Hydroxyferroroméite, a new secondary weathering mineral from Oms, France

Hydroxyferroroméite, ideally (Fe²⁺_1.5☐_0.5)Sb⁵⁺₂O₆(OH), is a new secondary mineral from the Correc d’en Llinassos, Oms, Pyrénées-Orientales Department, France. Hydroxyferroroméite occurs as yellow to yellow-brown powdery boxwork replacements up to about 50 μm across after tetrahedrite in a siderite–quartz matrix. No distinct crystals have been observed. The empirical formula (based on 7 (O + OH) per formula unit, pfu) is (Fe²⁺_1.07Cu²⁺_0.50Zn_0.03Sr_0.03Ca_0.01☐_0.36)_Ʃ2(Sb⁵⁺_1.88Si_0.09Al_0.02As_0.01)_Ʃ2O₆((OH)_0.86O_0.14). X-ray photoelectron spectroscopy was used to determine the valence states of Sb, Fe and Cu. Hydroxyferroroméite crystallises in the space group $Fd3¯m$ with the pyrochlore structure and hence is a new Fe²⁺-dominant member of the roméite group of the pyrochlore supergroup. It has the unit-cell parameters: a = 10.25(3) Å, V = 1077(6) ų and Z = 8. A model, based on bond-valence theory, for incorporation of the small Fe²⁺ cation into a displaced variant of the A site of the pyrochlore structure is proposed.