Order of osakaite–namuwite–lahnsteinite formation during alkalization of sulfate solutions

Experiments with dropwise alkalization of a 0.1 M ZnSO4 solution with 1M NaOH were carried out to establish the formation order of Zn-hydroxy-sulfate minerals (osakaite, namuwite, and lahnsteinite). During the controlled alkalization with 1M NaOH of a 0.1M ZnSO4 solution, namuwite (osakaite), lahnsteinite and wulfingite are successively formed. The obtained precipitates were characterized by powder X-ray diffraction (XRD) and SEM-EDS methods. The formation conditions and regions of stability of osakaite and namuwite are at near circumneutral pH (6.5–7), while lahnsteinite is stable at high pH (11–12). This explains the wider distribution of namuwite and osakaite in nature. The isomorphic substitution of Zn2+ cation with Cu2+ in the hydroxide layer of the Zn-hydroxy-sulfate minerals during controlled alkalization was studied using mixed 0.1M ZnSO4 + CuSO4 solution (molar ratio Zn:Cu = 4:1 and 1:1). During controlled alkalization of mixed Zn-Cu sulfate solutions, 25% copper cations are isomorphic incorporated in the namuwite (osakaite) and lahnsteinite structures. It was determined that this Cu2+ content is the upper limit of the isomorphism and corresponds to the occupation of 1/3 of octahedral positions.