A Mössbauer and FTIR study of synthetic amphiboles along the magnesioriebeckite – ferri-clinoholmquistite join

• Published in: Physics and chemistry of minerals 2005-06, Vol.32 (2), p.103-113
• Main Authors: Della Ventura, Giancarlo, Redhammer, Günther J., Iezzi, Gianluca, Hawthorne, Frank C., Papin, Arnaud, Robert, Jean-Louis
• Format: Article
• Language: English
• Subjects: Amphiboles; Earth Sciences; Geochemistry; Infrared spectroscopy; Iron 57; Organic chemistry; Parameters; Quadrupoles; Sciences of the Universe; Solid solutions; Spectra; Splitting
• ISSN: 0342-1791; 1432-2021
• DOI: 10.1007/s00269-005-0451-1

A series of amphiboles along the magnesioriebeckite—□Na2Mg3Fe3+2Si8O22(OH)2– ferri-clinoholmquistite—□Li2Mg3Fe3+2Si8O22(OH)2 - join, defined by the BLiB Na−1 exchange vector, were hydrothermally synthesized at 700°C, 0.4 GPa, NNO + 1 redox conditions. Powder XRD and SEM-EDAX showed a very high (> 90%) amphibole yield for all samples. X-ray patterns were indexed in the C2/m space group; refined cell-parameters show a linear decrease of a and β as a function of chemistry. IR spectra in the OH-stretching region show four main and rather sharp bands; these are assigned to Mg and Fe2+ at M(1,3), and indicate that the obtained amphiboles depart from the nominal octahedral composition (M1,3Mg3). The IR spectra also show that there is an increasing filling-up of the A-site for increasing Na in the system (increasing solid-solution toward, arfvedsonite). Mössbauer spectra show four well-defined quadrupole doublets which are assigned to Fe3+ at M2 and to Fe2+ at M1, M3 and M4, respectively. The Fe3+/Fe2+ content derived from fitted peak areas show variable Fe3+ concentration along the series. Mössbauer spectra also show a distinct alteration of 57Fe hyperfine parameters with changing Na–Li at M4. The most evident variation is observed for the quadrupole splitting of Fe3+ at M2, which increases by ≈50% from ferri-clinoholmquistite to magnesio-riebeckite; this suggest that the M2 octahedron in ferri-clinoholmquistite is much closer to the ideal geometry than the M2 octahedron in magnesio-riebeckite. Mössbauer spectra show also a well-defined increase in the Fe2+ quadrupole splitting of the M1 and M3 octahedra, which is attributed to the Na–Li distribution at the B-sites.