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Synthesis, crystal structure and crystal chemistry of ferri-clinoholmquistite, Li 2Mg 3Fe 3+ 2Si 8O 22(OH) 2

Abstract : This work reports the synthesis of ferri-clinoholmquistite, nominally Li2(Mg3Fe3+2)Si8O22(OH)2, at varying fO2 conditions. Amphibole compositions were characterized by X-ray (powder and single-crystal) diffraction, microchemical (EMPA) and spectroscopic (FTIR, Mössbauer and Raman) techniques. Under reducing conditions ( NNO+1, where NNO = Nickel–Nickel oxide buffer), the amphibole yield is very high (>90%), but its composition, and in particular the FeO/Fe2O3 ratio, departs significantly from the nominal one. Under oxidizing conditions ( NNO+1.5), the amphibole yield is much lower (<60%, with Li-pyroxene abundant), but its composition is close to the ideal stoichiometry. The exchange vector of relevance for the studied system is M2(Mg,Fe2+) M4(Mg,Fe2+) M2Fe3+–1 M4Li–1, which is still rather unexplored in natural systems. Amphibole crystals of suitable size for structure refinement were obtained only at 800 °C, 0.4 GPa and NNO conditions (sample 152), and have C2/m symmetry. The X-ray powder patterns for all other samples were indexed in the same symmetry; the amphibole closest to ideal composition has a = 9.428(1) Å, b = 17.878(3) Å, c = 5.282(1) Å, = 102.06(2)°, V = 870.8(3) Å3. Mössbauer spectra show that Fe3+ is strongly ordered at M2 in all samples, whereas Fe2+ is disordered over the B and C sites. FTIR analysis shows that the amount of CFe2+ increases for increasingly reducing conditions. FTIR data also provide strong evidence for slight but significant amounts of Li at the A sites.
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https://hal-insu.archives-ouvertes.fr/hal-00091035
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Submitted on : Tuesday, September 5, 2006 - 10:12:53 AM
Last modification on : Thursday, June 18, 2020 - 12:46:02 PM

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Gianluca Iezzi, Fernando Camara, Giancarlo Della Ventura, R. Oberti, G. Pedrazzi, et al.. Synthesis, crystal structure and crystal chemistry of ferri-clinoholmquistite, Li 2Mg 3Fe 3+ 2Si 8O 22(OH) 2. Physics and Chemistry of Minerals, Springer Verlag, 2004, 31, pp.6, 375-385. ⟨10.1007/s00269-004-0402-2⟩. ⟨hal-00091035⟩

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