The crystal structure of simonkolleite, Zn5 (OH)8 Cl2 (H2O), rhombohedral, space group R3̅m, a 6.3412(3), c 23.646(1) Å, V 823.4(1) Å3, Z = 3, has been refined to an R index of 1.6% based on 284 observed (5σ) reflections measured with MoKα X-radiation. There are two distinct Zn sites fully occupied by Zn: Zn(1) is octahedrally coordinated by six (OH) groups, <Zn(1)– OH> = 2.127 Å, and Zn(2) is tetrahedrally coordinated by three (OH) groups and one Cl atom, <Zn(2)–OH,Cl> = 2.041 Å. The (Znφ6) octahedra (φ: O, OH, Cl) form an edge-sharing dioctahedral sheet similar to that observed in dioctahedral micas. On each side of the vacant octahedron, a (Znφ4) tetrahedron is attached to three anions of the sheet and points away from the sheet. The decorated sheet can be written as [M3 (Tφ4)2 φ2], where M = Zn, T = Zn and φ = OH, Cl. Intercalated between adjacent sheets are interstitial (H2O) groups. The sheets are held together by hydrogen bonding from (OH) groups of one sheet to Cl anions of adjacent sheets, and to interstitial (H2O) groups. Simonkolleite can be considered as a decorated sheet structure of the form [Mφ2]4 with an interrupted sheet, and is related to the structures of cianciulliite, gordaite, namuwite, bechererite, chalcophyllite, ramsbeckite, chalcophanite and claringbullite.