Oxycalcioroméite, Ca₂Sb₂O₆O, from Buca della Vena mine, Apuan Alps, Tuscany, Italy: a new member of the pyrochlore supergroup

Abstract

Abstract The new mineral species oxycalcioroméite, Ca 2 Sb 5+ 2 O 6 O, has been discovered at the Buca della Vena mine, Stazzema, Apuan Alps, Tuscany, Italy. It occurs as euhedral octahedra, up to 0.1 mm in size, embedded in dolostone lenses in the baryte + pyrite + iron oxides ore. Associated minerals are calcite, cinnabar, derbylite, dolomite, hematite, 'mica', pyrite, sphalerite and 'tourmaline'. Oxycalcioroméite is reddish-brown in colour and transparent. It is isotropic, with n calc = 1.950. Electron microprobe analysis gave (wt.%; n = 6) Sb 2 O 5 63.73, TiO 2 3.53, SnO 2 0.28, Sb 2 O 3 10.93, V 2 O 3 0.68, Al 2 O 3 0.28, PbO 0.68, FeO 5.52, MnO 0.13, CaO 13.68, Na 2 O 0.83, F 1.20, O = F – 0.51, total 100.96. No H 2 O, above the detection limit, was indicated by either infrared or micro-Raman spectroscopies. The empirical formula, based on 2 cations at the B site, is (Ca 1.073 Fe 2+ 0.338 Sb 3+ 0.330 Na 0.118 Pb 0.013 Mn 0.008 ) Σ=1.880 (Sb 5+ 1.73 4Ti 0.19 4V 0.040 Al 0.024 Sn 0.008 ) Σ=2.000 (O 6.682 F 0.278 ) Σ6.960 . The crystal structure study gives a cubic unit cell, space group Fd m , with a 10.3042(7) Å, V 1094.06(13) Å 3 , Z = 8. The five strongest X-ray powder diffraction lines are [ d (Å) I (visually estimated) (hkl) ]: 3.105(m)(311); 2.977(s)(222); 2.576(m)(400); 1.824(ms)(440); and 1.556(ms)(622). The crystal structure of oxycalcioroméite has been solved by X-ray single-crystal study on the basis of 114 observed reflections, with a final R 1 = 0.0114. It agrees with the general features of the members of the pyrochlore supergroup.