Partial molar volumes of NiO and CoO liquids: implications for the pressure dependence of metal-silicate partitioning

Volumetric measurements have been conducted on 7 Ni- and Go-containing sodi um disilicate liquids within a compositional range varying from 0 to 9 mol% of NiO and from 0 to 23 mol% of CoO and over a large temperature interval (i.e., above their respective glass transition temperature and up to at lea st 1473 K). Their molar volumes and thermal expansivities have been determi ned by combining high-temperature measurements using the Pt-based double-bo b Archimedean method and low-temperature measurements using the method desc ribed by Webb et al. [S.L. Webb, R. Knoche, D.B. Dingwell, Determination of silicate liquid thermal expansivity using dilatometry and calorimetry, fur . J. Mineral. 4 (1992) 95-104] based on an assumed equivalence of the relax ation of volume and enthalpy at the glass transition. The molar volume of t he present liquids decreases with increasing NiO and CoO contents and the C o-containing liquids exhibit a greater molar volume than the Ni-containing liquids at equivalent molar concentrations. The present results were analys ed using a regression equation from which the partial molar volume of NiO a nd CoO liquids was obtained by the least squares method. This procedure yie lds partial molar volumes valid over the entire temperature range of 11.506 +/- 0.687 and 14.884 +/- 0.149 cm(3)/mol and temperature derivatives of 2. 684 +/- 1.6 x 10(-3) and 1.441 +/- 0.4 x 10(-3) cm(3)/mol K, respectively f or NiO and CoO at 800 K. The behavior of M-Fe metal-silicate exchange parti tion coefficient (M = Ni, Co), based on present molar volume determinations , has been estimated as a function of pressure over a wide temperature rang e. The metal-silicate exchange partition coefficients of both Ni and Co dec rease with increasing pressure within the entire temperature range consider ed in this study (i.e., 800-3000 It). (C) 1999 Elsevier Science B.V. All ri ghts reserved.