Abstract: Hydrohonessite has a composition that can be expressed by the formula [N i 8−x 2+ F e x 3+ (OH) 16 ][ S 2 x O 4 2− .y H 2 O.zNiS O 4 ] , where x is approximately 2.6, y is 7, and z is 1. The X-ray powder diffraction pattern has strongest lines at 11.0 (10), 5.56 (5), 3.68 (4), and 2.709 Å (3), and can be indexed on a hexagonal unit cell with a = 3.09 Å and c = 10.80 Å. It is optically uniaxial negative with ɛ = 1.59 and ω = 1.63; bright yellow in hand specimen and transmitted light. Hydrohonessite is the hydrated equivalent of honessite, and is related to carrboydite, motukoreaite, and mountkeithite. These minerals are related to brucite and pyroaurite-type minerals in that they have a layered structure consisting of brucite-like layers separated by about 7 Å of interlayer material consisting predominantly of water, but also containing anions that balance the charge of the brucite-like layer. Hydrohonessite is a secondary mineral resulting from the weathering of Ni-Fe sulphides, and appears to be stable between pH 6 and 7.