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

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 = 8K to T = 350K and by the differential scanning calorimetry (d.s.c.) method from T = 300K to T = 1000K. 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 (C-p,C-m/R) at T = 298.15 K for each sample is: Me(28), 82.07; Me(44), 82.09; Me(55), 83.95; Me(69), 85.80; Me(88), 84.54. The standard molar entropies, {S-m(o)(298.15 K) - S-m(o)(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 = Me(75)) are 84.85, 83.94 and 86.15, respectively. Values of the coefficients in the equation C-p,C-m/R = a + bT + cT(2) + dT(-1/2) + eT(-2) (valid from T = 300 K to T = 1000 K) are: (Me(x), a, b/K, c/K-2, d/K--1/2, e/K-2 Me(88)), 315.580, -0.0795676, 1.52825 . 10(-5), -3954.83, 1808460; Me(69), 261.285, -0.0415017, 8.73053 . 10(-7), -3028.28, 1083666; Me(55), 232.236, -0.0352222, 6.49875 . 10(-6), 2505.99, 601750; Me(44), 276.696, -0.0756614, 2.39722 . 10(-5), -3210.40, 1044363; Me(28), 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 Delta(sol)H(m)(o)/R . K are: Me(11), 32.1(4) +/- 0.7; Me(28), 32.3(4) +/- 0.4; Me(44), 33.6(6) +/- 0.8; Me(69), 35.2(9) +/- 0.8; Me(88), 32.8(7) +/- 0.3. The calculated enthalpies of formation for stoichiometric scapolites Delta(f)H(m)(o)/10(3) . R . K at T = 298.15 K are: Me(0), -1467.(4) +/- 1.(3); Me(11), -1491.(2) +/- 1.(2); Me(28), - 1527.(6) +/- 0.(9); Me(44), -1564.(1) +/- 1.(1); Me(55), -1587.(4) +/- 1.(1); Me(69), -1619.(7) +/- 1.(1); Me(75), -1633.(1) +/- 1.(1); Me(88), -1649.1 +/- 1.(0); Me(100), -1664 +/- 1.(6). The heat capacity, the entropy, and the enthalpy of solution have maximal values near Me(75) 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. (C) 1996 Academic Press Limited