High-T phase transition of synthetic ANaB(LiMg)CMg5Si8O22(OH)2 amphibole: an X-ray synchrotron powder diffraction and FTIR spectroscopic study |
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Authors: | Gianluca Iezzi Mario Tribaudino Giancarlo Della Ventura Fabrizio Nestola Fabio Bellatreccia |
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Institution: | (1) Dipartimento Scienze della Terra, University G. D’Annunzio, Via Dei Vestini, 30, 66013 Chieti, Italy;(2) Dipartimento Scienze della Terra, University of Parma, Parma, Italy;(3) Dipartimento Scienze Geologiche, University of Rome TRE, Rome, Italy;(4) Bayerisches Geoinstitut, University of Bayreuth, Bayreuth, Germany |
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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 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□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°C. The spontaneous scalar strain, fitted with a tricritical 2–6 Landau potential, gives a T
c of 325(10)°C (β parameter = 0.27). Comparison with the second-order P21
/m ⇔ C2/m phase transition at 255°C for synthetic amphibole ANa0.8B(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 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. |
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Keywords: | Synthetic Na0 95(Li0 95Mg1 05)Mg5Si8O22(OH)2 amphibole HT-FTIR spectroscopy HT-XRPD Phase transition Thermodynamic behaviour |
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