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

摘要

Copiapite is a mineral of iron- and sulphate-rich acidic environments and has a general formula AFe_4~(3+)(So_4)_6(OH)_2(H_2O)_(20), where A = Fe~(2+), 2/3Fe~(3+), 2/3Al~(3+), 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 = Al~(3+), Fe~(2+) or Fe~(2+) and we designate it as the structural type AL. The other group consists of copiapite with A = Mg~(2+) Zn~(2+) or Ni~(2+)+ and this is the structural type MG. The solid-solution series between Fe~(3+) and Al~(3+) copiapite is continuous. The series between Mg~(2+)-Al~(3+), Mg~(2+)-Fe~(3+) and Mg~(2+)-Ar~(3+)-Fe~(3+) 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 Mg~(2+) and Zn~(2+) copiapite is continuous only at 25 deg C. At 75 deg 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-Fe~(2+) and Mg-Fe~(2+) copiapite are not continuous and show complex behaviour of the intermediate compositions.