Crystal-chemical aspects of the Roméite group, A₂Sb₂O₆Y, of the pyrochlore supergroup

Four specimens of the roméite-group minerals oxyplumboroméite and fluorcalcioroméite from the Långban Mn-Fe deposit in Central Sweden were structurally and chemically characterized by single-crystal X-ray diffraction, electron microprobe analysis and infrared spectroscopy. The data obtained and those on additional roméite samples from literature show that the main structural variations within the roméite group are related to variations in the content of Pb²⁺ which is incorporated into the roméite structure via the substitution Pb²⁺ → A²⁺ where A²⁺ = Ca, Mn and Sr. Additionally, the cation occupancy at the six-fold coordinated B site, which is associated with the heterovalent substitution ^BFe³⁺ + ^Y□ → ^BSb⁵⁺ + ^YO² can strongly affect structural parameters. Chemical formulae of the roméite minerals group are discussed. According to crystal-chemical information, the species associated with the name 'kenoplumboroméite', hydroxycalcioroméite and fluorcalcioroméite most closely approximate end-member compositions Pb₂(SbFe³⁺)O₆□, Ca₂(Sb⁵⁺Ti) O₆(OH) and (CaNa)Sb₂O₆F, respectively. However, in accord with pyrochlore nomenclature rules, their names correspond to multiple end-members and are best described by the general formulae: (Pb,#)₂(Sb,#)₂O₆□, (Ca,#)₂(Sb,#)₂O₆(OH) and (Ca,#)Sb₂(O,#)₆F, where '#' indicates an unspecified charge-balancing chemical substituent, including vacancies.