The activities of NiSi0.5O2, CoSi0.5O2, and MnSi0.5O2 in magmatic liquids have been calibrated from experimentally determined distributions of Ni, Co, and Mn between olivine and silicate melt. Chemical potentials of these species in silicate liquids are extracted from an expression for the chemical potentials of minor components in olivine and are described by an extension of the regular solution model of Ghiorso and Sack (1994). The extended liquid mixing model, together with the mixing model for multicomponent olivine, retrieves olivine/liquid DM ol liq for Ni, Co, and Mn with average absolute values of deviations equal to 14%, 13%, and 14%, respectively. Partitioning of Ca between olivine and liquid, using previously calibrated chemical potentials of CaMgSiO4 in olivine and CaSiO3 in silicate melt, is reproduced with an average relative deviation of 18%. Extrapolation of these models successfully predicts experimentally determined partitioning of Ni and Co between olivine and ultramafic liquids to > 2000°C and 100 kbar, thereby casting doubt on the importance of pressure-induced coordination changes to the geochemistry of Ni and Co under these conditions. Calculated partitioning of Ni and Co between silicate liquid and molten metal indicates that these elements remain markedly siderophile to temperatures greater than 3000 K. Calculations suggest that the siderophile behavior of Ni and Co may be reduced by increasing pressure. Forward modeling of phase equilibria and Ni partitioning during crystallization of mafic melts from Hawaii suggests that the Ni/MgO systematics of Hawaiian basaltic glasses are consistent with a parental liquid having 17% MgO and 760 ppm Ni. Activity coefficients of NiSi0.5O2, CoSi0.5O2, and MnSi0.5O2 in natural magmas are generally greater than unity, owing in a large part to positive energetic interactions between transition metals and SiO2. The primary determining compositional variable for these liquid activity coefficients and partition coefficients is molar silica content. The effects of changing temperature and liquid composition on olivine/ liquid partitioning of Ni, Co, and Mn are comparable and complementary, leading to more compatible behavior at lower temperatures and in more evolved liquids. Activity coefficients are greatest for NiSi0.5O2, and smaller for CoSi0.5O2 and MnSi0.5O2. Compositional variation of mineral/liquid partitioning and of activity coefficients owing to interactions with SiO2 are likely to be important for many other trace elements.