Heat capacity, relative enthalpy, and calorimetric entropy of silicate minerals; an empirical method of prediction

Abstract

A procedure to estimate the standard molar heat capacity, relative enthalpy, and calorimetric entropy of silicate minerals has been devised and evaluated. These are estimated by summing, in appropriate proportions, fictive molar isobaric heat capacities, relative enthalpies, and entropies evaluated for structural components of the mineral phases such as MgO-4, MgO-6, or MgO-8 where the Mg is in 4-,6-, or 8-fold coordination, respectively. The fictive molar heat capacities and entropies were obtained from a large body of experimental calorimetric data on heat capacity, entropy, and relative enthalpy for minerals. The summation technique has a precision better than2To for heat capacity and relative enthalpy, and 5Vo for entropy, relative to the data base, between 29E and 1500 K. The accuracy of prediction of molar heat capacity and relative enthalpy for mineral phases and specific heat for rocks is expected to be within 3Tousing this technique. The accuracy of prediction of molar calorimetric entropy is expected to be within 5% using this technique. Tables of evaluated coefficients for the structural components of mineral phases are given. Three examples of calculations are given. (1) The estimated heat capacity of an illite is calculated and compared with experimental data which was not used in the evaluation. (2) The estimated relative enthalpy of acmite is calculated and compared with experimental data also not used in the evaluation. (3) The estimated calorimetric molar entropy of an illite is calculated and compared with data not used in the evaluation. In each of these cases, the estimated values deviate less than 2.5Vo from the observed values.