A yellow cryptocrystalline mineral from Rock Creek, B. C., is a nonstoichiometric nickel hydroxide corresponding to the artificial compound with an ideal formula 4Ni(OH) 2 XNiOOH, hexagonal cell dimensions a=3.07, c=23.2A; rhombohedral dimensions a=7.93A, alpha =22 degrees 19', V=63.18, Z=1/4 S.G. (calc.) 2.96. The three strongest observed lines of the X-ray diffraction powder pattern are 7.58 A (10), 003; 3.79 A (6), 006; 2.585 A (6), 012 (hexagonal cell indices). The nickel hydroxide probably originated as one of the products of hydrolysis of nickel-bearing solutions that derived their metal either from the silicates of the serpentine or from the secondary nickel minerals, annabergite and garnierite.