Samples of the pharmacosiderite group were synthesized either directly, from aqueous solutions at 160 °C, or by ion exchange over extended periods of time at 100 °C. In more than 200 experiments, no pure pharmacosiderite sample was obtained, and a protocol was developed to remove scorodite and arsenical iron oxides from the samples. In this way, K-, Na-, Ba-, and Sr-dominant pharmacosiderite samples were prepared. The chemical compositions of the two samples used for further experiments were Ba0.702Fe4[(AsO4)0.953(SO4)0.047]3(OH)3.455O0.545·5.647H2O and K1.086Fe4[(AsO4)0.953(SO4)0.047]3 (OH)3.772O0.228·4.432H2O. The Ba-dominant pharmacosiderite is tetragonal at room temperature, and the K-dominant pharmacosiderite is cubic. Upon heating, both samples lose zeolitic H2O (shown by thermogravimetry), and this loss is accompanied by unit-cell contraction. In Ba-dominant pharmacosiderite, this loss also seems to be responsible for a symmetry change from tetragonal to cubic. The slight unit-cell contraction in Ba-dominant pharmacosiderite at <100 °C might be attributed to either negative thermal expansion or minor H2O loss; our data cannot differentiate between these two possibilities. Both samples persisted in a crystalline state up to 320 °C (the highest temperature of the powder XRD experiment), showing that pharmacosiderite is able to tolerate almost complete removal of the zeolitic H2O molecules. Low-temperature heat capacity measurements show a diffuse magnetic anomaly for K-dominant pharmacosiderite at ≈5 K and a sharp lambda transition for Ba-dominant pharmacosiderite at 15.2 K. The calculated standard entropy at T = 298.15 is 816.9 ± 5.7 J/molK for K-dominant pharmacosiderite (molecular mass 824.2076 g/mol, see formula above) and 814.1 ± 5.5 J/molK for Ba-dominant pharmacosiderite (899.7194 g/mol).

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Abstract. The sulfidic waste dumps of the historical mining sites Giftkies and Kaňk (Czech Republic) have been exposed to a temperate climate over decades. This exposure generated low-pH conditions caused by metal sulfide decomposition. Tin sulfides of the stannite–kësterite series [Cu2(Fe,Zn)SnS4] are common Sn minerals in the ores at the investigated sites. They decompose under acidic and oxidizing conditions and form in situ secondary precipitates. Compositional analyses of primary and secondary minerals were collected by electron microprobe to track the environmental mobility of the released elements during weathering. Transmission electron microscopy revealed a diffusion-driven alteration of stannite to Sn-rich chalcopyrite and the precipitation of native copper and silver from stannite. In assemblages containing arsenopyrite, an in situ and amorphous Sn–Fe–As (SFA)-rich phase precipitated close to the Sn sulfide. The SFA precipitate contains very little sulfur, which was probably released to the aqueous phase as oxidized species, whereas small amounts of Cu and Zn were captured by the SFA. This precipitate is metastable and acts as a temporaneous sink for mobile elements (Cu, Zn) and elements derived from acid-soluble silicates and phosphates (Ca, Si, Al, and P). With advanced weathering, complex redox reactions result in the precipitation of magnetite as an oxidation product of the sulfidic material under oxidative conditions. The stable minerals goethite and cassiterite mark the end of the weathering sequence and crystallized from the amorphous SFA precipitate.

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