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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)
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- 2003-01-01T00:00:00+01:00 (literal)
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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)
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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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