Synthesis and stability of some members of the pharmacosiderite group, AFe4(OH)4(AsO4)3·nH2O (A = K, Na, 0.5Ba, 0.5Sr)

Abstract

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).

Crystal chemistry of ivanyukite-group minerals, A3–xH1+x[Ti4O4(SiO4)3](H2O)n (A = Na, K, Cu), (n = 6–9, x = 0–2): crystal structures, ion-exchange, chemical evolution

“…The MT materials based upon the pharmacosiderite framework topology (Behrens et al, 1998a) possess outstanding ion-exchange properties (Majzlan et al, 2019). Chapman & Roe (1990) first synthesized the analogue of ivanyukite-K and reported on the formation of its H-and Cs-exchanged phases.…”

Secondary uranyl arsenates–phosphates and Sb–Bi-rich minerals of the segnitite–philipsbornite series in the oxidation zone at the Prakovce-Zimná Voda REE–U–Au quartz-vein mineralisation, Western Carpathians, Slovakia

“…Detailed investigations of localities with secondary ferric arsenates (e.g. Sejkora et al , 2006) show that minerals of the pharmacosiderite supergroup occur at these sites as a minor, generally inconspicuous, phase and that at some localities, pharmacosiderite can be an important, or the dominant, carrier of As (Drahota et al , 2009, Majzlan et al , 2019). Such occurrences seem to be linked to slow weathering under near-neutral conditions.…”

“…Such occurrences seem to be linked to slow weathering under near-neutral conditions. Hence, even at sites where substantial acidity is developed, pharmacosiderite could crystallise in microenvironments that are able to neutralise the acidity (Majzlan et al , 2019). In the presence of Ba, formation of bariopharmacosiderite is favoured and seems to persist even under long-lasting circumneutral conditions (Herrmann et al , 2018).…”

Microwave-Assisted Synthesis of Titanosilicates Using a Precursor Produced from Titanium Ore Concentrate

“…Other interesting examples of materials with useful applications in the field of sorption are the synthetic analog of the sitinakite mineral [14,15]-CTS, the alkali cation analogs of the pharmacosiderite mineral (KFe 4 (AsO 4 )(OH) 4 •7H 2 O) [16,17]-GTS-1 [18,19], and the ivanyukite mineral [20,21]-SIV [18,[22][23][24]. All these synthetic materials have attracted interest in the fields of catalysis [25,26], adsorption [27], separation and ion exchange [15,20,[28][29][30][31] due to their branched channel system in the crystal structure with sufficiently wide internal channels (Figure 1).…”