Kinetic decomposition of Ni₂SiO₄ in oxygen potential gradients

Abstract

Nickel orthosilicate (Ni₂SiO₄) has been found to decompose into its component binary oxides in oxygen potential gradients at 1373 K. Nickel oxide was formed at the high oxygen potential boundary, while silica was detected at the low oxygen potential side. Significant porosity and fissures were observed near the Ni₂SiO₄/SiO₂ interface and the SiO₂ layer. The critical oxygen partial pressure ratio required for decomposition varied from 1.63 to 2.15 as the oxygen pressures were altered from 1.01 × 10⁵ to 2.7 × 10^-4 Pa, well above the dissociation pressure of Ni₂SiO₄. Platinum markers placed at the boundaries of the Ni₂SiO₄ sample indicated growth of NiO at the higher oxygen potential boundary, without any apparent transport of material to the low oxygen potential side. However, significant movement of the bulk Ni₂SiO₄ crystal with respect to the marker was not observed. The decomposition of the silicate occurs due to the unequal rates of transport of Ni and Si. The critical oxygen partial pressure ratio required for decomposition is related both to the thermodynamic stability of Ni₂SiO₄ with respect to component oxides and the ratio of diffusivities of nickel and silicon. Kinetic decomposition of multicomponent oxides, first discovered by Schmalzried, Laqua, and co-workers [H. Schmalzried, W. Laqua, and P. L. Lin, Z. Natur Forsch. Teil A 34, 192 (1979); H. Schmalzried and W. Laqua, Oxid. Met. 15, 339 (1981); W. Laqua and H. Schmalzried, Chemical Metallurgy-A Tribute to Carl Wagner (Metallurgical Society of the AIME, New York, 1981), p. 29] has important consequences for their use at high temperatures and in geochemistry.