Abstract
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. Galena9s 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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