western Alps): A variable-temperature neutron powder diffraction study

Neutron powder diffraction data have been collected between 293 and 893 K on phengite 3T (space group P3₁12) from the Dora-Maira massif, using the high-resolution powder diffractometer HRPD at the ISIS Facility (UK). The complexity of the structure and the presence of some talc impurity allowed only Rietveld refinements softly constrained to previous X-ray bond lengths to be carried out. However, the contrast between Al, Mg and Si neutron scattering lengths enables conclusions in favour of octahedral and tetrahedral cation order to be made. The cell parameters expand linearly up to 893 K [α_a = 5.7(1), α_c = 21.4(5), α_v = 33.1(5) × 10^-6 K^-1] and no hysteresis has been observed; the thermal expansion values are compared with those of muscovite 2M₁ and discussed in terms of stability. The unsatisfactory charge balance usually observed in dioctahedral micas is discussed on the light of OH → O substitution, M(1) extra cations and interlayer cation deficiency.