The crystal structure of tedhadleyite, ideally Hg2+
O4I2(Cl,Br)2, triclinic, A
, a 7.0147(5), b 11.8508(7), c 12.5985(8) Å, α 115.583(5), β 82.575(2), γ 100.619(2)°, V 927.0(2) Å3, Z = 2, was solved by direct methods and refined to an R1 index of 4.5% for 2677 unique reflections. There are six symmetrically distinct Hg sites in tedhadleyite: Hg(1) is occupied by Hg2+ and Hg(2-6) are occupied by Hg+ that forms three [Hg-Hg]2+ dimers with Hg-Hg separations between 2.527 and 2.556 Å. These [Hg-Hg]2+ dimers have strong covalent bonds to O atoms, forming pseudo-linear O-Hg-Hg-O arrangements, and weak bonds to halogen and O atoms at high angles to the dimer axis. The [O-Hg-Hg-O] groups share anions to form four-membered square rings of composition [Hg8O4] that link along  via [O-Hg-Hg-O] groups and along  via [O-Hg-O] groups, forming rectangular rings of composition [Hg14O8]. The rings form a corrugated layer that interweaves with a symmetrically related layer whereby the [O-Hg(6)-Hg(6)-O] linking groups of one layer pass through the centres of the square [Hg8O4] rings of the other layer to form [Hg11O4] complex slabs parallel to (010) that link through Hg-I and Hg-Br,Cl bonds.