| Abstract |
The possibilities of exchange reactions in two types of layer Zn-hydroxy sulfate minerals with neutral and negatively charged hydroxide layer - namuwite (Zn-4(OH)(6)(SO4)center dot 4H(2)O) and gordaite (NaZn4 (OH)(6) (SO4)Cl center dot 6H(2)O) - were studied. As starting materials, synthetic analogs of these minerals are used. Through direct synthesis, besides the known namuwite, gordaite, and its Ca2+ form, a series of new cationic (K+, NH4+) and anionic (Cl- and Br-) gordaite forms were synthesized. The K+, NH4+ content is respectively about 1/3 and 2/3 of the nominal compositions. The determined characteristics of Br-forms showed similarity with corresponding chloride forms. The results of the experimental study of the interaction between gordaites and namuwite and various halide solutions showed the realization of three types of solid-state exchange reactions: cation exchange, simultaneous cation-anion exchange, and water molecule-halide anion exchange. All samples were characterized by powder XRD and EDS chemical analysis, and the transformation mechanism was monitored by SEM investigation. By cation exchange, the all cationic gordaite forms were formed, and in addition, Sr2+-form was also obtained. Mutual conversions between the isovalent cations Na+-K+, Ca2+-Sr2+, and between all possible combinations of heterovalent cation pairs were established. Regardless of the charge of the hydroxide layer - neutral (namuwite) or negatively charged (gordaites), due to the stronger interaction of halide anion with the interlayer cations than that with the hydroxide layer's tetrahedral Zn2+ cation, the anion exit together with the leaving cations was observed, causing the simultaneous cation-anion exchange or mutual substitution of H2O molecules and halide anions. In the first case, the gordaite Br-forms from chloride one are formed. However, under treatment of the all various gordaite forms with iodide solutions, no exchange was observed. The big size of I- anion probably prevents its occupation of the apical position of hydroxide layer's Zn-tetrahedron and provokes the formation of new pure zinc-hydroxy-sulfate-hydrate phase with unknown specific structure. In the second case, the solid-state mutual substitution of H2O molecules and halide anions halide in the apex of the Zn-tetrahedron resulted in the formation of different zinc hydroxy sulfate hydrates, depending on the treating water amount. The reversible reaction caused a formation of different cationic gordaite forms under treatment of namuwite with chloride solutions of Na+, K+, NH4+, Ca2+, and Sr2+. The treatment of gordaites and namuwite with chloride solutions of some of the other alkaline and alkaline earth metals Li+, Mg2+, and Ba2+ resulted in gordaite dissolution and subsequent pseudomorphic simonkolleite crystallization. The simonkolleite formation is probably due to geometric difficulties for Li+ and Mg2+ interlayer incorporation caused by their great hydration capability and for Ba2+ due to its ability to form insoluble barite. |
