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American Mineralogist; January 2005; v. 90; no. 1; p. 155-161; DOI: 10.2138/am.2005.1577
© 2005 Mineralogical Society of America
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The Mn, Mg-intracrystalline exchange reaction in donpeacorite (Mn0.54Ca0.03Mg1.43Si2O6) and its relation to the fractionation behavior of Mn in Fe, Mg-orthopyroxene

Marilena Stimpfl*

University of Arizona, Lunar and Planetary Laboratory, Tucson, Arizona 85721, U.S.A.

Correspondence: * E-mail: mstimpfl{at}lpl.arizona.edu

The equilibrium intracrystalline distribution of Mn and Mg between the M1 and M2 sites of a Mn-rich/Fe-free orthopyroxene (donpeacorite) was investigated by means of annealing experiments at temperatures between 980 and 800 °C and single-crystal X-ray diffraction. The data show that Mn, as does Fe2+ in Fe-Mg orthopyroxene, preferentially orders at the M2 site. However, comparison of the distribution coefficient kD(Mn-Mg) determined in this study with kD* measured for Fe-Mg orthopyroxene shows that Mn has a much stronger preference for the M2 site relative to Fe2+. This result implies that the practice to partition Fe2+ + Mn = Fe* as one species, typically implemented to determine the quenched-site occupancies in Fe-rich/Mn-poor orthopyroxene, should be abandoned and that Mn should be considered totally ordered at M2. The partitioning method, i.e., Fe vs. Fe*, has implications for the determination of cooling rates from the observed ordering state of orthopyroxene, particularly for Fe-poor compositions (Fs < 0.16).




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