Peterbaylissite, idealized as Hg (super 1+) 3 (CO 3 )(OH).2H 2 O, is orthorhombic, space group Pcab (61), with unit-cell parameters refined from powder data: a 11.130(2), b 11.139(3), c 10.725(3) Aa, V 1330(1) Aa 3 , a:b:c = 0.9992:1:0.9628, Z = 8. The strongest six reflections in the X-ray powder pattern [d in Aa(I)(hkl)] are: 4.84(50)(012), 2.969(70)(231), 2.786(70)(040,400), 2.648(100)(223), 2.419(60)(241,024,412), and 1.580(50)(623). The mineral is an extremely rare constituent in a small prospect pit near the long-abandoned Clear Creek mercury mine, New Idria district, San Benito County, California. The mineral is most closely associated with ferroan magnesiochromite and is found as isolated and clustered grains on a crust composed of ferroan magnesite and quartz. Other mercury-bearing phases found on the holotype specimen include cinnabar, metacinnabar and native mercury. Individual crystals of peterbaylissite range in size from 20 mu m up to 0.2 mm, but the average length of crystals is approximately 0.1 mm. Crystals are subhedral to euhedral, elongate , and possess a wedge-like shape with a conchoidal outline. Platy crystals also are present but are rare. The mineral is opaque, black to very dark red-brown color, with a dark brown-black streak. Physical properties include: submetallic to adamantine luster, irregular fracture, brittle, nonfluorescent, hardness less than 5, calculated density 7.14 g/cm 3 (for both the empirical and idealized formulae). In polished section, peterbaylissite is weakly to moderately bireflectant and is nonpleochroic. In reflected plane-polarized light, it is grey with a slight blue tinge and possesses abundant orange and brilliant yellow-white internal reflections. The anisotropy is weak, with dull and dark grey and brown rotation tints. Measured values of reflectance for two grains, in air and in oil, are tabulated. Electron-microprobe analyses yielded Hg 2 O 87.4(1.5) wt.% The empirical formula, derived from crystal-structure analysis and electron-microprobe analyses, is Hg (super 1+) (sub 3.00) (CO 3 )(OH).2H 2 O, based on O = 6. The idealized formula requires Hg 2 O 87.54, CO 2 6.16, H 2 O 6.30, total 100.00 wt%. Important features of the crystal structure are summarized. This is the first natural occurrence of Hg (super 1+) as an isolated ion in an oxysalt. The mineral is named for Professor Emeritus Peter Bayliss, University of Calgary, for his many important contributions to structural and experimental mineralogy, and for his long-standing service to the International Centre for Diffraction Data.