Structural relaxation in the MnCO3–CaCO3 solid solution: a Mn K-edge EXAFS study |
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Authors: | Y J Lee R J Reeder R W Wenskus E J Elzinga |
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Institution: | (1) Department of Geosciences, State University of New York at Stony Brook, Stony Brook, NY 11794-2100, USA e-mail: rjreeder@notes.cc.sunysb.edu, US |
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Abstract: | Mn K-edge EXAFS spectroscopy of solid-solution samples encompassing the complete MnCO3–CaCO3 series shows that first-shell Mn–O distances deviate little from the 2.19-Å distance observed in pure MnCO3. Very slight lengthening is observed only in the limiting case of dilute Mn(II) calcite solid solutions, where the Mn–O distance is 2.21 Å. The observed nearly complete structural relaxation and the composition independence of the Mn–O distance are consistent with the Pauling model behavior of solid solutions, and agree with previous studies showing a high degree of relaxation around hetero-sized substituents in the calcite structure. Strain occurs through bond bending, which is facilitated by the exclusively corner-sharing topology of calcite. Observed distances from Mn to more distant neighbors show significant variation across the solid-solution series that resembles Vegard's law-type behavior but reflects averaging. The high degree of relaxation suggests modest enthalpies of mixing in the solution, consistent with calorimetric studies. |
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Keywords: | Calcite Rhodochrosite EXAFS Solid solutions Relaxation Impurities |
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