Summary: Nickelblödite, the nickel analogue of blödite, has been discovered in nickel mines at Kambalda and Carr Boyd Rocks in Western Australia. The Kambalda sample, found in an underground opening, has a composition corresponding to Na2·02(Ni0·79Mg0·14Fc0·05)(SO4)2·00·3·17H2O. The sample from Carr Boyd Rocks, collected from an open pit, is a more magnesian variety, with a composition corresponding to Na1·93 (Ni0·55Mg0·46Cu0·02Co0·02)(SO4)2·01·4·39H2O. The mineral occurs as a surface efflorescence on nickel-rich sulphide ore in both cases. Nickelblödite is light green and transparent, and occurs as tabular crystallites up to 150 μm in diameter. The Kambalda material is biaxially negative with 2V = 60–70° α = 1·513, β(calc) = 1·518 and γ = 1·520. D 2·43, indentation hardness VHN 139. The Carr Boyd material has lower refractive indices and hardness. Strongest lines of the indexed powder pattern (Kambalda sample) are 4·466(9), 4·193(7), 3·720(6), 3·223(10), 3·190(8), 2·589(6). These measurements conform to a monoclinic unit cell with α = 10·87, b = 8·07, c = 5·46Å, and β = 100·72°. The possibility of H3O+ substitution is discussed.