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American Mineralogist; May 2004; v. 89; no. 5-6; p. 785-789
© 2004 Mineralogical Society of America
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The fate of structure-bound Mn2+ during the decomposition of dolomite and in the resulting conversion products: An EPR study

Josef Granwehr1,4, Peter G. Weidler2 and Andreas U. Gehring3,*

1 Laboratory for Physical Chemistry, ETH Zurich, 8093 Zurich, Switzerland
2 Forschungszentrum Karlsruhe GmbH, Institute for Technical Chemistry, Water and Geotechnology Section (ITC-WGT), D-76021 Karlsruhe, Germany
3 Institute of Geophysics, ETH Zurich, 8093 Zurich, Switzerland

Correspondence: * E-mail: gehring{at}sl.ethz.ch

Dolomite from the Jhamarkotra phosphate mine (India) contains 680 ppm Mn2+ as indicated by electron-spin resonance spectroscopy (EPR). Dipolar broadening of the EPR signal prevents a quantitative assignment of Mn2+ to the Ca and Mg sites. Upon heating to 700 °C, over 99% of the dolomite is decomposed and all the Mn2+ is released from the dolomite structure. Approximately 95% of the original Mn2+ is oxidized and forms Mn-oxide. The remaining Mn2+ preferentially migrates into CaO. This behavior is interpreted by the Goldschmidt rule stating that smaller ions are taken up at the sites of larger ions. During subsequent hydration of CaO into Ca(OH)2, the Mn2+ remains stable. The protection of Mn2+ against oxidation is explained by the topotaxical alteration of the host minerals. Carbonation of the Ca(OH)2 leads to Mn2+ oxidation caused by the dissolution of the hydroxide prior to the formation of CaCO3. Divalent Mn monitored by EPR can be used to unravel the behavior of trace elements on the molecular level during mineral alteration.






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