In a fused magma liquid from which silicate minerals crystallize, the metal ions can be four-coordinate. In olivine crystals, the M(II) co-ordination sites are octahedral. Partition coefficients, which are defined as K_p = [M(II)]_olivine/[M(II)]_melt, follow the order Ni(II) > Co(II) > Fe(II) > Mn(II). Account for this in terms of ligand-field theory. (See I.M. Dale and P. Henderson, 24th Int. Geol. Congress, Sect. 10, 1972, 105.)