ABSTRACT. Gortdrumite superficially resembles chalcocite but is more distinctly anisotropic, and is present in a vein containing ferroan dolomite and baryte which cuts dolomitized limestones from the Gortdrum Orebody of County Tipperary, Ireland. It has an empirical formula of approximately (Cu,Fe)6Hg2S 5. A distinctive powder diffraction X-ray pattern has been obtained which indicates a probable orthorhombic crystal lattice with a 14.96, b 7.90, and c 24.1 A. The reflectivity at 589 nm is 25.1 to 27.9~ and the VHNlo hardness range 186 to 230.THE Gortdrum orebody lies in basal Carboniferous sedimentary rocks adjacent to a major east-north-east trending normal fault. In the near-surface zone the mineralization consists dominantly of chalcopyrite and mercurian tennantite, and occurs as disseminations and veinlets within shattered massive bioclastic limestones. At greater depths the limestones are comparatively shaly and less competent and the mineralization is more finely disseminated and composed primarily of bornite and chalcocite. Pyrite is a minor constituent of the ores but other sulphides are uncommon. Very small amounts of cinnabar are present, mainly as smears on fracture surfaces, and the accessory minerals include native amalgam, stromeyerite, sphalerite, wittichenite, and cobaltite (Steed, 1975). The orebody was mined for copper, mercury, and silver from 1967 to 1975 but the open-pit workings are now flooded.In locally dolomitized limestones about 75 feet below the pre-mining rock surface, a small composite ferroan dolomite-baryte vein containing disseminated cinnabar occurred. Three samples from this vein were studied in polished section and it was observed that in addition to cinnabar there are small grains, some separate and some intergrown, of chalcopyrite, bornite, chalcocite, and the unknown phase. Such an intimate association of copper sulphides and cinnabar is notably atypical for the deposit.The grains (less than 200 #m by 50 #m) of the unknown mineral are present in all three polished sections, and axe generally isolated within the Copyright the Mineralogical Society dolomite or intergrown with bornite. In reflected light the phase resembles chalcocite in colour and reflectivity, and it has a similar polishing hardness. In contrast to chalcocite, however, it is strongly anisotropic and in some grains shows finely developed lamellar twinning. The polarization colours in air range from grey-white with a bluish tint to blue and there are no internal reflections.Reflectivity and hardness. Reflectivity determinations were made on ten separate grains in random orientations, using a C.O.M. approved tungsten carbide standard (calibrated by the German Physical Standards Laboratory, Gdttingen) for comparison. The R ~o range determined in air at 589 nm is 25.1 to 27.9 ~o.These results indicate a slightly lower reflectivity than that for chalcocite, given by Bowie et al.
