Crystal-structure and FTIR spectroscopy of synthetic Na(NaMg)Mg5Si8O22(OH)2, a P21/m amphibole. (Abstract/Comunicazione in rivista)

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  • Crystal-structure and FTIR spectroscopy of synthetic Na(NaMg)Mg5Si8O22(OH)2, a P21/m amphibole. (Abstract/Comunicazione in rivista) (literal)
Anno
  • 2003-01-01T00:00:00+01:00 (literal)
Alternative label
  • G. Iezzi (1), F. Cámara (2), G. Della Ventura (3), R. Oberti (2), F. Holtz (4) (2003)
    Crystal-structure and FTIR spectroscopy of synthetic Na(NaMg)Mg5Si8O22(OH)2, a P21/m amphibole.
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  • G. Iezzi (1), F. Cámara (2), G. Della Ventura (3), R. Oberti (2), F. Holtz (4) (literal)
Pagina inizio
  • 11801 (literal)
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  • 5 (literal)
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  • 2 (literal)
Note
  • Abstract (literal)
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  • (1) Bayerisches Geoinstitut, Universität Bayreuth, (2) CNR-Istituto di Geoscienze e Georisorse, Pavia, (3) Dipartimento Scienze Geologiche, Università di Roma Tre, (4) Institut für Mineralogie, Universität Hannover (literal)
Titolo
  • Crystal-structure and FTIR spectroscopy of synthetic Na(NaMg)Mg5Si8O22(OH)2, a P21/m amphibole. (literal)
Abstract
  • Na(NaMg)Mg5Si8O22(OH)2 is an interesting amphibole composition never found in nature, which has been the object of much experimental work (Maresch and Langer, 1976; Raudsepp et al., 1991). It typically gives very high-yield run-products, consisting of extremely acicular and well formed crystals, generally less than 1 or 2 ?m wide. X-ray powder analysis suggested various symmetries and space groups. The IR spectrum in the OH region consists of two major absorbances: a single well-resolved and rather sharp band at 3714 cm-1, and a broader band, possibly consisting of three overlapping components, centred at 3743 cm-1. No clear explanations for such a pattern have been provided so far. We prepared four samples, using hydrothermal techniques, at the following T and P conditions: 750, 800 and 850C and 0.4 GPa and 900C and 0.5 GPa. Runs at 800C and 900C yielded crystals suitable for single-crystal X-ray data collection. All samples have P21/m symmetry, similarly to what found by Oberti et al. (2002) for a similar composition with excess water synthesised by W. Maresch. This symmetry implies non equivalence of the two double-chains of tetrahedra facing base-to-base, which is mainly expressed by different degrees of stretching and kinking. There are also two independent anion sites (O3A and O3B) which may be occupied by OH. This feature explains the IR pattern. The band at 3743 cm-1 is assigned to the shorter O3B-H2 bond, whereas the 3715 cm-1 is assigned to the longer O3A-H1 bond. In addition, the broader shape of the higher-frequency, 3743 cm-1 band, suggests a stronger interaction of the O3B-H2 dipole with ANa, which is confirmed by structure refinement. Increasing T of syntheses implies progressive departure from the ideal stoichiometry via the BMgANa-1 exchange, which has been characterised by EMPA, structure refinement and FTIR spectroscopy. The latter in particular can now be used to detect unusual symmetries in X-ray powder patterns of synthetic amphiboles. Maresch, W.V, Langer, K. (1976). Contrib. Mineral. Petrol., 56, 27-34. Raudsepp, M., Turnock, A.C., Hawthorne, F.C. (1991). Eur. J. Mineral., 3, 983-1004. Oberti, R., Ottolini, L., Della Ventura, G., Prella, D. (2000). Plinius, 24, 157. (literal)
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