High-T phase transition of synthetic Na-A(B)(LiMg)(Mg5Si8O22)-Mg-C(OH)(2) amphibole: an X-ray synchrotron powder diffraction and FTIR spectroscopic study

The synthetic amphibole Na-0.95(Li0.95Mg1.05)Mg5Si8O22(OH)(2) was studied in situ at high-T, using IR OH-stretching spectroscopy and synchrotron X-ray powder diffraction. At room-T the sample has P2(1) /m symmetry, as shown by the FTIR spectrum. It shows in the OH region two well-defined and intense absorptions at 3,748 and 3,712 cm(-1), respectively, and two minor bands at 3,667 and 3,687 cm(-1). The main bands are assigned to the two independent O-H groups in the primitive structure. The two minor bands evidencing the presence of small amount of vacant A-site ((A)square(0.05)). With increasing T, these bands shift continuously and merge into a unique absorption at high temperature. A change as a function of increasing T is revealed by the evolution of the refined unit-cell parameters, whose trend shows a transition to C2/m at about 320-330 degrees C. The spontaneous scalar strain, fitted with a tricritical 2-6 Landau potential, gives a T-c of 325(10)degrees C (beta parameter = 0.27). Comparison with the second-order P2(1)/m double left right arrow C2/m phase transition at 255 degrees C for synthetic amphibole Na-A(0.8)B(Na0.8Mg1.2)(Mg5Si8O22)-Mg-C(OH)(2) indicates that the substitution of Na with Li at the B-sites strongly affects the thermodynamic character and the T-c of the phase transition. The comparison of LNMSH amphiboles with cummingtonitic ones shows that the high-T thermodynamic behaviour is affected by A-site occupancy.