The synthetic amphibole Na0.95(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 P21/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 cm1, respectively, and two minor bands at 3,667 and 3,687 cm1. 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 (Ah0.05). 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 320–330C. The spontaneous scalar strain, fitted with a tricritical 2–6 Landau potential, gives a Tc of 325(10)C (b parameter = 0.27). Comparison with the second-order P21/m , C2/m phase transition at 255C for synthetic amphibole ANa0.8 B (Na0.8Mg1.2)CMg5Si8O22(OH)2 indicates that the substitution of Na with Li at the B-sites strongly affects the thermodynamic character and the Tc 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.
High-T phase transition of ANaB(LiMg)CMg5Si8O22(OH)2 amphibole: an X-ray synchrotron powder diffraction and FTIR spectroscopic study.
IEZZI, Gianluca;
2005-01-01
Abstract
The synthetic amphibole Na0.95(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 P21/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 cm1, respectively, and two minor bands at 3,667 and 3,687 cm1. 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 (Ah0.05). 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 320–330C. The spontaneous scalar strain, fitted with a tricritical 2–6 Landau potential, gives a Tc of 325(10)C (b parameter = 0.27). Comparison with the second-order P21/m , C2/m phase transition at 255C for synthetic amphibole ANa0.8 B (Na0.8Mg1.2)CMg5Si8O22(OH)2 indicates that the substitution of Na with Li at the B-sites strongly affects the thermodynamic character and the Tc 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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
