Ilimaussite, a new rare earth-niobium-barium silicate from Ilímaussaq South Greenland

Abstract

A new mineral - Na₄Ba₂CeFeNb₂Si₈O₂₈.5H₂O - found in a hydrothermal vein in nepheline-sodalite syenite (naujaite) of the llímaussaq alkaline massif is described. The mineral is named ilimaussite after the locality where it was found. It forms brownish-yellow lamellae and is associated with ussingite, chkalovite and epistolite. The specific gravity is 3.6. The mineral is optically uniaxial positive; n_e = 1.695, n_o = 1.689. The mineral is hexagonal, a_o = 10.80, c_o = 20.31, V_o = 2052 ų, Z = 3. The chemical composition is: SiO₂ - 31.28, TiO₂ - 1.64, Nb₂O₅ - 13.20, TR₂O₃ - 10.60, Fe₂O₃ - 3.18, BaO - 23.62, Na₂O - 7.00, K₂O - 3.80, loss by ignition - 6.43; total 100. 75.