Nielsenite, PdCub>3b>, 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 Aub>3b>Cu and PdAuCu. Nielsenite occurs as discrete grains or in sulfide-bearing, droplet-shaped to irregular grains that are 5–50 μm (ave: 16 μm) 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: (Pdb>0.862b>Aub>0.058b>Ptb>0.049b>Feb>0.028b>Pbb>0.003b>)(Cub>2.996b>Feb>0.004b>)b>∑3b> or, ideally, PdCub>3b>. The mineral is tetragonal, space group P4mm, with a 3.7125(8), c 25.62(1) Å, V 353.2(1) Å3 for Z = 4. The strongest six lines on the X-ray powder-diffraction pattern [d in Å(I)(hkl)] are: 2.137(100)(117), 1.8596(70)(200), 1.8337(40)(0014), 1.3126(60)(220), 1.1188(55)(317), and 1.0663 (30)(2214). Nielsenite is considered to be isostructural with synthetic tetragonal PdCub>3b> (P4mm). The mineral is commonly associated with skaergaardite (PdCu), which is considered to form at ~600°C under conditions of fairly high f(Sb>2b>) (>7 log units). Synthetic PdCub>3b> forms at or below 508°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.
