During the last years, a series of experimental studies have been done to characterize the structural features of synthetic amphiboles in the Li<sub>2</sub>O-Na<sub>2</sub>O-MgO-SiO<sub>2</sub>-H<sub>2</sub>O (LNMSH system) under different <i>T</i>, <i>P</i> and X conditions. Camára et al. (2003) and Iezzi et al. (2004) showed that the Li-free Na<sub>0.8</sub>(Na<sub>0.8</sub>Mg<sub>1.2</sub>)Mg<sub>5</sub>Si<sub>8</sub>O<sub>22</sub>(OH)<sub>2</sub> compound is <i>P</i>2<sub>1</sub>/<i>m</i> at room-<i>T</i>, and Camára et al. (2003) characterized the displacive <i>P</i>2<sub>1</sub>/<i>m</i>  <i>C</i>2/<i>m</i> phase-transition by single-crystal XRD. The phase transition of this amphibole has a macroscopic second order character, with a transition temperature (<i>T<sub>C</sub></i>) of 257 °C. Iezzi et al. (2005) did a similar investigation on the Na(LiMg)Mg<sub>5</sub>Si<sub>8</sub>O<sub>22</sub>(OH)<sub>2</sub> composition by means of synchrotron radiation H<i>T</i>-XRPD, and showed that this compound also undergoes a displacive phase transition as a function of <i>T</i>, but the transition has a tricritical character and <i>T<sub>C</sub></i> = 326 °C. Furthermore Iezzi et al. (2006) showed that all amphiboles in the LNMSH system, independently of their Na-Li contents at the B site, have the <i>P</i>2<sub>1</sub>/<i>m</i> symmetry at room conditions. Natural Group 5 amphiboles which belong to the <sup>B</sup>(Na,Li)<sub>2</sub>, <sup>B</sup>(Na,Mn)<sub>2</sub> and <sup>B</sup>(Ca,Mn)<sub>2</sub> solid-solutions, whether their A site is filled or empty, have monoclinic <i>C</i>2/<i>m</i> symmetry. In contrast, the A-site filled synthetic samples belonging to the <sup>B</sup>(Na,Mg)<sub>2</sub> and <sup>B</sup>(Li,Mg)<sub>2</sub> solid-solutions, have <i>P</i>2<sub>1</sub>/<i>m</i> symmetry at room <i>T</i>, and undergo phase-transitions at variable <i>T<sub>C</sub></i>, depending on their chemical composition (Welch et al. 2007). The <i>P</i>2<sub>1</sub>/<i>m</i> symmetry is restricted to amphiboles with a double-chain composition close to Si<sub>8</sub> and is probably stabilized by small cations only at the B-group sites or by a B compositions where large monovalent cation (Na) and a small divalent cation (Mg) have a ratio close to 1:1. However, the dimension of the C-group cations also play a significant role in determining the transition behavior (cf. Welch et al. 2007 for more discussion). In this work, we explore the effects on the phase transitions of a large divalent cation as Ca which progressively replaces <sup>B</sup>Mg. The amphibole symmetry was monitored using <i>in situ</i> low- and high-<i>T</i> FTIR spectroscopy. Samples were prepared as conventional KBr disks (Iezzi et al., 2006); low-<i>T</i> spectra were collected using a nitrogen-cooled cryostat, while high-<i>T</i> spectra were collected using a Specac microfurnace. Previous work (Iezzi et al., 2004, 2006) show that OH-stretching FTIR spectroscopy is particularly useful in characterizing the symmetry changes associated with the <i>P</i>  <i>C</i> transition in these amphiboles. For varying <i>T</i> conditions there are well-defined modifications of the room-<i>T</i> pattern, consisting of progressive band broadening and band splitting. Inspection of the data shows that for increasing <sup>B</sup>Ca in the amphibole structure there is a linear decrease of the transition temperature <i>T<sub>C</sub></i> for the Mg-richer samples, as also evidenced by the companion X-Ray diffraction study (Iezzi et al., this meeting). References Cámara F., Oberti R., Iezzi G., Della Ventura G. (2003) Phys. Chem. Miner., 30, 570-581. Della Ventura G., Iezzi G., Bellatreccia F., Càmara F., Oberti R. (2004). Mitt. Oster. Min. Ges., 149: 23. Iezzi G., Della Ventura G., Oberti R., Cámara F., Holtz F. (2004) Am. Miner., 89, 640–646. Iezzi G., Tribaudino M., Della Ventura G., Nestola G., Bellatreccia F. (2005) Phys. Chem. Miner., 32, 515–523. Iezzi G., Della Ventura G., Tribaudino M. (2006) Am. Miner., 91, 425-429. Welch M.D., Cámara F., Della Ventura G., Iezzi G. (2007) Rev. Mineral. Geochem., 67, in press.

DELLA VENTURA, G., Bellatreccia, F., Iezzi, G., Piccinini, M. (2007). The P-C phase transition of synthetic amphiboles in the NCMSH system: an FTIR study, 266-266.

The P-C phase transition of synthetic amphiboles in the NCMSH system: an FTIR study

DELLA VENTURA, Giancarlo;BELLATRECCIA, FABIO;
2007-01-01

Abstract

During the last years, a series of experimental studies have been done to characterize the structural features of synthetic amphiboles in the Li2O-Na2O-MgO-SiO2-H2O (LNMSH system) under different T, P and X conditions. Camára et al. (2003) and Iezzi et al. (2004) showed that the Li-free Na0.8(Na0.8Mg1.2)Mg5Si8O22(OH)2 compound is P21/m at room-T, and Camára et al. (2003) characterized the displacive P21/mC2/m phase-transition by single-crystal XRD. The phase transition of this amphibole has a macroscopic second order character, with a transition temperature (TC) of 257 °C. Iezzi et al. (2005) did a similar investigation on the Na(LiMg)Mg5Si8O22(OH)2 composition by means of synchrotron radiation HT-XRPD, and showed that this compound also undergoes a displacive phase transition as a function of T, but the transition has a tricritical character and TC = 326 °C. Furthermore Iezzi et al. (2006) showed that all amphiboles in the LNMSH system, independently of their Na-Li contents at the B site, have the P21/m symmetry at room conditions. Natural Group 5 amphiboles which belong to the B(Na,Li)2, B(Na,Mn)2 and B(Ca,Mn)2 solid-solutions, whether their A site is filled or empty, have monoclinic C2/m symmetry. In contrast, the A-site filled synthetic samples belonging to the B(Na,Mg)2 and B(Li,Mg)2 solid-solutions, have P21/m symmetry at room T, and undergo phase-transitions at variable TC, depending on their chemical composition (Welch et al. 2007). The P21/m symmetry is restricted to amphiboles with a double-chain composition close to Si8 and is probably stabilized by small cations only at the B-group sites or by a B compositions where large monovalent cation (Na) and a small divalent cation (Mg) have a ratio close to 1:1. However, the dimension of the C-group cations also play a significant role in determining the transition behavior (cf. Welch et al. 2007 for more discussion). In this work, we explore the effects on the phase transitions of a large divalent cation as Ca which progressively replaces BMg. The amphibole symmetry was monitored using in situ low- and high-T FTIR spectroscopy. Samples were prepared as conventional KBr disks (Iezzi et al., 2006); low-T spectra were collected using a nitrogen-cooled cryostat, while high-T spectra were collected using a Specac microfurnace. Previous work (Iezzi et al., 2004, 2006) show that OH-stretching FTIR spectroscopy is particularly useful in characterizing the symmetry changes associated with the PC transition in these amphiboles. For varying T conditions there are well-defined modifications of the room-T pattern, consisting of progressive band broadening and band splitting. Inspection of the data shows that for increasing BCa in the amphibole structure there is a linear decrease of the transition temperature TC for the Mg-richer samples, as also evidenced by the companion X-Ray diffraction study (Iezzi et al., this meeting). References Cámara F., Oberti R., Iezzi G., Della Ventura G. (2003) Phys. Chem. Miner., 30, 570-581. Della Ventura G., Iezzi G., Bellatreccia F., Càmara F., Oberti R. (2004). Mitt. Oster. Min. Ges., 149: 23. Iezzi G., Della Ventura G., Oberti R., Cámara F., Holtz F. (2004) Am. Miner., 89, 640–646. Iezzi G., Tribaudino M., Della Ventura G., Nestola G., Bellatreccia F. (2005) Phys. Chem. Miner., 32, 515–523. Iezzi G., Della Ventura G., Tribaudino M. (2006) Am. Miner., 91, 425-429. Welch M.D., Cámara F., Della Ventura G., Iezzi G. (2007) Rev. Mineral. Geochem., 67, in press.
2007
DELLA VENTURA, G., Bellatreccia, F., Iezzi, G., Piccinini, M. (2007). The P-C phase transition of synthetic amphiboles in the NCMSH system: an FTIR study, 266-266.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11590/175739
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