Summary: Extensive substitution of Ni for Zn in hydrozincite from the Parc Mine, North Wales, has been observed. This is the first time that a substantial concentration of another transition metal in this mineral has been reported. The average composition of the nickeloan hydrozincite is Zn_4.63Ni_0.37(CO₃)₂(OH)₆. The relationship of this new variety to other secondary carbonate-containing nickel minerals is discussed, as is the possibility of substitution of other transition metal ions into the hydrozincite lattice.