Thermochemistry of the high structural state plagioclases

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Abstract

The enthalpies of solution of a suite of 19 high-structural state synthetic plagioclases were measured in a Pb2B2O5 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, Al2O3 and SiO2, 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 Helgesonet 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 Charluet al. (1978), and in good agreement with the value of −23.89 ± 0.82 given by Robieet al. (1979), based on acid calorimetry.

The excess enthalpy of mixing in high plagioclase can be represented by the expression, valid at 970 K: ΔHex(±0.16 kcal) = 6.7461 XabX2An + 2.0247 XAnX2Ab where XAb and XAn are, respectively, the mole fractions of NaAlSi3O8 and CaAl2Si2O8. This ΔHex, 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 XAn≥ 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: ΔGmix = ΔHex + RT XAbln[X2Ab(2 − XAb)]+ XAnln[XAn(1+XAn)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.

References (39)

  • C.W. Burnham

    Lattice constant refinement

    Carnegie Inst. Washington Yearb.

    (1962)
  • J.R. Goldsmith et al.

    Cation order in anorthite (CaAl2Si2O8) as revealed by gallium and germanium substitutions

    Acta Crystallogr.

    (1955)
  • J.R. Goldsmith et al.

    Scapolite-plagioclase stability relations at high pressures and temperatures in the system NaAlSi3O8CaAl2Si2O8CaCO3CaSO4

    Am. Mineral.

    (1978)
  • T.L. Grove

    Structural characterization of labradorite-bytownite plagioclase from volcanic, plutonic and metamorphic environments

    Contrib. Mineral. Petrol.

    (1977)
  • H.C. Helgeson et al.

    Summary and critique of the thermodynamic properties of rock-forming minerals

    Am. J. Sci.

    (1978)
  • B.S. Hemingway et al.

    Enthalpies of formation of low albite (NaAlSi3O8), gibbsite (Al(OH)3), and NaAlO2; revised values for ΔH0f,298 and ΔG0f,298 of some aluminosilicate minerals

    J. Res. U.S. Geol. Surv.

    (1977)
  • J.L. Holm et al.

    Thermodynamics of the disordering process in albite

    Am. Mineral.

    (1968)
  • G.L. HovIs et al.

    A solution calorimetric investigation of KNa mixing in a sanidine-analbite ion-exchange series

    Am. Mineral.

    (1977)
  • O.J. Kleppa et al.

    The role of solution calorimetry in the study of mineral equilibria

    Fortschr. Mineral.

    (1975)
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