Aurivilliusite, ideally Hg2+Hg1+OI, is monoclinic, C2/c, with unit-cell parameters refined from X-ray powder data: a = 17.580(6), b = 6.979(1), c = 6.693(3) Å, ß = 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 [d in Å (I)(hkl)] are: 8.547(70)(200), 3.275(100)(002), 2.993(80)(2̅21), 2.873(80)(600), 2.404(50b)(6̅02, 421, 2̅22) and 1.878(50)(2̅23). This extremely rare mineral was collected from a small prospect pit near the long-abandoned 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)∑0.99, 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), chemist-crystallographer 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.

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