Coulsellite, CaNa3AlMg3F14, a rhombohedral pyrochlore with 1:3 ordering in both A and B sites, from the Cleveland Mine, Tasmania, Australia

Coulsellite, CaNa 3 AlMg 3 F 14 , from the Cleveland tin mine at Luina, western Tasmania, has a rhombohedral distortion of the cubic pyrochlore A 2 B 2 X 6 Y structure, with a = 7.1756(1) A, α = 59.867(1)°, space group R 3 m , Z = 1. The corresponding hexagonal cell parameters are a = 7.1620(1) A, c = 17.5972(3) A. The crystals are multiply twinned about threefold axes of the pseudocubic cell. The structure was determined using X-ray data collected on a twinned crystal and refined to R obs = 0.027 for 452 observed reflections with I > 2σ( I ). The structure is possibly unique among published structures of pyrochlore-like minerals in having full 1:3 ordering of Ca:Na in the A sites and Al:Mg in the B sites. Transmission electron photomicrographs show a nanodomain structure due to twinning on a scale of ~5 nm.