AbstractSamples 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).