A series of amphiboles along the magnesioriebeckite - square Na2Mg3Fe3+ 2Si8O22(OH)(2)- ferri-clinoholmquistite - circle Li2Mg3Fe3+ 2Si8O22(OH)2 - join, defined by the Li-B(B) Na-1 exchange vector, were hydrothermally synthesized at 700 degrees 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 alpha and beta 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 (Mg-M1,3(3)). 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). Mossbauer 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. Mossbauer spectra also show a distinct alteration of Fe-57 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 approximate to 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. Mossbauer 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.
DELLA VENTURA, G., REDHAMMER G., J., Iezzi, G., HAWTHORNE F., C., Papin, A., Robert, J.L. (2005). A Mössbauer and FTIR study of synthetic amphiboles along the magnesioriebeckite – ferri-clinoholmquistite join. PHYSICS AND CHEMISTRY OF MINERALS, 32, 103-113 [10.1007/s00269-005-0451-1].
A Mössbauer and FTIR study of synthetic amphiboles along the magnesioriebeckite – ferri-clinoholmquistite join
DELLA VENTURA, Giancarlo;
2005-01-01
Abstract
A series of amphiboles along the magnesioriebeckite - square Na2Mg3Fe3+ 2Si8O22(OH)(2)- ferri-clinoholmquistite - circle Li2Mg3Fe3+ 2Si8O22(OH)2 - join, defined by the Li-B(B) Na-1 exchange vector, were hydrothermally synthesized at 700 degrees 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 alpha and beta 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 (Mg-M1,3(3)). 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). Mossbauer 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. Mossbauer spectra also show a distinct alteration of Fe-57 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 approximate to 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. Mossbauer 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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
