779 The Canadian Mineralogist Vol. 39, pp. 779-784 (2001) CLEARCREEKITE, A NEW POLYMORPH OF Hg 1+ 3 (CO 3 )(OH)•2H 2 O, FROM THE CLEAR CREEK CLAIM, SAN BENITO COUNTY, CALIFORNIA ANDREW C. ROBERTS § Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada LEE A. GROAT AND MATI RAUDSEPP Department of Earth and Ocean Sciences, The University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada T. SCOTT ERCIT Research Division, Canadian Museum of Nature, P.O. Box 3443A, Station “D”, Ottawa, Ontario K1P 6P4, Canada RICHARD C. ERD * United States Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, U.S.A. ELIZABETH A. MOFFATT Canadian Conservation Institute, 1030 Innes Road, Ottawa, Ontario K1A 0M5, Canada JOHN A.R. STIRLING Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada ABSTRACT Clearcreekite is a newly recognized polymorph of Hg 1+ 3 (CO 3 )(OH)•2H 2 O (along with the previously described peterbaylissite). The mineral is monoclinic, space group P2 1 /c (14), with unit-cell parameters refined from powder data: a 6.760(4), b 9.580(4), c 10.931(4) Å,  105.53(5)°, V 682.1(6) Å 3 , a:b:c = 0.7056:1:1.1410, Z = 4. The strongest six reflections in the X-ray powder-diffraction pattern [d in Å(I)(hkl)] are: 7.09(70)(011), 5.32(40)( ¯ 111), 4.62(90)(012), 2.831(100)(023), 2.767(100) (211, ¯ 221), and 2.391(40)(040, ¯ 204). 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 found as an isolated cluster of crystals in a shallow depression, associated with cinnabar and edoylerite, on a single specimen of brecciated silica–carbonate rock. Individual crystals do not exceed 0.17 mm in longest dimension and are subhedral, tabular, with major {001} and minor {010} forms. The mineral is transparent with a pale greenish yellow color and streak. Physical properties include: vitreous luster, uneven fracture, brittle, nonfluorescent, soft (grains punctured by an electron beam), calculated density 6.96 g/cm 3 (idealized formula). The mineral becomes dark brown-black and opaque when subjected to X-radiation, and the change is irreversible. Electron-microprobe analysis yielded 84.65 wt.% Hg 2 O. The empirical formula, derived from results of a crystal-structure analy- sis and of an electron-microprobe analysis, is Hg 1+ 2.92 (C 1.01 O 2.98 )(OH) 1.04 •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.%. The infrared-absorption spectrum confirms the presence of both CO 3 and H 2 O. The mineral is named after the type locality. Keywords: clearcreekite, new mineral species, polymorph, peterbaylissite, hydrated mercurous hydroxide–carbonate, X-ray data, electron-microprobe data, infrared-absorption data, Clear Creek claim, San Benito County, California. § E-mail address: aroberts@nrcan.gc.ca * retired