Abstract

Plumbopharmacosiderite, Pb0.5Fe3+4(AsO4)3(OH)4·5H2O, is a new hydrated and hydroxylated arsenate of Fe3+ and Pb belonging to the pharmacosiderite group, pharmacosiderite supergroup of minerals. It was found at the Monte Falò Pb-Zn mine near the village of Coiromonte in the Armeno municipality, Novara province, Italy. The mineral occurs on the surfaces of brittle fissures in a mica schist cross-cut by arsenopyrite veins as minute cubes up to 50 μm in size. Plumbopharmacosiderite is a product of oxidation of primary Pb-, Fe-sulfide minerals due to weathering of the primary ore and is found in close association with arsenopyrite, scorodite, beudantite, rare segnitite, marcasite, galena, sphalerite, and mimetite. Its color is pale green to yellowish-green, transparent with a vitreous to resinous luster. The streak is white. Plumbopharmacosiderite is brittle with an irregular fracture. The Mohs hardness is 2.5–3 and the calculated density is 2.89 g/cm3. The mineral is optically isotropic with refractive index n = 1.73(1). Plumbopharmacosiderite is non-fluorescent under 254 nm (short wave) and 366 nm (long wave) UV light. The empirical formula is: (Pb0.42K0.20Ba0.15Na0.03Ca0.01)Σ0.81(Fe3+3.69Al0.22)Σ3.91(As2.95Si0.01)Σ2.96O12(OH)Σ3.90·5H2O. The simplified formula is Pb0.5Fe3+4(AsO4)3(OH)4·5H2O. Plumbopharmacosiderite is cubic, with space group forumla and unit-cell parameters a 7.9791 (2) Å and V 508.00(6) Å3 for Z = 1. Its crystal structure was refined and conforms with that of bariopharmacosiderite. The new mineral and the name plumbopharmacosiderite were approved by the Commission on New Minerals, Nomenclature, and Classification (CNMNC) of the International Mineralogical Association (IMA) under the number IMA 2016-109.

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