gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/002646198548007 Fe-Mn Cation Ordering in Fayalite-Tephroite (FexMn1−x)2SiO4 Olivines: A Neutron Diffraction Study Redfern S. A. T. Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK Knight K. S. ISIS, Rutherford Appleton Laboratory, Oxon, OX11 0QX, UK Henderson C. M. B. Department of Earth Sciences, University of Manchester, Oxford Road, Manchester, MI3 9PL, UK Wood B. J. Department of Geology, University of Bristol, Queens Road, Bristol, BS8 1RJ, UK 10 1998 62 5 607 615 30 4 1998 Copyright © 1998, The Mineralogical Society 1998 Abstract

Time-of-flight neutron powder diffraction has been employed to determine precise occupancies of the M1 and M2 metal cation sites in synthetic olivines of compositions (Fe0.3Mn0.7)2SiO4, (Fe0.5Mn0.5)2−SiO4, and (Fe0.7Mn0.3)2SiO4. The distribution coefficient for Fe-Mn exchange in these samples has values of 4.864, 3.976, and 4.078, reflecting the preference of Mn2+ for the M2 site, over Fe2+. These results, and the behaviour of the difference in mean bond lengths of the two sites, indicate that, while showing a tendency towards ordering, the composition-dependence of the solid solution is near ideal.

 Keywords olivines netron powder diffraction cation sites bond length