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 Cu 3.71 Zn 0.29 (OH) 6 Cl 2 using single-crystal X-ray diffraction at 100, 200, 300, 353, 393 and 423 K. At 300 K paratacamite has space group R 3 ¯ with unit-cell parameters a 13.644 and c 14.035 Å and exhibits a pronounced subcell, a ′ = ½ a and c ′ = c , analogous to that of the closely related mineral herbertsmithite, Cu 3 Zn(OH) 6 Cl 2 . Between 353 and 393 K, paratacamite undergoes a reversible phase transformation to the herbertsmithite-like substructure, space group R 3 ¯ m , 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 ( R 3 ¯ ) of the initial and recovered 300 K states are almost identical. The complete reversibility of the transformation establishes that paratacamite of composition Cu 3.71 Zn 0.29 (OH) 6 Cl 2 is thermodynamically stable at ambient temperatures. The nature of the rhombic distortion of the M (2)O 6 octahedron is discussed by considering two possibilities that are dependent upon the nature of cation substitution in the interlayer sites.