Structural Effects of Cu/Zn Substitution in the Malachite-Rosasite System

Synthetic zincian malachite samples (Cu1-xZnx)(2)(OH)(2)CO3 with x = 0, 0.1, 0,2 and 0.3 were characterized by powder X-ray diffraction and optical spectroscopy. The XRD patterns of the samples up to x = 0.2 indicate single phase materials with an approximately linear dependence of the relined lattice parameters on the zinc content. In contrast, the sample with a nominal zinc content x = 0.3 shows the formation of a small amount of aurichalcite (Zn,Cu)(5)(OH)(6)(CO3)(2) as an additional phase. Based on the lattice parameter variations, the zinc content of the zincian malachite component in this sample is estimated to be x approximate to 0.27, which seems to represent the maximum possible substitution in zincian malachite under the synthesis conditions applied. The results are discussed in relation to preparation of Cu/ZnO catalysts and the crystal structures of the minerals malachite and rosasite. One striking difference between these two structurally closely related phases is the orientation of the Jahn-Teller elongated axes of the CuO6 octahedra in the unit cell, which seems to be correlated with the placement of the monoclinic beta angle. The structural and chemical relationship between these crystallographically distinct phases is discussed using a hypothetical intermediate Zn-2(OH)(2)CO3 phase of higher orthorhombic symmetry. In addition to the crystallographic analysis, optical spectroscopy proves to be a useful tool for estimation of the Cu:Zn ratio in (Cu1-xZnx)(2)(OH)(2)CO3 samples.