Aurivilliusite, ideally Hg2+Hg1+OI, is monoclinic, C 2/c, with unit-cell parameters refined from X-ray powder data: a= 17.580(6), b= 6.979(1), c= 6.693(3)Å, β = 101.71(4)°, V = 804.0(5)Å3, a:b:c= 2.5190:1:0.9590,Z = 8. The strongest six lines of the X-ray powder-diffraction pattern [din Å (I )(hkl)] are: 8.547(70)(200), 3.275(100)(002), 2.993(80)(21), 2.873(80)(600), 2.404(50b)(02, 421, 22) and 1.878(50)(23). This extremely rare mineral was collected from a small prospect pit near the longabandoned Clear Creek mercury mine, New Idria district, San Benito County, California, USA. It is intimately intermixed with another new undefined Hg-O-I phase (‘CCUK-15’), and is also closely associated with native mercury, cinnabar and edgarbaileyite in a host rock principally composed of quartz and magnesite. Aurivilliusite occurs in a cm-wide quartz vein predominantly as irregular-shaped thin patches ‘splattered’ on the quartz surface; patches vary in size from 10–20 μm up to 0.5 mm. The only known subhedral platy brightly reflecting crystal fragment, with major ﹛100﹜ form and distinct ﹛100﹜ cleavage, did not exceed 0.2 mm in longest dimension. The mineral is dark grey-black with a dark red-brown streak. Physical properties include: metallic lustre; opaque; non-fluorescent; brittle; uneven fracture; calculated density 8.96 g/cm3 (empirical formula), 8.99 g/cm3 (ideal formula). In polished section in plane-polarized reflected light, aurivilliusite resembles cinnabar, is extremely light sensitive, shows twinning and no internal reflections, and exhibits an unusual ‘red light’ coalescing phenomena. Averaged and corrected results of electron-microprobe analyses yielded HgO 40.10, Hg2O 38.62, I 22.76, Br 0.22, Cl 0.06, sum 101.76, less O = I + Br + Cl –1.46, total 100.30 wt.%, corresponding to Hg1.002+Hg1.001+ O1.01(I0.97Br0.01Cl0.01)Σ0.99, based on O + I + Br + Cl = 2 atoms per formula unit (a.p.f.u.). The original value for Hg, 74.27 wt.%, was partitioned in a HgO:Hg2O ratio of 1:1 after the discovery of the crystal-structure paper dealing with the synthetic equivalent of aurivilliusite. The mineral name is in honour of the late Dr Karin Aurivillius (1920 –1982), chemistcrystallographer at the University of Lund, Sweden, for her significant contributions to the crystal chemistry of Hg-bearing inorganic compounds. Aurivilliusite is related chemically to terlinguaite, Hg2+Hg1+OCl, but has a different structure and X-ray characteristics.

The crystal structure of tedhadleyite, ideally Hg2+Hg101+O4l2(Cl,Br)2,triclinic, AĪ, 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)º, 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 [100] via [O–Hg–Hg-O] groups and along [001] 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.