Crystal Chemistry of Alkali–Aluminum–Iron Sulfates from the Burnt Mine Dumps of the Chelyabinsk Coal Basin, South Urals, Russia

Technogenic steklite, KAl(SO₄)₂, and unnamed mineral phase (K,Na)₃Na₃(Fe,Al)₂(SO₄)₆ from burnt dumps of the Chelyabinsk Coal Basin have been investigated by single-crystal X-ray diffraction and electron microprobe analysis. Steklite is trigonal, space group <i>P</i><inline-formula><math display="inline"><semantics><mover accent="true"><mn>3</mn><mo>¯</mo></mover></semantics></math></inline-formula>, <i>a</i> = 4.7277(3), <i>c</i> = 7.9871(5) Å, <i>V</i> = 154.60(2) ų. The crystal structure was refined to <i>R</i>₁ = 0.026 (<i>wR</i>₂ = 0.068). It is based upon the [Al(SO₄)₂]⁻ layers formed by corner sharing of SO₄ tetrahedra and AlO₆ polyhedra. The anionic [Al(SO₄)₂]⁻ layers are parallel to the (001) plane and linked via interlayer K⁺ ions. The regular octahedral coordination of Al is observed that distinguishes technogenic steklite from that found in Tolbachik fumaroles. The (K,Na)₃Na₃(Fe,Al)₂(SO₄)₆ phase is trigonal, space group <i>R</i><inline-formula><math display="inline"><semantics><mover accent="true"><mn>3</mn><mo>¯</mo></mover></semantics></math></inline-formula>, <i>a</i> = 13.932(2), <i>c</i> = 17.992(2) Å, <i>V</i> = 3024.4(7) ų, <i>R</i>₁ = 0.073 (<i>wR</i>₂ = 0.108). The crystal structure is based upon the anionic chains [(Fe,Al)(SO₄)₃]³⁻ running parallel to the <i>c</i> axis and interconnected via K⁺ and Na⁺ ions. There are no known minerals or synthetic compounds isotypic to (K,Na)₃Na₃(Fe,Al)₂(SO₄)₆, due to the presence of separate K and Na sites in its structure.