Between July 20 and August 3, 1998, cruise SO–133 aboard the R/V Sonne completed detailed mapping and sampling of the seafloor around Lihir Island in Papua New Guinea. Sampling of Conical Seamount resulted in the discovery of an unusual occurrence of alacranite and realgar associated with polymetallic vein mineralization consisting of pyrite, sphalerite, galena, minor chalcopyrite and sulfosalts, along with intense clay and silica (amorphous) alteration. Alacranite occurs as subhedral to euhedral tabular orange crystals that are transparent to translucent with a vitreous to resinous luster. Crystals occur both isolated and as groupings within vugs and on matrix, and have a maximum length of 0.1 mm. Single-crystal diffraction data were collected using MoKα X-radiation and a CCD-based detector. The structure is monoclinic, space group C2/c, a 9.943(1), b 9.366(1), c 8.908(1) Å, β 102.007(2)°, and was refined to an agreement index (R1) of 6.6%. The structure determination indicates that the formula of alacranite is As4S4, Z = 4, rather than As8S9 as reported earlier from a Russian locality. Thus, alacranite is the third polymorph of this composition, after realgar and pararealgar. The structure contains covalently bonded As4S4 molecules that are linked by van der Waals forces to form the three-dimensional structure. The structure is identical to that of synthetic α–As4S4.
- arsenic sulfide
- structure determination
- Conical Seamount
- Lihir Island
- Papua New Guinea
↵* Geological Survey of Canada contribution number 1999253.
- © 2001 Mineralogical Association of Canada