Ronneburgite, K (sub 2) MnV (sub 4) O (sub 12) , a new mineral from Ronneberg, 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 (sub alpha ) = 1.925(5), n (sub beta ) = 1.960(10), n (sub gamma ) = 1.988(4), and 2V (sub x) = 82 degrees . Electron microprobe analysis (K (sub 2) O = 16.93, MnO = 12.44, MgO = 0.62, V (sub 2) O (sub 5) = 68.54; total = 98.53 wt%) gives the empirical formula K (sub 1.91) Mn (sub 0.93) Mg (sub 0.08) V (sub 4.00) O (sub 11.96) , which simplifies to K (sub 2) MnV (sub 4) O (sub 12) . Ronneburgite is monoclinic, space group P2 (sub 1) /n, with a = 8.183(3), b = 9.247(3), c = 8.651(2) Aa, and beta = 109.74(2) degrees . The measured density is 2.84 g/cm (super 3) ; the calculated density for Z = 2 is 2.85 g/cm (super 3) . Single-crystal X-ray structure determination (R (sub 1) = 0.046 for 1846 reflections) shows that ronneburgite contains infinite vierer-single metavanadate chains [(VO (sub 3) ) (sub n) ] (super n-) of corner-sharing VO (sub 4) 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 Mn (super 2+) -O and Mn (super 3+) -O indicate that some Mn (super 3+) and V (super 4+) ions are present. K+ ions and octahedral Mn (super n+) ions bridge between the metavanadate chains by coordinating to their oxygen atoms.