One in four of the Cu-filled cation sites in clinoatacamite, the monoclinic polymorph of atacamite and botallackite, is angle- rather than Jahn-Teller-distorted. Experiments show that this site alone is susceptible to substitution by a non-Jahn-Teller distorting cation of suitable radius and charge, such as Zn 2+ , and also Ni 2+ , Co 2+ , Fe 2+ , Cd 2+ and Mg 2+ . The crystal symmetry changes to rhombohedral when ∼⅓ of the Cu in this site is substituted by e.g. Zn, thus giving paratacamite; the Zn is essential for stability and forms, with larger proportions of Zn, a series to the end-member in which the site is fully occupied by Zn. This end-member, rhombohedral stoichiometric Cu 3 Zn(OH) 6 Cl 2 , has been characterized using natural specimens from Chile and Iran and is named herbertsmithite. The other cations mentioned behave similarly, producing stabilized rhombohedral paratacamites and end-members analogous to herbertsmithite, which if found in nature, as the Ni analogue has been, should be named species. Clinoatacamite, paratacamite and herbertsmithite have rather similar X-ray powder diffraction patterns, but are readily distinguished by infrared spectroscopy. Zn-stabilized paratacamite forms blue-green crystals of rhombohedral habit, with vitreous lustre, e = 1.828-1.830, o = 1.835, uniaxial negative, weakly pleochroic O > E, density: 3.75 g cm -3 , Mohs hardness: 3-3½; space group R , a = 13.654(5), c = 14.041(6) Å, Z = 24, with pronounced R m substructure; six strongest XRD lines 5.452 (100), 2.895 (20), 2.760 (74), 2.262 (52), 1.817 (18), 1.708 (21). Herbertsmithite forms dark green crystals of rhombohedral habit, with vitreous lustre, e = 1.817, o = 1.825, uniaxial negative, weakly pleochroic O > E, density: 3.95 g cm -3 , Mohs hardness: 3-3½;; space group R m with no superstructure observed, a = 6.834(1), c = 14.075(2) Å, Z = 3; six strongest XRD lines 5.466 (55), 4.702 (14), 2.764 (100), 2.266 (36), 1.820 (13), 1.709 (18).
No takes yet. Share an insight, caveat, or question.
Braithwaite et al. (2004) studied this question.