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 (Fe (sub 0.3) Mn (sub 0.7) ) 2 SiO 4 , (Fe (sub 0.5) Mn (sub 0.5) ) 2 SiO 4 , and (Fe (sub 0.7) Mn (sub 0.3) ) 2 SiO 4 . The distribution coefficient for Fe-Mn exchange in these samples has values of 4.864, 3.976, and 4.078, reflecting the preference of Mn (super 2+) for the M2 site, over Fe (super 2+) . 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.

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