Understanding the structure and stability of mineral phases which contain harmful elements, such as U and As, is vital to determine the likelihood of toxic element release into the environment. This proposal investigates a family of minerals called metatobernite we identified at an abandoned mine site in Cornwall. Metatorbernite has the ideal chemical formula Cu(UO)2(PO4)2.8H2)O, we found the minerals also contained As and have the chemical formula Cu(UO)2(PO4)0.5(AsO4)1.5.8H2O. Our studies show that the mineral phases containing As are less stable in solutions of high pH than the pure P containing phases. We have also observed that when heated to temperatures up to 200°C, they breakdown in different ways. Neutron diffraction studies into these mineral phases will allow us to determine their structures and help model their behaviour at different conditions of pH and temperature.