This piece features a central blade (and some lesser ones as well) of pyrite pseudomorphs after marcasite, together with dolomite. The blades are replaced by finely crystalline pyrite and with magnification one can see that some of the pyrite has a cool, almost skeletal look to it. The dolomite crystals are typical composite rhombohedra and in fact are dusted with tiny crystals of marcasite (a later generation, not pseudomorphed). In very good condition, the main blade is pristine and most of the others are also in excellent condition (one dolomite crystal is incomplete.) From the Steve Szilard collection, a fine Nanisivik pseudomorph.
About the Nanisivik Mine
The Nanisivik Mine is in the far north, near the northernmost edge of Baffin Island in the Canadian Arctic, at 73 degrees north latitude. It operated from 1976 to 2002, and was an important producer of exquisite pyrite specimens, along with beautiful specimens of other sulphide minerals, and quartz, dolomite and calcite. The mine’s location made for some extreme conditions. Weeks of total darkness in the winter could see temperatures down to -50C (-58F). In the summer, while the outdoor temperatures could be +10C (50F), the temperature in the mine was -15C (5F) thanks to a 500 metre-thick permafrost layer. The vugs containing crystals were ice-filled, and collecting involved the use of ice picks. This was challenging, but at least the ice protected the crystals during collecting. Specimens were removed with the ice remaining attached, and then melted gradually at the surface to expose the crystals. Collecting was usually impossible during summer months because the underground walls would become coated with thick layers of frost and ice (like the inside of a freezer), obscuring any chance of seeing crystal vugs. Today, the mine is most broadly remembered for the spectacular complex pyrite crystals it produced.
There is a great article on the locality:
Gait, R.I, G.W. Robinson, K. Bailey and D. Dumka, 1990. Minerals of the Nanisivik Mine, Baffin Island, Northwest Territories.* Mineralogical Record 21: 515-534.
(* this region of Nunavut was formerly part of the Northwest Territories)