Original paper

The farringtonite / Mg3(PO4)2-II transformation: A new curve for pressure calibration in piston-cylinder apparatus

Brunet, Fabrice; Vielzeuf, Daniel

European Journal of Mineralogy Volume 8 Number 2 (1996), p. 349 - 354

24 references

published: Apr 26, 1996
manuscript accepted: Dec 14, 1995
manuscript received: Feb 20, 1995

DOI: 10.1127/ejm/8/2/0349

BibTeX file

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Abstract

Abstract The polymorphic transformation farringtonite - Mg3(PO4)2-II has been bracketed in a gas-pressure apparatus between 6100 and 8300 bars in the temperature range 565-825 °C The equilibrium curve can be described as: P (bars) = - 772 + 8.13 T (K) ± 200 bars. The small value of the Clapeyron slope is comparable to that of the quartz-coesite transformation and makes the farringtonite - Mg3(PO4)2-. Il transformation a good candidate for the calibration of piston-cylinder assemblies at low pressure (P < 10 kbar). Thermochemical values for the Mg3(PO4)2-. Il polymorph have been retrieved from the reversals: S°298 = 180.7 J.mole-1 K-1 and ΔfH°298 = - 3745.8 kJ.mole-1. In an independent determination of the transition in NaCl-based piston-cylinder assemblies, pressure was overestimated by up to 400 bars. This might reflect friction inherent to the pressure-cell design and/or too low a rate of friction-decay compared to the fast kinetics of the polymorphic transformation

Keywords

phase transitionpressure calibrationMg3(PO4)2 polymorphsfarringtonitethermodynamic properties