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Mineralogical Magazine; June 2004; v. 68; no. 3; p. 527-539; DOI: 10.1180/0026461046830204
© 2004 Mineralogical Society of Great Britain and Ireland
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Herbertsmithite, Cu3Zn(OH)6Cl2, a new species, and the definition of paratacamite

R. S. W. Braithwaite1, K. Mereiter2, W. H. Paar3,* and A. M. Clark4

1 Chemistry Department, University of Manchester Institute of Science and Technology, Manchester M60 1QD, UK
2 Institut für Mineralogie, Kristallographie und Strukturchemie der Technischen Universität Wien, Getriedemarkt 9, A-1060 Wien, Austria
3 Department Geography, Geology and Mineralogy, Division Mineralogy and Material Sciences, Universität Salzburg, Hellbrunnerstrasse 34, A-5020 Salzburg, Austria
4 Mineralogy Department, The Natural History Museum, Cromwell Road, London SW7 5BD, UK

* E-mail: werner.paar{at}sbg.ac.at

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 Zn2+, and also Ni2+, Co2+, Fe2+, Cd2+ and Mg2+. The crystal symmetry changes to rhombohedral when ~1/3of 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 Cu3Zn(OH)6Cl2, 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, {varepsilon} = 1.828–1.830, {omega} = 1.835, uniaxial negative, weakly pleochroic O > E, density: 3.75 g cm–3, Mohs hardness: 3–33; space group R, a = 13.654(5), c = 14.041(6) Å, Z = 24, with pronounced Rm 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, {varepsilon} = 1.817, {omega} = 1.825, uniaxial negative, weakly pleochroic O > E, density: 3.95 g cm–3, Mohs hardness: 3–33; space group Rm 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).

KEYWORDS: herbertsmithite, paratacamite, anarakite, solid solution, site occupancy and selectivity, new mineral, IR spectroscopy, XRD, Cu chlorides, Anarak, Iran, Sierra Gorda, Chile




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