THE FARRINGTONITE MG-3(PO4)(2)-II TRANSFORMATION - A NEW CURVE FOR PRESSURE CALIBRATION IN PISTON-CYLINDER APPARATUS/

The polymorphic transformation farringtonite - Mg-3(PO4)(2)-II has bee n bracketed in a gas-pressure apparatus between 6100 and 8300 bars in the temperature range 565-825 degrees C. The equilibrium curve can be described as: P (bars) = - 772 + 8.13 T (K) +/- 200 bars. The small va lue of the Clapeyron slope is comparable to that of the quartz-coesite transformation and makes the farringtonite - Mg-3(PO4)(2)-II transfor mation a good candidate for the calibration of piston-cylinder assembl ies at low pressure (P < 10 kbar). Thermochemical values for the Mg-3( PO4)(2)-II polymorph have been retrieved from the reversals : S degree s(298) = 180.7 J.mole(-1)K(-1) and Delta(f)H degrees(298) = - 3745.8 k J.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 de sign and/or too low a rate of friction-decay compared to tile fast kin etics of the polymorphic transformation.