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

摘要

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)_(0.50)K_(0.48)Na_(0.06)]_(1.04)(Fe_(3.79)Al_(0.22))_(4.01)[(As_(2.73)P_(0.15))_(2.88)O_(12)](OH)_4·4H_(2.14)O. The five strongest lines in the X-ray powder diffraction pattern are [d_(obs)(A),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) A, V = 504.11(2) A~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 H3O-exhchanged pharmacosiderite and the general pharmacosiderite structure type.