Calderonite, a new lead-iron-vanadate of the brackebuschite group

Calderonite, ideally Pb 2 Fe 3 + (VO 4 ) 2 (OH), a new member of the brackebuschite group, has been found in the upper oxidation zone of two Pb-Zn hydrothermal deposits located at Santa Marta and Azuaga, Badajoz province, Spain. Brackebuschite and calderonite probably form a complete solid solution, locally with important substitution of Cu and Zn for Fe and Mn. The monoclinic cell parameters derived from powder X-ray diffraction (XRD) data are: a = 7.647(5) A, b = 6.094(1) A, c = 8.900(2) A, β = 112.0(2)° and V = 384.5(4) A 3 , and the six strongest lines, d-spacing (A),(I),(hkl), are: 4.893(4)(011), 4.166(3)(002), 3.242(10) (211), 3.058(3)(020), 2.980(5) (T03) and 2.746(5)(003). Electron microprobe analyses (EMPA) show a certain degree of compositional variation not only between the Santa Marta and Azuaga samples but also among grains from the same locality. A representative formula of Santa Marta calderonite, determined from EMPA, based on 9 O atoms: is (Pb 1 . 9 5 0 Ca 0 . 0 0 4 Ba 0 . 0 1 5 ) 1 . 9 6 9 (Fe 3 + 0.892Cu 0 . 0 5 9 Zn 0 . 0 0 8 Al 0 . 0 1 5 ) 0 . 9 7 4 (V 1 . 8 4 7 As 0 . 0 0 8 Si 0 . 0 3 9 P 0 . 0 5 7 ) 1 . 9 5 1 O 7 . 5 0 7 (OH) 1 . 4 9 3 Fe 3 + is principally substituted by Cu 2 + , and V 5 + by Si 4 + , ρ c = 6.05 g/cm 3 . The thermogravimetric analysis yields a weight loss of 1.91%, which corresponds to the 1.493 H needed to maintain the charge balance. Differential thermal analysis shows endothermic effects at 279 and 663 °C due to dehydroxylation. A single-crystal XRD refinement was carried out on a selected crystal with cationic content determined by EMPA and starting atomic positions from brackebuschite. Final R-value of 5.81% based on 952 reflections with I > 2σ 1 , assuming P2 1 /m symmetry, the cell dimensions are a = 7.649 A, b = 6.101 A, c = 8.904 A, β = 112.23°. Calderonite is red orange to red brown, semitransparent to translucent with vitreous luster and red streak and powder. The fracture is splintery. Optically, it is biaxial positive, with a 2V x = 86° and strong dispersion. In plane-polarized light, it is strongly pleochroic (X = light greenish brown, Y = brown, Z = reddish brown).