Thermodynamic Properties and Phase Equilibria of the Secondary Copper Minerals Libethenite, Olivenite, Pseudomalachite, Kröhnkite, Cyanochroite, and Devilline

Abstract

The thermodynamic properties of libethenite [Cu 2 (PO 4 )(OH)], olivenite [Cu 2 (AsO 4 )(OH)], pseudomalachite [Cu 5 (PO 4 ) 2 (OH) 4 ], cyanochroite [K 2 Cu(SO 4 ) 2 · 6H 2 O], krohnkite [Na 2 Cu(SO 4 ) 2 · 2H 2 O], and devilline [CaCu 4 (SO 4 ) 2 (OH) 6 · 3H 2 O] were determined by a combination of acid-solution calorimetry (enthalpy of formation) and relaxation calorimetry (heat capacity and entropy). The calculated Gibbs free energies (in kJ · mol −1 ) for these phases are −1229.3 ± 4.5, −848.7 ± 4.8, −2837.9 ± 10.8, −3441.4 ± 3.9, −2442.3 ± 3.6, −3843.2 ± 8.4, respectively. The phases studied were characterized by powder X-ray diffraction, infrared and Raman spectroscopy, and electron microprobe, as needed. We have also determined the crystal structure of devilline by single-crystal X-ray diffraction. Both of the copper phosphates investigated, libethenite and pseudomalachite, have a stability fieldinthe pH–p∈ space, but olivenite appears to be the only copper arsenate among those considered here (cornubite, clinoclase, euchroite) that has a stability field at T = 298.15 K and P = 1 bar. Chemical data from natural solid-solutions between libethenite–olivenite and pseudomalachite–cornwallite suggest that both solid solutions deviate slightly from ideality. The arsenic-rich members of these solutions tend to accept zinc in larger amounts. We have performed thermodynamic simulations showing that devilline crystallizes only when gypsum is nearby, which elevates the activity of Ca 2+ in the aqueous solutions.