Nielsenite, PdCu3, a new mineral species discovered in the Skaergaard intrusion, Kangerdlugssuaq area, East Greenland, occurs in a tholeiitic gabbro associated with plagioclase, clinopyroxene, orthopyroxene, ilmenite, titanian magnetite, fayalite and accessory chlorite-group minerals, ferrosaponite, a member of the annite-phlogopite series, hornblende, actinolite, epidote-group minerals, calcite, ankerite, apatite and baddeleyite. The mineral is found in composite microglobules principally composed of bornite-chalcocite, chalcocite, along with rare digenite, chalcopyrite, cobaltpentlandite, Co-rich pentlandite, and sphalerite. Associated platinum-group minerals include skaergaardite, keithconnite, vasilite, zvyagintsevite, (Cu,Pd,Au), (Pd,Cu,Sn) and (Pt,Fe,Cu,Pd) alloys and unnamed Au3Cu and PdAuCu. Nielsenite occurs as discrete grains or in sulfide-bearing, droplet-shaped to irregular grains that are 5-50 mu m (ave: 16 mu m) in size. The mineral is steel-grey in color with a metallic luster, a black streak and a sectile tenacity. No discernible forms or faces were observed. Neither cleavage nor fracture was observed, and no micro-indentation measurements were made. The mineral is non-pleochroic and exhibits neither discernible internal reflections nor evidence for twinning. It appears bright creamy white under reflected light. Reflectance values (in %) in air (in oil) are: 57.6 (47.5) at 470 nm, 60.85 (50.8) at 546 nm, 62.8 (53.0) at 589 nm and 66.7 (57.5) at 650 nm. The average result of 11 analyses is (in wt.%): Pd 29.86, Pt 3.08, Au 3.70, Cu 61.96, Fe 0.59, Pb 0.17, total 99.36%. The empirical formula (normalized to 4 apfu) is: (Pd0.862Au0.058Pt0.049Fe0.028Pb0.003)(Cu2.996Fe0.004)(Sigma 3) or, ideally, PdCu3. The mineral is tetragonal, space group P4mm, with a 3.7125(8), c 25.62(1) angstrom, V 353.2(1) angstrom(3) for Z = 4. The strongest six lines oil the X-ray powder-diffraction pattern [d in angstrom(I)(hkl)] are: 2.137(100)(117), 1.8596(70)(200), 1.8337(40)(00 (14) under bar), 1.3126(60)(220), 1.1188(55)(317), and 1.0663 (30)(22 (14) under bar). Nielsenite is considered to be isostructural with synthetic tetragomal PdCu3 (P4mm). The mineral is commonly associated with skaergaardite (PdCu), which is considered to form at -600 degrees C under conditions of fairly high f(S-2) (>7 log units). Synthetic PdCu3 forms at or below 508 degrees C and is characterized by a relatively high degree of Pd-Cu disorder, consistent with observations made for nielsenite. The name honors Troels F.D. Nielsen, geologist with the Geological Survey of Denmark and Greenland.
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