Hydroniumpharmacoalumite, (H3O)Al4[(OH)4(AsO4)3] · 4-5 H2O, a new mineral of the pharmacosiderite supergroup from Rodalquilar, Spain
Hochleitner, Rupert; Fehr, Karl T.; Kaliwoda, Melanie; Günther, Amanda; Rewitzer, Christian; Schmahl, Wolfgang W.; Park, Sohyun
published: Apr 1, 2015
ArtNo. ESP154019202005, Price: 29.00 €
Hydroniumpharmacoalumite, ideally (H3O)Al4[(OH)4(AsO4)3] · 4-5 H2O, is a new mineral from the Maria Josefa gold mine, Rodalquilar, Andalusia region, Spain. The mineral formed as an oxidised, secondary alteration product of arsenic-bearing ore minerals in fractures of an alunitised gold-bearing volcanic rock. It occurs as colourless to white, intergrown cubic crystals with natropharmacoalumite, alunite, jarosite, scorodite, native gold and goethite. Individual crystals are up to 0.1 mm on edge, although most crystals are smaller. The mineral is transparent with a vitreous to adamantine lustre and a white streak. It is brittle with an irregular fracture, cleavage could not be observed. The Mohs hardness is ∼2.5 with a calculated density of 2.486 g cm 3 for the empirical formula. Electron microprobe analyses yielded Na2O 0.43 %, K2O 0.10 %, Al2O3 30.50 %, Fe2O3 0.36 %, As2O5 52,01 % and H2O was calculated as 16.60 % totalling 100 %. The empirical formula (based on 3 As apfu and with H3O+Na+K = 1.00) is [(H3O)0.90Na0.09K0.01]Σ1.00(Al3.97Fe0.03)Σ4.00(AsO4)3(OH)4 · 2.75H2O. The five strongest lines in the X-ray powder diffraction pattern are [dobs(Å),Iobs,(hkl)]: 7.727,100,(100); 4.461,10,(111); 3.863,40,(200); 2.732,12,(220); 1.932,16,(400). Hydroniumpharmacoalumite is cubic, space group P-43 m, with a = 7.7269(2) Å, V = 461.33(4) Å and Z = 1. The structure is consistent with the general pharmacosiderite structure type, with hydronium (oxonium) as the dominant cation in cavities of strongly distorted Al octahedra and As tetrahedra and especially with the structure of the iron-analogue hydroniumpharmacosiderite (Mills et al. 2010a, b).