Crerarite a Pt-Bi-S mineral: A new occurrence and re-investigation of its chemical formula and crystal structure
|Location||International Geological Congress,oslo 2008|
|Author||McDonald, Andrew M.۱; Krstic, Sasa۲; Vandenberg, Benjamin۲|
|Holding Date||04 October 2008|
Crerarite is a rare PGM originally described from the Lac Sheen Ni-Cu-PGE deposit in the Abitibi-Temiscaminque district, Quebec, Canada where it was discovered in loose boulders associated with pyrrhotite, actinolite and other PGM (moncheite, michenerite). The mineral was considered to have the simplified formula, (Pt,Pb)Bi3(S,Se)4-x and based on a single-crystal study, a NaCl-type structure (Fm3m, a = 5.86 Å) with a high concentration of vacancies (10-20%).
During examination of ore samples from chalcopyrite-millerite stringers hosted by Sudbury breccia and Levack gneiss from the 148 Zone of the Levack North Property (probable up-dip extension of the McCreedy East 153 deposit) in the north range of the Sudbury basin, a phase whose chemistry and appearance closely matched that of crerarite was encountered. The mineral occurs in subhedral, angular to elongate grains that are 30 to 250 µm in size, is association with a chlorite-group mineral, quartz, a clinoamphibole and chalcopyrite. It appears to be intimately associated with chalcopyrite (with sharp grain boundaries between the two) and infrequently contains rounded, elongate inclusions of Bi-rich moncheite. This discovery precipitated a comprehensive study (single-crystal & Gandolfi XRD, electron microprobe) involving both this new material and its discovery in-situ, along with a re-examination of the type material, the goal being to more completely characterize the mineral.
Results from single-crystal XRD analysis of crerarite from the type locality indicate the mineral is triclinic, P-1 with a = 9.3521(7), b = 9.3898(6), c = 14.517(1) Å, α = 105.647(1) β = 96.780(1), γ = 108.390(1)°; a rationale relationship with this cell and that originally proposed for the mineral could not be discerned. Results also indicate that, considering observed bond distances refined-site occupancies and negative Bi-Pb crystal-chemical variations, Pt→Pb substitution is unlikely.
A revised, simplified chemical formula for crerarite is suggested as: Pt2-x(Bi,Pb)11(S,Se)11.