Substitution in basic secondary Cu(II) chloride minerals

This thesis reports results from a comprehensive crystallographic and spectroscopic investigation of natural and synthetic samples of the basic Cu(II) chloride minerals, with focus on substitution phenomena in the group. A series of composition-induced phase transformations occur in the group. They are possibly anatacamite P1→ clinoatacamite P21/n → paratacamite R3→ herbertsmithite R3m, when Zn is the dominant substituting cation in the formula Cu4-xZnx(OH)6Cl2. The role of paratacamite in this series is poorly understood. It was originally described with the formula Cu2(OH)3Cl, but it is likely its structure is stabilised by the presence of essential Zn. An analogous series with Ni substitution to the end-member R3m phase known as gillardite, which is isostructural with herbertsmithite, also occurs. Based on the group theory, two series of space group symmetries are possible, P1→ R3→ R3m and P1→ C2/m → P21/c → R3m. These possibilities are explored through an analysis of the inherent structural changes related to compositional effects. A crystal from the type specimen of paratacamite (British Museum specimen BM86958), with composition Cu3.71Zn0.29(OH)6Cl2, was analysed by single-crystal X-ray diffraction at several temperatures (100, 200, 300, 353, 393 and 423 K). Its structure at 300 K is confirmed in space group R3for the unit cell a ≈ 13.6, c ≈ 14.0 Å, with a pronounced substructure corresponding to a′ ≈ ½a and c′ ≈ c, in space group R3m (analogous to the structure of herbertsmithite). Paratacamite undergoes a reversible phase transformation to the R3m substructure at elevated temperatures. This establishes that paratacamite is thermodynamically stable at 300 K for the composition studied. The loss of the superstructure at elevated temperature indicates that the substitution phenomenon is statistical between both interlayer metal sites, rather than being preferential at M(1) as has been suggested in the literature. It is suggested that the observed (2+2+2) octahedral configuration at M(2) is a consequence of a superimposition of non-tetragonally elongated Zn(OH)6 octahedra with dynamic (4+2) Jahn-Teller distorted Cu(OH)6octahedra occupying two orientations. In the course of this investigation the single-crystal X-ray structure of two new analogues of paratacamite were determined. One is an Mg-rich specimen from the Quebrada Mine, Camerones, Chile, Cu 3(Mg,Cu)(OH)6Cl2, and the other is a Ni-rich specimen from the Carr Boyd Rocks Mine, Western Australia, Australia, Cu3(Ni,Cu)(OH)6Cl2. The supercell analogous to that reported for paratacamite was identified and the structure was solved in space group R3. Both analogues exhibit a substructure with a' ≈ ½a, c' ≈ c in space group R3m. They are the first examples of naturally occurring substituted paratacamite congeners to be reported. Substitution phenomena in the Mg analogue is confirmed as being statistical in nature by refinement of the site scattering factors of interlayer sites M(1) and M(2). The substitution behaviour in the Ni analogue, as well as paratacamite containing from Zn from the holotype specimen was assumed to be statistical throughout this investigation, but the possibility remains that Zn and Ni preferentially occupy one of the interlayer sites. Furthermore, the single-crystal X-ray structure of naturally occurring Cu3(Co,Cu)(OH)6Cl2from the Torrecillas Mine, Salar Grande provence, Chile, is reported with unit cell parameters and structure analogous to that of herbertsmithite. Raman spectroscopy analyses, supported by single-crystal X-ray diffraction of samples exhibiting a range of compositions, have revealed several trends associated with variation in composition. An examination of both natural and synthetic samples indicate that the transformation series proceeds as P1→ P2 1/c → R3m, with decreasing Cu2+ content in Cu4-xMx(OH)6Cl2. The composition-induced changes in the paratacamite Raman spectrum with high interlayer Cu2+ content, suggests a distortion towards that of anatacamite. With excess substitution for interlayer Cu2+, the structure converges with that of the R3m aristotype. This corresponds to the space group series P1→ R3→ R3m, with decreasing Cu2+content as described above. The stability of paratacamite appears to be dependent on the type of substituting cation. Finally, the synthetic series of Zn- and Ni-substituted members was explored in order to determine the behaviour of the solid state activity coefficient (γ). In clinoatacamite, Zn 2+ occupation exhibits non-ideal behaviour for dilute solid solutions and γ > 1. With increasing Zn content, γ approaches unity near the composition Cu3.80Zn0.20(OH)6Cl2. (NOTE: SOME OF THE SCIENTIC SYMBOLS CAN NOT BE REPRESENTED CORRECTLY IN THE ABSTRACT. PLEASE READ WITH CAUTION AND REFER TO THE ORIGINAL THESIS.