Townendite, Na8ZrSi6O18, a new uranium-bearing lovozerite group mineral from the Ilímaussaq alkaline complex, Southern Greenland

Townendite, a new U-bearing mineral belonging to the trigonal lovozerite group has been found in the lujavrite layers of nepheline syenites in the Ilimaussaq alkaline complex in Southern Greenland. Its composition from electron microprobe analysis is Na6.08Ca0.16Mn0.17Fe0.13Zr0.67Y0.13Sn0.04U0.02Ti0.01Si6 O16.35(OH)1.65. Townendite has rhombohedral symmetry, R 3 m , with cell parameters (hexagonal setting) a = 10.345(2) A, c = 13.103(2) A. The structure has been refined using single-crystal X-ray data to R 1 = 0.056 for 532 reflections with F > 4σ( F ). Lovozerite group minerals are cyclosilicates in which Si6(O,OH)18 rings of tetrahedra share corners with isolated MO6 octahedra (M = Zr, Ti, Ca) to form a 3D framework, with large cations, mainly Na+, located in cavities in the framework. Townendite differs from other compositionally related lovozerite group minerals in having a high occupancy (92%) of another set of octahedrally coordinated sites that share faces with the MO6 octahedra, giving linear face-shared trimers parallel to [001]. These sites are occupied predominantly by Na+. Full occupancy of all sites corresponds to the ideal composition Na8ZrSi6O18. Townendite is a significant contributor, along with steenstrupine-(Ce), to U resources in the Ilimaussaq alkaline complex.