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

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.

A comprehensive spectroscopic study of synthetic Fe2+, Fe3+, Mg2+ and Al3+ copiapite by Raman, XRD, LIBS, MIR and vis–NIR

“…This similarity was also observed by Majzlan and Michallik. [41] A broad H 2 O absorption band appears in the 2800-3600 cm −1 spectral range (Fig. 5(a)).…”

“…This broad band was also reported by Majzlan and Michallik. [41] Adjacent to the broad band, a sharp (OH) − stretching peak appears for all copiapite samples, as marked by the dash line in Fig. 5(a).…”

“…[41] In a mid-IR emissivity study of ferricopiapite, however, the water bending mode was observed at 1649 and 1444 cm −1 . [63] As indicated by the dashed line in Fig.…”

“…The mid-IR absorbance study by Majzlan and Michallik reported the ν 3 peak positions at 1061, 1081, 1135 and 1223 cm −1 . [41] The thermal emission spectra of ferricopiapite show ν 3 peaks at 1220, ∼1116 and 1050 cm −1 . [63] A mid-IR reflection study of copiapite-group samples finds the ν 3 peaks at positions similar to that found in the thermal emission study.…”

“…[40,41] Supersaturated aqueous solutions were prepared by mixing the required reagents with diluted H 2 SO 4 that controls the pH values. The pH value of the solution was measured using a pH meter with a resolution of 0.1 pH units.…”