Thermodynamic properties of copper silicate: dioptase: Cu 6Si 6O 18·6H 2O

Abstract

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 C o p, m and entropy S o m 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 Cu 2+ ions. Smoothed thermodynamic properties: molar heat capacity, molar entropy increment Δ T O S o m, and (Φ o m+Δ T′ O H o m/ 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: C o p. m / R = 37.98+177.33·10 -3·( T/K)-1.66·(K/ T)2·10 5. 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 Δ f G o m(298.15 K) = -(7324.6±27.6) kJ·mol -1 and Δ f G o m(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(Cu 2+)/ c o} diagram have been studied.