A temperature-induced reversible transformation between paratacamite and herbertsmithite

The crystal chemistry of paratacamite has been re-evaluated by studying a crystal from the holotype specimen BM86958 of composition Cu3.71Zn0.29(OH)6Cl2 using single-crystal X-ray diffraction at 100, 200, 300, 353, 393 and 423 K. At 300 K paratacamite has space group TeX with unit-cell parameters a 13.644 and c 14.035 Å and exhibits a pronounced subcell, a"² = ½a and c"² = c, analogous to that of the closely related mineral herbertsmithite, Cu3Zn(OH)6Cl2. Between 353 and 393 K, paratacamite undergoes a reversible phase transformation to the herbertsmithite-like substructure, space group TeX , unit-cell parameters a 6.839 and c 14.072 Å (393 K). The transformation is characterised by a gradual reduction in intensity of superlattice reflections, which are absent at 393 and 443 K. On cooling from 443 to 300 K at ~10 K min−1, the superlattice reflections reappear and the refined structures ( TeX ) of the initial and recovered 300 K states are almost identical. The complete reversibility of the transformation establishes that paratacamite of composition Cu3.71Zn0.29(OH)6Cl2 is thermodynamically stable at ambient temperatures. The nature of the rhombic distortion of the M(2)O6 octahedron is discussed by considering two possibilities that are dependent upon the nature of cation substitution in the interlayer sites.