Petterdite, the Cr-dominant analogue of dundasite, a new mineral species from Dundas, Tasmania, Australia and Callenberg, Saxony, Germany

Petterdite is a newly discovered hydrated lead chromium hydroxyl carbonate, the Cr3+-dominant analogue of dundasite, with a formula PbCr2(CO3)(2)(OH)(4).H2O. The type locality is the Red Lead mine, on the Zeehan-Dundas mining field in northwestern Tasmania, Australia. The mineral also occurs in small amounts at the Callenberg nickel deposit, Sawony, Germany. Petterdite forms thin crusts made up of tiny platy crystals up to about 15 mum across, associated with crocoite and anglesite at the type locality, and with crocoite, cerussite, bindheimite, pyromorphite, and relict galena at Callenberg. The petterdite crusts are pale greyish to pinkish violet and non-fluorescent; they have an earthy to pearly luster. The streak is pale violet, and the Mohs hardness is estimated to be about 2. Crystals are poorly developed, flattened on {010}, and slightly elongate along [001] or, less commonly, [100]. Most crystal terminations, in the direction of [001], have a frayed appearance. Cleavage is fair parallel to {100}, and possibly also parallel to (010). Density could not be measured. Petterdite is biaxial negative, alpha 1.704(5), beta approximate to 1.802, and gamma 1.842(5). The 2V(calc.), is close to 62 degrees, and the optical orientation is X = a, Y = b, Z = c, with pleochroism X = Y colorless to pale greyish pink, Z greyish pink. Chemical analysis by a combination of electron microprobe and CHN gave (wt%) PbO 43.33, SrO 1.40, Cr2O3 22.64, Al2O3 3.65, Sb2O5 0.67, CO2 18.3, H2O (by difference) 10.01, total 100.00 wt%. The simplified formula is PbCr23+(CO3)(2)(OH)(4).H2O. Petterdite is orthorhombic, with unit-cell parameters a 9.079(3). b 16.321 (9), c 5.786(7) Angstrom, V857(1) Angstrom (3). With Z = 4, the calculated density is 3.95 g/cm(3). The strongest six lines in the X-ray powder-diffraction pattern [d(obs)(Angstrom)(I-obs)hkl] are: 7.937(100)(110), 4.686(50b)(021,111), 3.633(70)(131), 3.270(40)(221), 2.718(40)(022,060), 2.6909(40)(241,301). The crystal structure has not been determined owing to the small size of the crystals. Petterdite is assumed to have space group Pbnm by analogy with dundasite, Both minerals have similar optical and physical properties. They are also isostructural with dresserite and strontiodresserite. Petterdite is a supergene mineral formed by the alteration of galena by solutions containing chromium and carbonate. The mineral is named after William Frederick Petterd (1849-1910) in recognition of his work on the mineralogy of Tasmania.