Hydroniumpharmacosiderite, a new member of the pharmacosiderite supergroup from Cornwall, UK : structure and description

Hydroniumpharmacosiderite, ideally (H 3O)Fe 4(AsO 4) 3(OH) 4.4H 2O, is a new mineral from Cornwall, UK, probably from the St. Day group of mines. It occurs as a single yellowish green, slightly elongated cube, measuring 0.17 mm × 0.14 mm × 0.14 mm. The mineral is transparent with a vitreous lustre. It is brittle with a cleavage on {001}, has an irregular fracture, a white streak and a Mohs hardness of 2-3 (determined on H 3O-exchanged pharmacosiderite). Hydroniumpharmacosiderite has a calculated density of 2.559 g cm -3 for the empirical formula. The empirical formula, based upon 20.5 oxygen atoms, is: [(H 3O) 0.50K 0.48Na 0.06] 1.04(Fe 3.79Al 0.22) 4.01[(As 2.73P 0.15) 2.88O 12](OH) 4"¢4H 2.14O. The five strongest lines in the X-ray powder diffraction pattern are [d obs(Å), I obs,(hkl)]: 8.050,100,(001); 3.265,35,(112); 2.412,30,(113); 2.830,23,(202); 4.628,22,(111). Hydroniumpharmacosiderite is cubic, space group P43m with a = 7.9587(2) Å, V = 504.11(2) Å 3 and Z = 1. The crystal structure was solved by direct methods and refined to R 1 = 0.0481 for 520 reflections with I > 2σ(I). The structure is consistent with determinations for H 3O-exhchanged pharmacosiderite and the general pharmacosiderite structure type.