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3.10 c. A new curve for calibrating piston-cylinder assemblies at low pressure (F. Brunet, in collaboration with D. Vielzeuf/Clermont-Ferrand)

The curve of the farringtonite to Mg3(PO4)2-II polymorphic transformation has been tightly bracketed in an internally heated pressure vessel (IHPV) at Clermont-Ferrand (France). In the temperature range 565-825 °C and for pressures between 6100 and 8300 bars, the equilibrium curve can be described as: P(bars) = -772 + 8.13 T (K) ± 200 bars.

The same equilibrium curve has been bracketed in a piston-cylinder (PC) apparatus (at the Bayerisches Geoinstitut) using low-friction NaCl-based assemblies (for design, see 3/4 inch NaCl assemblies, Annual Report 1993). The comparison of the PC and IHPV data sets indicates that the friction correction is 300 to 400 bars for our 3/4 inch NaCl-based cells at 800 °C and 8.4 kbar. The farringtonite to Mg3(PO4)2-II transformation is unique for calibrating piston-cylinder assemblies at low pressure (below 10 kbar). Furthermore, this curve fulfills the requirements (e.g. low Clapeyron slope and fast kinetics of transformation) for an accurate pressure calibration.

Finally, the IHPV data allowed us to extract the thermochemical properties Δf298 = ~ 3745.8 kJ.mole-1 and S°298 = 180.7 J.mole-1.K-1 for Mg3(PO4)2-II, the high-pressure form with an olivine-related structure.

Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany
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