Jukonite, a questionable hydrated Ca-Fe arsenate mineral from Grotta della Monaca (Sant'Agata d'Esaro, Cosenza, Italy)

We report the first Italian occurrence of yukonite, a rare hydrated arsenate of calcium and ferric iron, from Grotta della Monaca cave, S. Agata di Esaro, Cosenza, Italy. We have studied samples of cotype yukonite from the Daulton mine, Yukon, Canada, for comparison. At Grotta della Monaca, yukonite occurs in compact masses, dark reddish brown to brownish yellow in color. At the TEM scale, it consists of a chaotic assemblage of small grains (maximum dimension <20 nm) embedded in an amorphous matrix, occasionally admixed with scorodite. SAED patterns of these grains consist of weak and diffuse diffraction rings, indicating low crystallinity, and preventing any definition of symmetry or lattice parameters. Results of a combination of microanalytical, spectroscopic and thermogravimetric techniques provide chemical compositions: Ca1.76Fe2+0.09Fe3+3.12 [(As0.81Si0.10P0.09)O4]3(OH)3.76•4H2O for yukonite from Grotta della Monaca and Ca1.76Fe2+0.10Fe3+3.56[(As0.89Si0.08P0.03) O4]3(OH)5.16•3H2O for yukonite from the Daulton mine, close to the stoichiometry Ca2Fe3(AsO4)3(OH)4•4H2O. The thermogravimetric data indicate a total H2O content of ~17 wt%; FTIR data show that most of this occurs as H2O, but some OH could be present as well. The FTIR data show, in addition, the presence of ammonium and organic matter, suggesting that poorly crystalline organic matter is trapped in the material examined. Both chemical and XRD data point to a close relation with arseniosiderite [Ca2Fe3(AsO4)3O2•3H2O]. Both occur as oxidation products of As-bearing sulfides and seem to be composed of basically the same type of layers; arseniosiderite represents a fully crystalline substance, whereas yukonite is a semicrystalline material with a large variation in composition due to the admixture of amorphous material.