Tristramite, a new mineral of the rhabdophane group, has the composition (Ca0.54U4+
0.17)Σ1.00 [(PO4)0.79(SO4)0.12(CO3)0.07]Σ0.98·1.77H2O. It occurs as a late-stage replacement or matrix to brecciated uraninite (var. pitchblende) associated with sulphides in hydrothermal veins related to Hercynian granites in south-west England. It is hexagonal, space group P6222, with a 6.913 ± 0.003 Å and c 6.422 ± 0.006 Å. The strongest lines of the indexed powder pattern are 2.99 (100), 2.83 (100), 2.14 (50), 1.850 (50), 5.99 (40), 4.37 (40), 3.46 (30). For comparison, new indexed powder data for rhabdophane from Fowey Consols, Cornwall, are included. The mineral is pale yellow to greenish yellow, uniaxial positive with ω 1.644 and ε 1.664, and does not fluoresce in either short-or long-wave ultraviolet light. The habit is acicular or fibrous and no cleavage has been observed. Density (g/cm3) 3.8–4.2 (meas.), 4.18 (calc.).