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Herbertsmithite, Cu3Zn(OH)6Cl2, a new species, and the definition of paratacamite

Published online by Cambridge University Press:  05 July 2018

R. S. W. Braithwaite ,
K. Mereiter ,
W. H. Paar  and
A. M. Clark
R. S. W. Braithwaite
Affiliation:
Chemistry Department, University of Manchester Institute of Science and Technology, Manchester M60 1QD, UK
K. Mereiter
Affiliation:
Institut für Mineralogie, Kristallographie und Strukturchemie der Technischen Universität Wien, Getriedemarkt 9, A-1060 Wien, Austria
W. H. Paar*
Affiliation:
Department Geography, Geology and Mineralogy, Division Mineralogy and Material Sciences, Universität Salzburg, Hellbrunnerstrasse 34, A-5020 Salzburg, Austria
A. M. Clark
Affiliation:
Mineralogy Department, The Natural History Museum, Cromwell Road, London SW7 5BD, UK
*
*E-mail: werner.paar@sbg.ac.at
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Abstract

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 ∼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 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, 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 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, e = 1.817, o = 1.825, uniaxial negative, weakly pleochroic O > E, density: 3.95 g cm-3, Mohs hardness: 3-3½;; 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

herbertsmithiteparatacamiteanarakitesolid solutionsite occupancy and selectivitynew mineralIR spectroscopyXRDCu chloridesAnarakIranSierra GordaChile
Type
Research Article
Information
Mineralogical Magazine , Volume 68 , Issue 3 , June 2004 , pp. 527 - 539
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2004

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