This paper reports the results of hydrothermal synthesis in the system Na2O-MgO-FeO-Fe2O3-SiO2- H2O. Four samples of stoichiometric magnesioriebeckite composition, ideally square 3Na(2)Mg(3)Fe(2)(3+)O(22)(OH)(2), were run at 700-800 degrees C, 0.4 GPa, and redox conditions varying from NNO (Nickel-Nickel Oxide) to NNO + 2.3 log fo(2) Powder XRD and SEM-EDX show a high (> 85%) amphibole yield for all samples; however, in no case was the end-member composition attained. EMP analyses show that the amphiboles obtained deviate strongly from nominal stoichiometry toward magnesio-arfvedsonite [NaNa2Mg4Fe3+Si8O22(OH)(2)]. Powder XRD patterns were indexed in the space group C2/m; refined cell-parameters reflect variations in the amphibole composition, and the cell volume is correlated linearly with the A-site occupancy. Mossbauer spectra show that in all samples, Fe3+ is completely ordered at M2, whereas Fe2+ occurs at the M1, M3, and M4 sites. The Fe3+/Fe2+ ratio is a function of f(o2): for increasing oxidation conditions, there is significant increase in Fe-M2(2+) and decrease in Fe2+, notably in Fe-M4(2+). Mossbauer spectra also show significant variation in Fe-M1(2+) and Fe-M3(2+) quadrupole splitting as a function of the Fe3+ content in the amphibole. IR spectra in the OH-stretching region show a well-resolved quadruplet at frequencies < 3680 cm(-1), assigned to octahedral (M1.3)(Mg, Fe2+)-OH-(A)square configurations, and a broad band consisting of four overlapping components related to (M1,3)(Mg, Fe2+) configurations associated with occupied A-sites. Quantitative evaluation of the relative band intensities suggests a linear increase of A-site occupancy with decreasing fo(2) of synthesis. The composition of the amphiboles synthesized, can be best described by a combination of the (C)(Mg,Fe2+)(1) (B)(Mg,Fe2+) Fe-C(-1)3+ Na-B(-1) and the Na-A(1) (C)(Mg,Fe2+)(1) (A)square(1) Fe-C(-1)3+ exchange vectors. The experimental trend is in accord with the trend documented for natural amphiboles, and suggests that the amphibole composition can in fact be used to monitor changes in f(o2) during crystallization.

DELLA VENTURA, G., Iezzi, G., REDHAMMER G., J., HAWTHORNE F., C., Scaillet, B., Novembre, D. (2005). Synthesis and crystal-chemistry of alkali amphiboles in the system Na2O-MgO-FeO-Fe2O3-SiO2-H2O as a function of fO2. AMERICAN MINERALOGIST, 90, 1375-1383.

Synthesis and crystal-chemistry of alkali amphiboles in the system Na2O-MgO-FeO-Fe2O3-SiO2-H2O as a function of fO2

DELLA VENTURA, Giancarlo;
2005-01-01

Abstract

This paper reports the results of hydrothermal synthesis in the system Na2O-MgO-FeO-Fe2O3-SiO2- H2O. Four samples of stoichiometric magnesioriebeckite composition, ideally square 3Na(2)Mg(3)Fe(2)(3+)O(22)(OH)(2), were run at 700-800 degrees C, 0.4 GPa, and redox conditions varying from NNO (Nickel-Nickel Oxide) to NNO + 2.3 log fo(2) Powder XRD and SEM-EDX show a high (> 85%) amphibole yield for all samples; however, in no case was the end-member composition attained. EMP analyses show that the amphiboles obtained deviate strongly from nominal stoichiometry toward magnesio-arfvedsonite [NaNa2Mg4Fe3+Si8O22(OH)(2)]. Powder XRD patterns were indexed in the space group C2/m; refined cell-parameters reflect variations in the amphibole composition, and the cell volume is correlated linearly with the A-site occupancy. Mossbauer spectra show that in all samples, Fe3+ is completely ordered at M2, whereas Fe2+ occurs at the M1, M3, and M4 sites. The Fe3+/Fe2+ ratio is a function of f(o2): for increasing oxidation conditions, there is significant increase in Fe-M2(2+) and decrease in Fe2+, notably in Fe-M4(2+). Mossbauer spectra also show significant variation in Fe-M1(2+) and Fe-M3(2+) quadrupole splitting as a function of the Fe3+ content in the amphibole. IR spectra in the OH-stretching region show a well-resolved quadruplet at frequencies < 3680 cm(-1), assigned to octahedral (M1.3)(Mg, Fe2+)-OH-(A)square configurations, and a broad band consisting of four overlapping components related to (M1,3)(Mg, Fe2+) configurations associated with occupied A-sites. Quantitative evaluation of the relative band intensities suggests a linear increase of A-site occupancy with decreasing fo(2) of synthesis. The composition of the amphiboles synthesized, can be best described by a combination of the (C)(Mg,Fe2+)(1) (B)(Mg,Fe2+) Fe-C(-1)3+ Na-B(-1) and the Na-A(1) (C)(Mg,Fe2+)(1) (A)square(1) Fe-C(-1)3+ exchange vectors. The experimental trend is in accord with the trend documented for natural amphiboles, and suggests that the amphibole composition can in fact be used to monitor changes in f(o2) during crystallization.
2005
DELLA VENTURA, G., Iezzi, G., REDHAMMER G., J., HAWTHORNE F., C., Scaillet, B., Novembre, D. (2005). Synthesis and crystal-chemistry of alkali amphiboles in the system Na2O-MgO-FeO-Fe2O3-SiO2-H2O as a function of fO2. AMERICAN MINERALOGIST, 90, 1375-1383.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11590/150226
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