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Order and miscibility in the otavite–magnesite solid solution

¹ Bayerisches Geoinstitut, Universitaet Bayreuth, D-95440 Bayreuth, Germany
² Friedrich-Schiller-Universitaet Jena, Institut fuer Geowissenschaften, Burgweg 11, D-07749 Jena

Correspondence: ^* E-mail: tiziana.boffa-ballaran{at}uni-bayreuth.de

The effects of cation substitution and ordering in the otavite (CdCO3)–magnesite (MgCO₃) solid solution have been investigated on samples synthesized at 1 GPa in the temperature range 500–800 °C for run durations up to 120 h. A complete, disordered solid solution, with Rc symmetry, was obtained at 800 °C, whereas Mg and Cd show partial ordering within the cadmium dolomite stability field, with R symmetry, at intermediate compositions in the temperature range 500–650 °C. Rietveld refinements for X-ray diffraction data show that variation of the a-axis is linear as a function of composition, independent of the degree of order, whereas the c-axis shows a positive deviation from linearity as a function of composition, decreasing with increasing degree of order. Octahedral bond distances of the 800 °C series vary linearly with composition. Site occupancies were used to determine the long-range order parameter, Q, for samples with R symmetry. Q⁴ varies linearly as a function of temperature, suggesting a tricritical phase transition with a critical transition temperature T_c of 702(10) °C. EDX-TEM compositional microanalyses of samples within two-phase regions are in good agreement with Rietveld refinements, and allow better constraint of phase boundaries.

Key Words: X-ray powder diffraction • otavite–magnesite solid solution • carbonates • phase transition • cation ordering

This article has been cited by other articles:

				F. A. Bromiley, T. B. Ballaran, and M. Zhang An infrared investigation of the otavite-magnesite solid solution American Mineralogist, May 1, 2007; 92(5-6): 837 - 843. [Abstract] [Full Text] [PDF]