THERMODYNAMIC PROPERTIES OF SCAPOLITES AT TEMPERATURES RANGING FROM 10-K TO 1000-K

The heat capacities of five mineral samples From the scapolite solid-s olution series, Na4Al3Si9O24Cl (marialite) to Ca4Al6Si6O24CO3 (meionit e), 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 i ncluded) 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 mo lar 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 thermody namic functions for the five samples are also presented. The enthalpie s of solution for five natural scapolites were measured in 2PbO . B2O3 melts at T = 973 K by Calvet-type calorimetry. The values of Delta(so l)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 scapol ites 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 th e enthalpy of solution have maximal values near Me(75) which may accou nt in part for the relatively common occurrence of that composition in natural assemblages. Earlier measurements on leucite have been extend ed by the Komada/Westrum phonon dispersion model and corrected to end- member composition. (C) 1996 Academic Press Limited