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)(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 with unit-cell parameters a 13.644 and c 14.035 and exhibits a pronounced subcell, a' = A1/2a and c'A = c, analogous to that of the closely related mineral herbertsmithite, Cu3Zn(OH)(6)Cl-2. Between 353 and 393 K, paratacamite undergoes a reversible phase transformation to the herbertsmithite-like substructure, space group , unit-cell parameters a 6.839 and c 14.072 (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 similar to 10 K min(-1), the superlattice reflections reappear and the refined structures () 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)(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.