Thermodynamic properties of scapolites at temperatures ranging from 10 K to 1000 K

Norikazu Komada, David P. Moecher, Edgar F. Westrum, Bruce S. Hemingway, Mikhail Yu Zolotov, Yury V. Semenov, Igor L. Khodakovsky

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Abstract

The heat capacities of five mineral samples from the scapolite solid-solution series, Na4Al3Si9O24Cl (marialite) to Ca4Al6Si6O24CO3 (meionite), were measured by the adiabatic method from T=8 K to T=350 K and by the differential scanning calorimetry (d.s.c.) method from T = 300 K to T = 1000 K. The meionite (Me) content in per cent {Me=100 Ca*/(Ca* + Na*)} (where the asterisk indicates that possible substituents are included) and molar heat capacity (Cp,m/R) at T=298.15 K for each sample is: Me28, 82.07; Me44, 82.09; Me55, 83.95; Me69, 85.80; Me88, 84.54. The standard molar entropies, {S,om(298.15 K)-Som(0 K)} R-1 (R=8.31451 J·K-1·mol-1), at T=298.15 K for the respective compositions are: 85.05±0.26, 83.78±0.50, 85.22±0.24, 85.76±0.21, and 84.17±0.59. The calculated standard molar entropies (as above) at T=298.15 K for the end-members marialite and meionite, and for an intermediate composition (mizzonite=Me75) are 84.85, 83.94 and 86.15, respectively. Values of the coefficients in the equation Cp,m/R = a + bT+ cT2 + dT-1/2 + eT-2 (valid from T = 300 K to T =1000 K) are: (Mex, a, b/K, c/K2, d/K-1/2, e/K-2 Me88), 315.580, -0.0795676, 1.52825.10-5, -3954.83, 1808460; Me69, 261.285, -0.0415017, 8.73053.10-7, -3028.28, 1083666; Me55, 232.236, -0.0352222, 6.49875.10-6, 2505.99, 601750; Me44, 276.696, -0.0756614, 2.39722.10-5, -3210.40, 1044363; Me28, 149.917, 0.0229399, -1.23180.10-5, 1208.87, -318470. Smoothed thermodynamic functions for the five samples are also presented. The enthalpies of solution for five natural scapolites were measured in 2PbO·B2O3 melts at T= 973 K by Calvet-type calorimetry. The values of ΔsolHom/R·K are: Me11, 32.14±0.7; Me28, 32.34±0.4; Me44, 33.66±0.8; Me69, 35.29±0.8; Me88, 32.87±0.3. The calculated enthalpies of formation for stoichiometric scapolites ΔfHom/103·R·K at T= 298.15 K are: Me0, -1467.4±1.3; Me11, -1491.2±1.2; Me28, -1527.6±0.9; Me44, -1564.1±1.1; Me55, -1587.4±1.1; Me69, -1619.7±1.1; Me75, -1633.1±1.1; Me88, -1649.1±1.0; Me100, -1664±1.6. The heat capacity, the entropy, and the enthalpy of solution have maximal values near Me75 which may account in part for the relatively common occurrence of that composition in natural assemblages. Earlier measurements on leucite have been extended by the Komada/Westrum phonon dispersion model and corrected to end-member composition.

Original languageEnglish (US)
Pages (from-to)941-973
Number of pages33
JournalJournal of Chemical Thermodynamics
Volume28
Issue number9
DOIs
StatePublished - Sep 1996
Externally publishedYes

ASJC Scopus subject areas

  • Atomic and Molecular Physics, and Optics
  • Materials Science(all)
  • Physical and Theoretical Chemistry

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    Komada, N., Moecher, D. P., Westrum, E. F., Hemingway, B. S., Zolotov, M. Y., Semenov, Y. V., & Khodakovsky, I. L. (1996). Thermodynamic properties of scapolites at temperatures ranging from 10 K to 1000 K. Journal of Chemical Thermodynamics, 28(9), 941-973. https://doi.org/10.1006/jcht.1996.0083