Crystal structure of Na₃Fe(SO₄)₃: A high-temperature product (∼400 °C) of sideronatrite [Na₂Fe(SO₄)₂OH⋅3H₂O]

Abstract

The iron sulfate Na₃Fe(SO₄)₃ studied here has been obtained as a high-temperature (HT) product (-400 °C) from the thermal decomposition of sideronatrite from Sierra Gorda (Chile) having composition Na₂Fe(SO₄)₂(OH)⋅3H₂O. The structure determination was carried out using synchrotron X‑ray powder diffraction. Structural data refined by the Rietveld method, up to R_p = 11.95%, are: space group R3̅, lattice parameters a = b = 13.6231(1) Å and c = 9.0698(1) Å, V = 1457.76(2) ų, and Z = 6. The structure of Na₃Fe(SO₄)₃ can be described in terms of FeO₆ octahedra connected to sulfate tetrahedra by corner-sharing to form infinite chains [Fe(SO₄)₃]_∞, running along c. These chains are joined together by Na atoms to build up a three-dimensional network of strong (Fe-O-S) and weak (Na-O) bonds. The topological relationships of Na₃Fe(SO₄)₃ to the structure of some analog minerals are also discussed.