Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Ventura, G. D. Articles by Cámara, F. Search for Related Content GeoRef GeoRef Citation

The P2₁/m-C2/m phase transition in synthetic amphiboles in the system Li₂O–Na₂O–MgO–SiO₂–H₂O: a high-T FTIR study

¹ Dipartimento di Scienze Geologiche, Università Roma Tre, Largo S. Leonardo Murialdo 1, 00146, Roma, Italy
² Dipartimento di Scienze della Terra, Università "G. D’Annunzio", 66013 Chieti Scalo, Italy
³ Department of Seismology and Tectonophysics, Istituto Nazionale di Geofisica e Vulcanologia, Roma, 00100 Italy
⁴ CNR-Istituto di Geoscienze e Georisorse, unità di Pavia, via Ferrata 1, 27100 Pavia, Italy

^* Corresponding author, e-mail: dellaven{at}uniroma3.it

The P2₁/m C2/m phase-transition has been studied by high-T FTIR analysis on a series of synthetic amphiboles in the Li₂O–Na₂O–MgO–SiO₂–H₂O (LNMSH) system. Spectra were collected in the T range 25–450 °C on KBr disks. All examined amphiboles have P2₁/m symmetry at room T. Their OH-stretching FTIR spectrum consists of two main bands at ~3740 and 3715 cm^–1. At the transition temperature (T_c), these bands merge into one single absorption centred at ~3720 cm^–1, and no further change is observed beyond this T. Significant modifications consisting in peak shifting and band broadening are also observed in the MIR (medium infrared) 1300–640 cm^–1 region. T_c values for the different compositions were estimated based on various methods; the most reliable procedure is considered to be the fit of Landau 2-4-6 potentials using band shifts observed in the MIR region. The T_c values obtained for all samples are consistent with previous results obtained on two members of the series examined here by single-crystal or synchrotron powder HT-XRD (high-T X-ray diffraction). They correlate linearly with the aggregate cation radius at M(4) [T_c (°C) = 803–533 r_M(4); R2 = 0.97]. This work thus provides a measure of the role played by the size of the M(4) polyhedron in determining the T_c in simple chemical systems where the B-site occupancy (and geometry) is the only variable. The slope of the equation is far less steep in the LNMSH system than in cummingtonite; crystal-chemical reasons for this behaviour are discussed, and the local order between A and monovalent B cations is suggested to be the major constraint. In more complex systems, inspection of the available data shows that other factors such as the aggregate size of the strip of octahedra must be taken into account.

Key-words: synthetic amphiboles, LNMSH system, HT-FTIR spectroscopy, phase transition, lithium.