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Compositional dependence of cation distribution in some synthetic (Mg,Zn)(Al,Fe³⁺)₂O₄ spinels

¹ Dipartimento di Scienze della Terra, Università di Roma "La Sapienza", Piazzale Aldo Moro 5, I-00185 Roma, Italy.
² Department of Mineralogy, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden. – e-mail: henrik.skogby{at}nrm.se
³ Dipartimento di Mineralogia e Petrologia, Università di Padova, Corso Garibaldi 37, I-35137 Padova, Italy.

e-mails: gianni.andreozzi{at}uniromal.it, lucchesi{at}uniromal.it

The compositional dependence of the intracrystalline cation distribution was investigated in four synthetic spinels belonging to binary solid solutions. Spinel single crystals were flux-grown in the (Mg,Zn)(Al,Fe³⁺)₂O₄ system, and cation distribution was determined by means of single-crystal X-ray structural refinement, electron-microprobe analysis and Mössbauer spectroscopy. Experimental data were processed and a detailed topochemical model was obtained for each sample: ^IV(Mg_0.76Al_0.24)^VI(Al_1.76Mg_0.24)O₄; ^IV(Zn_0.65Mg_0.22Al_0.13)^VI(Al_1.87Mg_0.13)O₄; ^IV(Mg_0.73Al_0.18Fe³⁺_0.09)^VI(Al_1.62Mg_0.27Fe³⁺_0.11)O₄ and ^IV(Mg_0.62Zn_0.15Al_0.15Fe³⁺_0.08) ^VI(Al_1.61Mg_0.23Fe³⁺_0.16)O₄. Zinc was found to occupy only the tetrahedrally coordinated site and its presence strongly influenced intersite cation partitioning. In Zn-bearing crystals both intracrystalline exchanges Mg Al and Mg Fe³⁺ turned out to be limited. Zinc in octahedral coordination, which is rare in natural spinels, was shown to be favoured by high Fe³⁺ contents and high equilibration temperatures. The cation distributions determined experimentally were found to be in close agreement with those calculated using the general thermodynamic model for spinel solid solutions by O'Neill & Navrotsky (1984).

Key-words: spinel, gahnite, franklinite, ionic radii, intracrystalline distribution.

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