Atacamite from paleofumarole cone height 1004 (Tolbachik volcano, Kamchatka): thermodynamic properties

Abstract

Atakamite with the empirical formula (Cu1.97Zn0.01)Cl0.94(OH)3.02, originating from the paleofumaroles of the monogenic volcano Vysota 1004 (Tolbachik, Kamchatka, Russia), has been studied by thermal and electron microprobe analyses, X-ray powder diffraction, IR and Raman spectroscopy, Calve microcalorimetry. Using X-ray diffraction and IR spectroscopy, the process of thermal decomposition of atacamite was studied. The enthalpy of formation from the elements for atacamite of the theoretical composition Cu2Cl(OH)3(−810.2 ± 7.7 kJ/mol) was determined by melt dissolution calorimetry and the Gibbs energy of formation (−657.0 ± 7.7 kJ/mol) was calculated. Based on the data obtained, thermodynamic modeling of the stability of atacamite in the Cu–O–Cl–H system was carried out, and the boundaries of its stability were calculated under conditions of high alkalinity and high acidity of the mineral-forming medium.