Dehydration and rehydration of Zn-hydroxy sulfate minerals with interrupted decorated hydroxide sheets

Abstract The dehydration-rehydration processes of three Zn-hydroxy sulfate minerals with interrupted decorated sheet structures – osakaite, gordaite and Ca-gordaite were investigated. The obtained products and processes were characterized by DTA-TG-MS, in situ and ex situ PXRD and FTIR. In osakaite structure the two different positions of water molecules cause different ways of dehydration. The derivation of each interlayer molecule results in the formation of discrete phases, with 4, 3 and 1 H 2 O, without any evidence for dihydrate formation. The process was conducted by both heating and adsorption. The rehydration of these metaphases proceeds reciprocally to the dehydration with relatively high rate at middle and high RH values. The derivation of water molecules from the Zn tetrahedron occurs only on heating. The derivation of only half of the apical molecules was observed. The rehydration of formed hemihydrate metaphase proceeds very slowly even at high RH values. The fully dehydrated phase thus proved to be impossible to form. Structural schemes of monohydrate and hemihydrate phases were proposed. In osakaite structure type all changes occur on the surface of the hydroxide layer, preserving its electrically neutral character, with further possible implications for facilitating intercalation of polar molecules. The negatively charged surface of gordaite hydroxide layer, caused by the occupation of apical position of Zn tetrahedra by Cl − determine dehydration behavior, similar to that of montmorillonite and vermiculite structures. On heating, gordaite and Ca-gordaite form three and two pillared structures with different sizes of interlayer space and different amount of water molecules for each phase. The rate of dehydration and/or rehydration and particular quantity of derived water molecules is controlled by the interlayer cations. In accordance with the high ionic potential of calcium, the dehydration of Ca-gordaite occurs only on heating and the rehydration proceeds at very low RH conditions. To the contrary, the much lower ionic potential of sodium causes the formation of dehydrated gordaite phases by both heating and by adsorption. Similarly to the montmorillonite and vermiculite – these two gordaite minerals can be expected to be used for both cation exchange for interlayer cations and anionic exchange of apical Cl − anion.