Experimental and theoretical study of (Fe 2+ , Mg)(Al, Fe 3+ ) 2 O 4 spinels: Activity-composition relationships, miscibility gaps, vacancy contents

Details Experimental and theoretical study of (Fe 2+ , Mg)(Al, Fe 3+ ) 2 O 4 spinels: Activity-composition relationships, miscibility gaps, vacancy contents Experimental and theoretical study of (Fe 2+ , Mg)(Al, Fe 3+ ) 2 O 4 spinels: Activity-composition relationships, miscibility gaps, vacancy contents

Aug 1986

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986gecoa..50.1765l&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 50, Issue 8, pp.1765-1783

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Scientific paper

The compositions of (Fe 2+ , Mg)(Al, Fe 3+ ) 2 O 4 spinels equilibrated with a l M (Fe 2+ , Mg)Cl 2 aqueous solution at 800°C, 4 kbars were determined. General considerations of reciprocal systems allow derivation of the exchange isotherm between a chloride aqueous solution and (Mg, Fe 2+ )Al 2 O 4 spinels. They enable calculation of G of the reaction: FeCl 2 + MgAl 2 O 4 = MgCl 2 + FeAl 2 O 4 G = 2.9 kcal at 800°C, 4 kbars and provide the activity-composition relationships for the binary join FeAl 2 O 4 -MgAl 2 O 4 , which shows a substantial positive deviation from ideality. Some tie-lines between coexisting aluminous and ferric spinels were also obtained in the (Fe 2+ , Mg)(Al, Fe 3+ ) 2 O 4 system. These experimental data are modeled by a Gibbs free energy formulation of the spinel solid solution (Lehmann and Roux, 1984), where the corrective function g 2 , necessary to reproduce the deviations from ideality, is artificially split into two parts: 1. (1) A homogeneous second degree polynomial in the composition variables, containing only the terms specific to the reciprocal nature of the system, whose coefficients are deduced from G of the exchange reaction: MgAl 2 O 4 + FeFe 2 O 4 = MgFe 2 O 4 + FeAl 2 O 4 G = 4.5 kcal at 800°C, 4 kbars 2. (2) A homogeneous second degree polynomial in the site occupancy fractions, to model the non-ideal behavior of the (Fe 2+ , Mg)Al 2 O 4 and (Fe 2+ , Mg)Fe 2 O 4 spinels and the miscibility gap along the Fe(Al, Fe 3+ ) 2 O 4 join. A model of reciprocal spinel solution involving defect end-members is used to estimate the vacancy contents of the spinels in equilibrium with sesquioxides. In this case, the corrective function necessary to take into account the reciprocal nature of the system is no longer a second degree polynomial, but a rational fraction.

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