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The crystal structures, solid solutions and infrared spectra of copiapite-group minerals

Published online by Cambridge University Press:  05 July 2018

J. Majzlan  and
R. Michallik
J. Majzlan*
Affiliation:
Institute of Mineralogy and Geochemistry, Albert-Ludwig University of Freiburg, Albertstraße 23b, D-79104 Freiburg, Germany
R. Michallik
Affiliation:
Institute of Mineralogy and Geochemistry, Albert-Ludwig University of Freiburg, Albertstraße 23b, D-79104 Freiburg, Germany
*
E-mail: [email protected]
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Abstract

Copiapite is a mineral of iron- and sulphate-rich acidic environments and has a general formula AFe3+4 (SO4)6(OH)2(H2O)20, where A = Fe2+, 2/3Fe3+, 2/3Al3+, Mg, Zn. The structure is built by infinite tetrahedral-octahedral chains and isolated octahedrally coordinated A sites. Our synthetic and natural copiapite samples can be divided into two large groups based on the orientation of the structural fragments. One group comprises copiapite phases where A = Al3+, Fe2+ or Fe3+ and we designate it as the structural type AL. The other group consists of copiapite with A = Mg2+, Zn2+ or Ni2+ and this is the structural type MG. The solid-solution series between Fe3+ and Al3+ copiapite is continuous. The series between Mg2+-Al3+, Mg2+-Fe3+ and Mg2+-Al3+-Fe3+ copiapite are not continuous; the samples with intermediate compositions contain two copiapite phases, one of the type AL and one of the type MG. The series between Mg2+ and Zn2+ copiapite is continuous only at 25°C. At 75°C, the Zn-rich portion of this systemcrystallizes a copiapite-like phase whose structure may be a superstructure of copiapite. The series between Al-Fe2+ and Mg-Fe2+ copiapite are not continuous and show complex behaviour of the intermediate compositions.

Keywords

copiapitecrystal structuressolid solutionsinfrared spectra
Type
Research Article
Information
Mineralogical Magazine , Volume 71 , Issue 5 , October 2007 , pp. 553 - 569
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2007

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