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Science 14 September 1990:
Vol. 249. no. 4974, pp. 1275 - 1278
DOI: 10.1126/science.249.4974.1275
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Articles

Negative Pressure-Temperature Slopes for Reactions Formign MgSiO3 Perovskite from Calorimetry

E. Ito 1, M. Akaogi 2, L. Topor 3, and A. Navrotsky 3

1 Institute for Study of the Earth's Interior, Okayama University, Misasa, Tottoriken, 682-02, Japan
2 Department of Chemistry, Gakushuin University, Tokyo 171, Japan
3 Department of Geological and Geophysical Sciences, Princeton University, Princeton, NJ 08544

A new and sensitive differential drop solution calorimetric technique was developed for very small samples. A single experiment using one 5.18-milligram sample of perovskite, synthesized at 25 gigapascals and 1873 Kelvin, gave 110.1 ± 4.1 kilojoules per mole for the enthalpy of the ilmenite-pervoskite transition in MgSiO3. The thermodynamics of the reaction of MgSiO3 (ilmenite) to MgSiO3 (perovskite) and of Mg2SiO4 (spinel) to MgSiO3 (pervoskite) and MgO (periclase) were assessed. Despite uncertainties in heat capacity and molar volume at high pressure and temperature, both reactions clearly have negative pressure-temperature slopes, -0.005 ± 0.002 and -0.004 ± 0.002 gigapascals per Kelvin, respectively. The latter may be insufficiently negative to preclude whole-mantle convection.


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