Thermochemistry of mixing strontianite [SrCO3(s)] and aragonite [CaCO3(s)] to form CaxSr1-xCO3(s) solid solutions

Details Thermochemistry of mixing strontianite [SrCO3(s)] and aragonite [CaCO3(s)] to form CaxSr1-xCO3(s) solid solutions Thermochemistry of mixing strontianite [SrCO3(s)] and aragonite [CaCO3(s)] to form CaxSr1-xCO3(s) solid solutions

Mar 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996gecoa..60..933c&link_type=abstract

Geochimica et Cosmochimica Acta, Volume 60, Issue 6, p. 933-940.

Computer Science

3

Scientific paper

Enthalpies of mixing [ΔH298mix] of aragonite and strontianite to form CaxSr1-xCO3(s) solid solutions were obtained at 298 K from drop-solution enthalpies [ΔH975ds] of pellets of the respective solid solutions into molten 2PbO · B2O3 at 975 K. The measured ΔH298mix values are positive for all measured values of x, are nearly symmetric around x = 0.50, and reach a maximum value of +3.82 ± 0.94 kJ mol-1. Previous electrochemical studies have reported that ΔG298ex values are also positive over the range 0.0 < x < 0.9 and reach a maximum value of +3.0(±1.6) kJ mol-1 at x ≈ 0.7. The general similarity between the ΔH298mix and the ΔG298ex values indicates that the excess entropy of mixing is small or zero, consistent with the regular-solution treatment. Within this regular-solution treatment, the interaction parameter is W = 13.5(±1.3) kJ mol-1, which yields a very narrow range of stable miscible compositions at Earth surface conditions. Compositions of aragonite or strontianite with even a few percent impurity are not stable and will unmix to form a mechanical mixture of Ca-rich strontianite and a Sr-rich aragonite. It is, in general, difficult to accurately estimate the compositions of coexisting, miscible solids such as the CaxSr1-xCO3 solids at Earth-surface conditions. The predictions are quite sensitive to small uncertainties in the data and such estimates are probably beyond the capabilities of either electrochemical cell measurements or calorimetry.

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