Abstract

Calderonite, ideally Pb2Fe3+(VO4)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) Å, b = 6.094(1) Å, c = 8.900(2) Å, β = 112.0(2)° and V = 384.5(4) Å3, and the six strongest lines, d-spacing (Å),(I),(hkl), are: 4.893(4)(011), 4.166(3)(002), 3.242(10) (2̅11), 3.058(3)(020), 2.980(5) (1̅03) 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 (Pb1.950Ca0.004Ba0.015)1.969(Fe3+0.892Cu0.059Zn0.008Al0.015)0.974 (V1.847As0.008Si0.039P0.057)1.951O7.507(OH)1.493. Fe3+ is principally substituted by Cu2+, and V5+ by Si4+. ρc = 6.05 g/cm3. 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 > 2σI, assuming P21/m symmetry, the cell dimensions are a = 7.649 Å, b = 6.101 Å, c = 8.904 Å, 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 2VX = 86° and strong dispersion. In plane-polarized light, it is strongly pleochroic (X = light greenish brown, Y = brown, Z = reddish brown).

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