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Mineralogical Magazine; October 2007; v. 71; no. 5; p. 553-569; DOI: 10.1180/minmag.2007.071.5.553
© 2007 Mineralogical Society of Great Britain and Ireland
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The crystal structures, solid solutions and infrared spectra of copiapite-group minerals

J. Majzlan* and R. Michallik

Institute of Mineralogy and Geochemistry, Albert-Ludwig University of Freiburg, Albertstraße 23b, D-79104 Freiburg, Germany

* E-mail: Juraj.Majzlan{at}minpet.uni-freiburg.de

Copiapite is a mineral of iron- and sulphate-rich acidic environments and has a general formula AFe43+(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 system crystallizes 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: copiapite, crystal structures, solid solutions, infrared spectra






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