Coulsellite, CaNa3AlMg3F14, from the Cleveland tin mine at Luina, western Tasmania, has a rhombohedral distortion of the cubic pyrochlore A2B2X6Y structure, with a = 7.1756(1) Å, α = 59.867(1)°, space group R3̅m, Z = 1. The corresponding hexagonal cell parameters are a = 7.1620(1) Å, c = 17.5972(3) Å. 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 Robs = 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.