Widgiemoolthalite is a rare hydrated nickel(II) carbonate mineral with the chemical formula (Ni,Mg)5(CO3)4(OH)2·5H2O. Usually bluish-green in color, it is a brittle mineral formed during the weathering of nickel sulfide. Present on gaspéite surfaces, widgiemoolthalite has a Mohs scale hardness of 3.5 and an unknown though likely disordered crystal structure. Widgiemoolthalite was first discovered in 1992 in Widgiemooltha, Western Australia, which is to date its only known source. It was named the following year by the three researchers who first reported its existence, Ernest H. Nickel, Bruce W. Robinson, and William G. Mumme.