SPERRYLITE
Sperrylite – PtAs2 – is an uncommon, but not rare, arsenide, constituting the most common and widely distributed platinum ore mineral. Well-formed crystals of sperrylite, especially if in a matrix, fetch very high prices in the collector’s mineral market.
It is classified in the Pyrite Group and may contain Fe, Sb, Cu, Rh, and Ir as impurities.
1. Characteristics
Crystal system: Cubic diploidal.
Color: Tin white.
Habit: Well-formed cubes and combined shapes of cube +- pentagon dodecahedron +- octahedron.
Cleavage: {001} poor.
Tenacity: Brittle.
Twinning: No.
Fracture: Conchoidal.
Mohs Hardness: 6 – 7
Parting: No.
Streak: Dark gray to black.
Lustre: Metallic intense.
Diaphaneity: Opaque.
Density (g/cm³): 10.58
2. Geology and Deposits
Sperrylite occurs almost exclusively in metamorphosed ultramafic intrusive rocks containing platinum, such as metadunites.
Because it is quite resistant to alteration, it can occur detrital in placers.
3. Mineral Associations
It occurs associated with native elements such as native gold, argentiferous gold, native platinum, leadamalgam, niggliite, native osmium, and native copper.
Associates with oxides such as magnetite, ilmenite and chromite.
Also with many sulfides, including pyrite, chalcopyrite, pyrrhotite, pentlandite, sphalerite, galena, stibnite, millerite, bornite, cubanite, violaite, gersdorffite, linnaeite, omeiite (Os,Ru)As2), sudburyite (PdSb), merenskyite (Pd,Te2), kotulskite (Pd(Te,Bi)2), cooperite (PtS), testibiopalladite (PdTe(Sb,Te)), talnakhite (Cu9(Fe,Ni)8S16), polyxene (Pt,Fe,Cu,Ni,Pd,Rh), cabrite (Pd2CuSn), laurite (RuS2) and hollingworthite ((Rh,Pt,Pd)AsS).
Also with quartz and epidote.
4. Transmitted Light Microscopy
This does not apply, as sperrylite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: The hardness of sperrylite upon polishing is very high, quite similar to that of pyrite. A long and careful grinding and polishing process is necessary, but an excellent quality polish can be achieved. Platinum is much less hard; braggite is slightly softer.
PLANE POLARIZED LIGHT – PPL
Reflection color: Pure white.
Compared to pyrite, the color is whiter.
Compared to native platinum, the color is distinctly darker.
Compared to stibiopalladinite, the color is slightly darker.
Pleochroism: No.
Reflectivity: 55,07%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Isotropic.
Internal reflections: No.
May be confused with: pyrite, when it occurs in very small grains where color determination is difficult.
Native platinum is less hard and generally does not exhibit idiomorphic forms.
Gersdorffite, in some cases, can also be similar to sperrylite, requiring the use of other analytical techniques.
General Characteristics:
Grain shape: Sperrylite generally presents as well-formed crystals with many faces, some of which may be curved.
Cleavage is sometimes observable.
Conchoidal fracture can be seen in some cases.
Twinning and zoning do not occur.
Myrmekitic intergrowths of platinum or magnetite with sperrylite may occur.
Sperrylite is replaced by native platinum, cooperite, braggite, chalcopyrite, pentlandite, cubanite, sphalerite, native gold, hesita, and Pd minerals.
Sperrylite replaces stibiopalladinite.
Inclusions of sperrylite may occur in native platinum.
Inclusions in sperrylite may be chalcopyrite, Rh-sperrylite, geversite, hollingworthite, native copper, parkerite, iridosmium, and Ni arsenide.
