Sperrylite

Sperrylite is a platinum arsenide mineral, composed of PtAs₂. It is an opaque, metallic mineral with a tin-white color, crystallizing in the isometric system within the pyrite group. Its crystals can be cubic, octahedral, or pyritohedral, and it also appears in massive and reniform habits. The mineral has a Mohs hardness of 6 to 7 and a notably high specific gravity of 10.6.

American chemist Francis Louis Sperry discovered sperrylite in 1888–1889 while performing a fire assay on ore from the Vermillion Mine, part of the Sudbury Basin discoveries. He was testing the ore’s gold content for the Canadian Copper Company, which was interested in the property.

The most significant occurrence of sperrylite is in the nickel ore deposits of the Sudbury Basin in Ontario, Canada. It is also found in the layered igneous complex of the Bushveld region in South Africa and the Oktyabr'skoye copper-nickel deposit near Noril'sk, Russia.

Geologically, sperrylite is the most common platinum mineral and typically occurs alongside a wide variety of other rare minerals, such as cooperite, laurite, kotulskite, merenskyite, iridium-osmium alloys, sudburyite, omeiite, testibiopalladite, and niggliite. It resists decomposition from normal weathering and has therefore been reported in scattered alluvial deposits. It was first identified in tiny crystals alongside rhodolite garnet and corundum during alluvial gem mining in streams draining Mason Mountain, Macon County, North Carolina (Hidden, 1898). In Finland, sperrylite has been found in sulfide deposits usually associated with layered mafic-ultramafic complexes.

Structurally, sperrylite belongs to the pyrite group, sharing similar crystal habits and structure. Analyses often show minor amounts of rhodium, and trace amounts of copper, iron, and antimony, along with intergrowths of Pt-Fe, have been reported from some locations. It crystallizes in space group Pa3, with a lattice constant of a = 5.9681(1) Å (Szymański, 1979). Its crystal structure closely resembles that of platarsite (ideally PtAsS). Sperrylite crystals vary greatly in shape and size and are usually enclosed in various host minerals, often closely associated with base-metal sulfides. They commonly occur at the edges of, and are partially enclosed by, pentlandite, pyrrhotite, or chalcopyrite (Seabrook, 2004).

The mineral typically appears as loose aggregates of bright silver cubes, some with octahedral modifications. Most grains are anhedral, though a few euhedral grains can be found. Sperrylite forms through contact metamorphism, indicated by the development of triple-point annealing contacts with pyrrhotite grains. These grains are later surrounded by veins of pyrite. It is cubic (2/m3) and commonly seen as well-developed cubes or cuboctahedra, some so highly modified that edges and corners appear rounded. Nicol and Goldschmidt (1903) identified seventeen crystal forms, including four different trapezohedra, a trisoctahedron, five pyritohedra, and four diploids. Crystals up to 2.5 cm have been reported.

Physically, sperrylite is a tin-white mineral with a brilliant metallic luster and a grey to black streak. It has indistinct cleavage on {001}, a conchoidal fracture, and is brittle. Its hardness ranges from 6 to 7, and it is quite dense, with a calculated specific gravity of 10.78. It has an isometric crystal structure, is non-magnetic, and non-radioactive.

Sperrylite was first described by H. H. Wells in 1889 from material collected at the Vermilion mine in what is now the Sudbury district, Ontario, Canada. Wells named it after Francis L. Sperry, chief chemist of the Canadian Copper Company in Sudbury, who originally collected the material in 1887 (Mitchell, 1985). The mineral occurred in weathered material alongside colorless, transparent cassiterite (SnO₂), thought to have derived from the oxidation of stannite (Cu₂(Fe,Zn)SnS₄).