Ronneburgite, K₂MnV₄O₁₂, a new mineral from Ronneburg, Thuringia, Germany: Description and crystal structure

Ronneburgite is a new chain vanadate mineral from Ronneburg, Thuringia, Germany. It occurs as reddish-brown crystals of equant, flattened, or short prismatic habit, up to 0.5 mm in length. The crystals have an adamantine luster and are translucent. Ronneburgite is biaxial negative with refractive indices n_α = 1.925(5), n_β = 1.960(10), n_γ = 1.988(4), and 2V_x = 82°. Electron microprobe analysis (K₂O = 16.93, MnO = 12.44, MgO = 0.62, V₂O₅ = 68.54; total = 98.53 wt%) gives the empirical formula K_1.91Mn_0.93Mg_0.08V_4.00O_11.96, which simplifies to K₂MnV₄O₁₂. Ronneburgite is mono-clinic, space group P2₁/n, with a = 8.183(3), b = 9.247(3), c = 8.651(2) Å, and β = 109.74(2)°. The measured density is 2.84 g/cm³; the calculated density for Z = 2 is 2.85 g/cm³. Single-crystal X-ray structure determination (R₁ = 0.046 for 1846 reflections) shows that ronneburgite contains infinite vierer-single metavanadate chains [(VO₃)_n]ⁿ⁻ of corner-sharing VO₄ tetrahedra along [101]. Although the empirical formula and calculated bond valences indicate that V is 5+ and Mn is 2+, the crystal color, a “poor” structural compatibility index, and Mn-O distances between those expected for Mn²⁺-O and Mn³⁺-O indicate that some Mn³⁺ and V⁴⁺ ions are present. K⁺ ions and octahedral Mnⁿ⁺ ions bridge between the metavanadate chains by coordinating to their oxygen atoms.