Partitioning behavior of Co and Ni between coexisting orthopyroxene (En 60 Fs 40 ) and rnonosulfide solid solutions, Mss, has been studied at 900 degrees C in the presence of magnetite and tridymite using evacuated silica tube techniques. The distribution coefficient of Ni, K NiD , (K NiD = [X NiS /X FeS ]/[X (sub NiSiO 3 ) /X (sub FeSiO 3 ) ]) based on three reversed experiments is 10.8 + or - 1.5 and of Co, K CoD , based on three experiments is 5.0 + or - 1.2. These K D values indicate that both Ni and Co prefer the sulfide phase relative to orthopyroxene and that Ni has a greater preference for the sulfide phase at 900 degrees C than Co. From the available distribution data and from the crystal chemical properties of Co and Ni, it is concluded that the geochemical behavior of Co could be temperature dependent perhaps due to a change in its electronic spin state in sulfides. Comparison of the present K NiD values with that observed by Clark and Naldrett (1972) between Mss and fayalite suggests that K NiD between Mss and olivine could depend on the composition (Fe/Fe + Mg) of olivine.

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