THERMODYNAMIC PROPERTIES OF COPPER SILICATE, DIOPTASE, CU6SI6O18.6H2O

The heat capacity of natural dioptase has been measured between T = 6 K and T = 600 K by combined cryogenic-adiabatic and differential-scanning calorimetry. At T = 298.15 K, the standard molar heat capacity Cop, m and entropy Som for dioptase are (88.898±0.087)·R and (88.682±0.087)·R (for R = 8.3144 J·K-1·mol-1), respectively. The heat capacity exhibits an extremely sharp λ-transition at T = 14.5 arising from the antiferromagnetic ordering of the magnetic moments of the Cu2+ ions. Smoothed thermodynamic properties: molar heat capacity, molar entropy increment ΔTOSom, and (Φom+ΔT′OH om/T), where T′ = 298.15 K, have been calculated. Between T = 298.15 K and T = 600 K, the heat capacity of dioptase measured by d.s.c. may be represented by the equation:. Cop. m/R = 37.98+177.33·10-3·(T/K)-1.66·(K/T)2 ·105. The enthalpies of dehydration and formation from the oxides were determined by a set of thermochemical cycles using decomposition and subsequent dissolution of the decomposition product in molted lead borate. The molar enthalpy of dehydration is (164.9±17.6) kJ·mol-1. The molar enthalpy of formation of dioptase from the oxides, determined by oxide-melt solution calorimetry, is -(42.2±24.8) kJ·mol-1. The values of ΔfGom(298.15 K) = -(7324.6±27.6) kJ·mol-1 and ΔfGom(298.15 K) = -(8165.2±26.8) kJ·mol-1 have been calculated. On the basis of the thermodynamic results obtained, the stability relations of dioptase and copper carbonates in terms of a pH against lg{c(Cu2+)/co} diagram have been studied. © 1993 Academic Press Ltd.