In the last few years a series of experimental studies has renewed the interest in the Na (NaMg) Mg5 Si8 O22(OH)2 amphibole end-member, studied long ago by Gibbs et al. (1962) and Maresch and Langer (1976). Na (NaMg) Mg5 Si8 O22(OH)2 has the P21/m space group at room temperature (Iezzi et al., 2004), and undergoes a displacive P21/m ? C2/m phase-transition at 257 °C (Camára et al., 2003). The room-T OH-stretching IR spectrum of Na (NaMg) Mg5 Si8 O22(OH)2 is characterised by two distinct groups of bands, which can be assigned to the two non-equivalent O-H dipoles in the P21/m structure; this implies that the IR OH-spectrum of these amphiboles is a powerful tool for a fast detection of P21/m symmetry. We relate here a powder high-T FTIR study of amphiboles synthesised in the Na2O-Li2O-MgO-SiO2-H2O system. Spectra were collected in the T range 25-380 °C on KBr disks, using a Linkam THS600 heating stage mounted on a NicPlan microscope at the University of Roma Tre. At 800 °C and 0.4 GPa, the BNa ? BLi exchange is complete along the Na (NaMg) Mg5 Si8 O22(OH)2 - Na (LiMg) Mg5 Si8 O22(OH)2 join. Both XRD and FTIR spectroscopy show that all solid-solution terms have P21/m symmetry at room-T. In particular, the IR spectra consist of two main bands around 3740 and 3715 cm-1, the exact position of which being a function of BLi (NNN effect), and two minor bands at 3690 and 3667 cm-1. At the transition temperature, the two main bands merge into one single absorption around 3720 cm-1. No significant change is observed beyond this T. The behaviour of the IR OH-stretching bands is in agreement with the second-order character of the P21/m ? C2/m phase-transition, which was obtained by single-crystal XRD studies of Na (NaMg) Mg5 Si8 O22(OH)2. The IR spectra show that the TC of the phase-transition is linearly dependent on the amphibole composition: for BLi = 0 apfu it is close to 250 °C (in agreement with Cámara et al., 2003), whereas for BLi = 1.0 apfu it is close to 340 °C; this latter value is coherent with preliminary powder-XRD synchrotron data on the same sample. Therefore, in this system where the composition of the A and C-group sites is virtually constant, the P21/m ? C2/m phase-transition is determined by the aggregate mean ionic radius at the B-group sites. REFERENCES GIBBS, G.V., MILLER, J.L. & SHELL, H.R. (1962): Am. Mineral., 47: 75-82. MARESCH, W.V & LANGER, K. (1976): Contrib. Mineral. Petrol.: 56, 27-34. IEZZI, G., DELLA VENTURA, G., CAMARA, F., OBERTI, R. & HOLTZ, F. (2004): Am. Mineral., 89, 640-646. CAMARA, F., OBERTI, R., IEZZI, G. & DELLA VENTURA. G. (2003): Phys. Chem. Miner., 30: 570-581.
The P21/m ? C2/m phase-transition of synthetic amphiboles in the system Na2O-Li2O-MgO-SiO2-H2O: an HT-FTIR study.
Oberti;
2004
Abstract
In the last few years a series of experimental studies has renewed the interest in the Na (NaMg) Mg5 Si8 O22(OH)2 amphibole end-member, studied long ago by Gibbs et al. (1962) and Maresch and Langer (1976). Na (NaMg) Mg5 Si8 O22(OH)2 has the P21/m space group at room temperature (Iezzi et al., 2004), and undergoes a displacive P21/m ? C2/m phase-transition at 257 °C (Camára et al., 2003). The room-T OH-stretching IR spectrum of Na (NaMg) Mg5 Si8 O22(OH)2 is characterised by two distinct groups of bands, which can be assigned to the two non-equivalent O-H dipoles in the P21/m structure; this implies that the IR OH-spectrum of these amphiboles is a powerful tool for a fast detection of P21/m symmetry. We relate here a powder high-T FTIR study of amphiboles synthesised in the Na2O-Li2O-MgO-SiO2-H2O system. Spectra were collected in the T range 25-380 °C on KBr disks, using a Linkam THS600 heating stage mounted on a NicPlan microscope at the University of Roma Tre. At 800 °C and 0.4 GPa, the BNa ? BLi exchange is complete along the Na (NaMg) Mg5 Si8 O22(OH)2 - Na (LiMg) Mg5 Si8 O22(OH)2 join. Both XRD and FTIR spectroscopy show that all solid-solution terms have P21/m symmetry at room-T. In particular, the IR spectra consist of two main bands around 3740 and 3715 cm-1, the exact position of which being a function of BLi (NNN effect), and two minor bands at 3690 and 3667 cm-1. At the transition temperature, the two main bands merge into one single absorption around 3720 cm-1. No significant change is observed beyond this T. The behaviour of the IR OH-stretching bands is in agreement with the second-order character of the P21/m ? C2/m phase-transition, which was obtained by single-crystal XRD studies of Na (NaMg) Mg5 Si8 O22(OH)2. The IR spectra show that the TC of the phase-transition is linearly dependent on the amphibole composition: for BLi = 0 apfu it is close to 250 °C (in agreement with Cámara et al., 2003), whereas for BLi = 1.0 apfu it is close to 340 °C; this latter value is coherent with preliminary powder-XRD synchrotron data on the same sample. Therefore, in this system where the composition of the A and C-group sites is virtually constant, the P21/m ? C2/m phase-transition is determined by the aggregate mean ionic radius at the B-group sites. REFERENCES GIBBS, G.V., MILLER, J.L. & SHELL, H.R. (1962): Am. Mineral., 47: 75-82. MARESCH, W.V & LANGER, K. (1976): Contrib. Mineral. Petrol.: 56, 27-34. IEZZI, G., DELLA VENTURA, G., CAMARA, F., OBERTI, R. & HOLTZ, F. (2004): Am. Mineral., 89, 640-646. CAMARA, F., OBERTI, R., IEZZI, G. & DELLA VENTURA. G. (2003): Phys. Chem. Miner., 30: 570-581.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
