FE-MN CATION ORDERING IN FAYALITE-TEPHROITE (FEXMN1-X)(2)SIO4 OLIVINES - A NEUTRON-DIFFRACTION STUDY

Time-of-flight neutron powder diffraction has been employed to determi ne precise occupancies of the M1 and M2 metal cation sites in syntheti c olivines of compositions (Fe0.3Mn0.7)(2)SiO4, (Fe0.5Mn0.5)(2-) SiO4, and (Feo(0.7)Mn(0.3))(2)SiO4. The distribution coefficient for Fe-Mn exchange in these samples has values of 4.864, 3.976, and 4.078, refle cting 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 c omposition-dependence of the solid solution is near ideal.