Santabarbaraite: a new amorphous phosphate mineral

Santabarbaraite is a new amorphous ferric iron hydroxy phosphate hydrate fromValdarno, Tuscany, Italy, the type locality, and Wannon Falls, Victoria, Australia. The mineral is the result of in situ oxidation of vivianite, occurring as pseudomorphs after vivianite crystals that are up to 3 mm across in concretionary nodules at the type locality and somewhat larger at Wannon Falls. Santabarbaraite is brown to light-brown in hand specimen, but appears yellowish amber under the microscope and has a similar streak. It is translucent with a distinct vitreous to greasy lustre and is nonfluorescent. It is brittle with a distinct parting along the perfect cleavage of vivianite. The hardness was not measured but the measured density of the type material is 2.42 g/cm3. Optically isotropic, the mineral has a refractive index of 1.695(5). The empirical formula for the type material is (Fe2.64Mn0.13Mg0.07) (PO4)2(OH)2.45 · 5.1H2O, giving a simplified formula of Fe3+3(PO4)2(OH)3·5H2O. Santabarbaraite from both localities was analysed by several techniques of X-ray absorption (XANES and EXAFS) and infrared spectroscopy, as well as DTA, DTG and TG. The results show that all the Fe in santabarbaraite is trivalent, associated with the presence of both H2O and hydroxyl. This is consistent with an oxidation series from vivianite through metavivianite to santabarbaraite, involving progressive oxidation of Fe2+ accompanied by conversion of H2O ligands to OH ions. Such a process leads to a gradual collapse of the vivianite structure as hydrogen bonds are eliminated. Santabarbaraite is the end product of this process and can be thus considered the phosphorus analogue of ferrisymplesite.