Thermal decomposition of zinc hydroxy-sulfate-hydrate minerals

The thermal decomposition of three structurally and chemically close compounds: Zn4(OH)6(SO4)·4H2O, NaZn4(OH)6(SO4)Cl·6H2O and CaZn8(OH)12(SO4)2Cl2·9H2O, known as namuwite, gordaite and “Ca-gordaite,” was comparatively studied. The thermal events, the released volatiles and solid residues were studied in the range of 20–1000 °C using TG–DTA-MS and powder XRD analyses. The thermal decomposition process of all three compounds includes successive dehydration, dehydroxylation and evolving of SO2, O2 and Cl. The first stage of dehydration occurs in the low-temperature region by the retention of the layered structure with an undisturbed hydroxide layer and a shrunk interlayer. The release of last interlayer water molecules induces the dehydroxylation of the hydroxide layer. As a result, ZnO and a series of secondary hydroxides and hydroxy-salts: Zn3(OH)2(SO4)2 (in the decomposition of the three compounds), β-Zn(OH)Cl (in the case of both gordaites) and Ca(OH)2 (for Ca-gordaite) were formed. The existence and formation of Zn3(OH)2(SO4)2 for the first time were proved by powder XRD. The decomposition of all secondary hydroxyl-containing compounds during the second stage of the dehydroxylation leads to formation of Zn3O(SO4)2, ZnCl2 and CaSO4 at middle temperature region. In the high-temperature region, the evolving of SO2, O2 and HCl occurred due to the decomposition of the (SO4)-groups and ZnCl2 hydration, respectively. At temperatures up to 1000 °C, ZnO was found to be the main final product of the thermal decomposition of the three minerals. In the case of gordaites, an anhydrite and a Na–Zn–SO4 X-ray amorphous phase were also presented.