Thermochemistry of the high structural state plagioclases

Abstract

The enthalpies of solution of a suite of 19 high-structural state synthetic plagioclases were measured in a Pb 2B 2O 5 melt at 970 K. The samples were crystallized from analyzed glasses at 1200°C and 20 kbar pressure in a piston-cylinder apparatus. A number of runs were also made on Amelia albite and Amelia albite synthetically disordered at 1050–1080°C and one bar for one month and at 1200°C and 20 kbar for 10 hr. The component oxides of anorthite, CaO, Al 2O 3 and SiO 2, were remeasured. The ΔH of disorder of albite inferred in the present study from albite crystallized from glass is 3.23 kcal, which agrees with the 3.4 found by Holm and Kleppa (1968). It is not certain whether this value includes the ΔH of a reversible displacive transition to monoclinic symmetry, as suggested by Helgeson et al. (1978) for the Holm-Kleppa results. The enthalpy of solution value for albite accepted for the solid solution series is based on the heat-treated Amelia albite and is 2.86 kcal less than for untreated Amelia albite. The enthalpy of formation from the oxides at 970 K of synthetic anorthite is −24.06 ± 0.31 kcal, significantly higher than the −23.16 kcal found by Charlu et al. (1978), and in good agreement with the value of −23.89 ± 0.82 given by Robie et al. (1979), based on acid calorimetry. The excess enthalpy of mixing in high plagioclase can be represented by the expression, valid at 970 K: ΔH ex (±0.16 kcal) = 6.7461 X ab X 2 An + 2.0247 X An X 2 Ab where X Ab and X An are, respectively, the mole fractions of NaAlSi 3O 8 and CaAl 2Si 2O 8. This ΔH ex, together with the mixing entropy of Kerrick and Darken's (1975) Al-avoidance model, reproduces almost perfectly the free energy of mixing found by Orville (1972) in aqueous cation-exchange experiments at 700°C. It is likely that Al-avoidance is the significant stabilizing factor in the high plagioclase series, at least for X An ≥ 0.3. At high temperatures the plagioclases have nearly the free energies of ideal one-site solid solutions. The Al-avoidance model leads to the following Gibbs energy of mixing for the high plagioclase series: ΔG mix = ΔH ex + RT X Abln[X 2 Ab(2 − X Ab)]+ X Anln [X An(1+X An) 2] 4 . The entropy and enthalpy of mixing should be very nearly independent of temperature because of the unlikelihood of excess heat capacity in the albite-anorthite join.