Tychite, Na₆Mg₂(SO₄)(CO₃) ₄: Structure analysis and Raman spectroscopic data

Tychite, hexasodium dimagnesium sulfate tetracarbonate, is a member of the northupite [Na₃Mg(CO₃)₂Cl] group and has a framework structure consisting of four basic units: CO₃^2- groups (3 symmetry), SO₄^2- tetrahedra (23 symmetry), MgO₆ (3 symmetry) and NaO₆ octahedra (2 symmetry). Among them, the CO₃^2-, SO₄^2-, and MgO ₆ units are regular in shape, but the NaO₆ octahedron, showing three different Na-O bond lengths, is considerably distorted. The Na atoms in tychite are slightly overbonded relative to those in northupite. The Raman spectroscopic data show three and four vibration modes for the CO ₃^2- and SO₄^2- groups, respectively; these are comparable to the corresponding Raman modes observed in other carbonates and sulfates.