Thermal decomposition of zinc hydroxy-sulfate-hydrate minerals

The thermal decomposition of three structurally and chemically close compounds: Zn-4(OH)(6)(SO4)center dot 4H(2)O, NaZn4(OH)(6)(SO4)Cl center dot 6H(2)O and CaZn8(OH)(12)(SO4)(2)Cl-2 center dot 9H(2)O, 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 A degrees 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, O-2 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: Zn-3(OH)(2)(SO4)(2) (in the decomposition of the three compounds), beta-Zn(OH)Cl (in the case of both gordaites) and Ca(OH)(2) (for Ca-gordaite) were formed. The existence and formation of Zn-3(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, O-2 and HCl occurred due to the decomposition of the (SO4)-groups and ZnCl2 hydration, respectively. At temperatures up to 1000 A degrees 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.