Computer Science
Scientific paper
Mar 1977
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1977gecoa..41..369n&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 41, Issue 3, pp.369-377
Computer Science
51
Scientific paper
The enthalpies of solution of several synthetic garnets on the join Mg 3 Al 2 Si 3 O 12 -Ca 3 Al 2 Si 3 O 12 (pyrope-grossular) and of several synthetic clinopyroxenes on the join CaMgSi 2 O 6 -CaAl 2 SiO 6 (diopside-Ca-Tschermak's molecule) were measured in a melt of composition 2PbO · B 2 O 3 at 970 K. The determinations were made with sufficient precision so that thermochemical characterizations of the solid solutions could be achieved. The pyrope-grossular solutions show positive enthalpies of mixing. The non-ideality in the range 0-30 mole % grossular is relatively the largest and is in good agreement with the predictions of and (1974) based largely on cation partitioning of natural high grade metamorphic garnets with biotite, and with the deductions of et al. (1975) based on measurement of the compositions of synthetic pyrope-rich garnets equilibrated with anorthite, Al 2 SiO 5 and quartz. However, the garnets show smaller excess enthalpies at higher grossular contents. This would lead to an asymmetric solvus with a critical temperature lower than predicted by the symmetrical regular solution model of and (1974). The composition-dependent non-ideality can be understood by simple ionic size considerations in solid substitution and is analogous to the situations for the calcite-dolomite and enstatite-diopside solvi. The heats of solution of pyropes crystallized in the range 1000-1500°C were all the same, within the precision of measurement, and thus we have found no evidence for temperature-dependent cation disordering as a possible explanation of the high entropy of pyrope, as suggested by et al. (1975). Positional disorder of dodecahedral Mg is a more probable reason. The diopside-CaTs join is also non-ideal, with the larger positive enthalpy deviations near the diopside end. The calorimetric data in the magnesian range are consistemt with the model for completely disordered tetrahedral Si and Al which results from the free energy derivations of wood (1975) based on syntheses of diopside-rich aluminous pyroxenes in the presence of anorthite and quartz. At higher Al concentrations the calorimetric data seem more consistent with the `local charge-balance' model of (1975). No evidence for temperature-dependent disorder was found for either the diopside or CaTs end-members.
Charlu T. V.
Kleppa O. J.
Newton Robert C.
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