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American Journal of Science, Vol. 307, March 2007, P.590-611

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Galena stability to 26 KBAR

Kevin T. Wheeler*, David Walker* and Marie C. Johnson**

* Lamont Doherty Earth Observatory and Department of Earth and Environmental Sciences, Columbia University, Palisades, New York 10964 USA; kwheeler{at}ldeo.columbia.edu; dwalker{at}ldeo.columbia.edu
** Department of Geography and Environmental Engineering, U S Military Academy, West Point, New York 10996

The melting point of galena was measured by differential thermal analysis to 25 kbar. The melting point is 1191 °C at 5.9 kbar and increases to ~1315 °C by 25 kbar along a concave-down trajectory. The form of the PbS melting curve resembles those of similarly structured compounds that show a range of initial slopes and curvatures. Initial liquidus slopes of several B1-structured compounds including PbS correlate well with melting volume change. However, the roughly 25 J/mol*K entropy of melting required for PbS by the volume of melting, initial liquidus slope, and Clapeyron equation is approximately double literature estimates. Similar discrepancies exist for the other B1-structured compounds. Liquidus curvature correlates well with the initial ratio of liquid to solid compressibility. PbS liquid compressibility of ~12*10-12 cm2/ dyne is estimated from galena compressibility and initial melting slope. The transformation of galena from cubic to orthorhombic structure with pressure was determined by electrical resistance measurements up to 1000 °C and occurs at about 26 kbar with little temperature variation. Galena's maximum stability is at the inferred triple point among galena, liquid, and orthorhombic-structured PbS at ~26 kbar and ~1315 °C.






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