PLATINUM (Native)
Native platinum is an extremely rare native element that, in addition to platinum ore, is a source of Os, Ir, and other rare elements.
Platinum – Pt – is one of the rarest elements in the Earth’s crust. Native platinum is consequently very rare, usually found as rolled grains, in nuggets. The largest masses of native platinum have reached 30 kg. Well-formed crystals are very rare. Many samples cataloged as “native platinum” are actually isoferroplatinum (Pt3Fe) or mixtures of isoferroplatinum with tetraferroplatinum (PtFe).
Native platinum never occurs in its pure form, but always in the form of alloys, with Fe (up to 20%), Cu, Au, Ni, Irm, Pd, Rh, Rt, and Os. These elements may be present as solid solution, inclusions, or coarse contamination. Since they never occur in their pure form, platinum ends up being the ore for their extraction.
1. Characteristics
Crystal system: Cubic hexaoctahedral.
Color: Steel gray, bright white, tin white, dark gray.
Habit: Granules, nuggets. Rare, distorted cubic crystals, up to 1.5 cm.
Cleavage: No.
Tenacity: Malleable, ductile, and supple.
Twinning: contact twins, on {111}.
Fracture: Irregular.
Mohs Hardness: 4 – 4.5
Parting: No.
Streak: Bright light gray.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 14 – 19, pure 21.4
2. Geology and Deposits
Primary deposits of native platinum are mafic and ultramafic rocks (dunites), where native platinum occurs associated with chromite and olivine. It also occurs primarily associated with magmatic sulfides, tellurides, antimonides, and arsenides.
Secondary platinum can occur in hydrothermal quartz veins or in contact metamorphic deposits.
Secondary deposits are alluvial deposits, the “placers,” where it is associated with gold and which were, for a long time, the only sources of platinum.
3. Mineral Associations
It is associated with oxides (chromite), sulfides (pyrite, pentlandite, millerite, pyrrhotite), native gold and other platinum minerals: arsenopalladinite, braggite, cooperite, sperrylite, stibiopalladinite, stumpflite, geversite, irarsite, iridosmium, merenskyite and michenerite.
4. Transmitted Light Microscopy
This does not apply, as native platinum is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Careful and prolonged grinding is necessary, but the polishing of native platinum is excellent. Hardness on polishing varies considerably depending on the chemical composition and geological occurrence. The more Fe, the less hard. In some samples, the polishing scratches are very persistent. The hardness on polishing is greater than that of sphalerite and less than that of pyrrhotite.
PLANE POLARIZED LIGHT – PPL
Reflection color: White, sometimes with a cream or bluish tint that arises due to neighboring grains and/or changes in chemical composition.
Pleochroism: No.
Reflectivity: 65.16% (very high!)
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Isotropic, but there is never complete extinction, in any position, due to the polishing scratches.
Internal reflections: No.
May be confused with: Other metals and alloys of the Platinum Group; it is necessary to use other analytical methods. Native silver is lighter.
Native palladium is more bluish.
Sperrylite is much lighter.
Iridium is more yellowish.
General Characteristics:
Grain shape: grains can be euhedral, polygonal or cubic, but are generally xenomorphic. Very different forms are exhibited by native platinums originating from platinum contained in platinum sulfides or arsenides. In this case, reniform, concretionary masses are formed, with a concentric structure (visible only after chemical attack!). These masses may have a high palladium content. Quartz-platinum vein platinums can develop various gel textures, such as long, branched threads, small “cockades” and thin-walled cells, finely dispersed, reticulated or concentric intergrowths with hematite, tuffs with a radial structure or forms resembling framboidal pyrites.
Polishing scratches are very common. Depending on the chemical composition, grooves are practically impossible to eliminate.
Twins on (111) are abundant, but visible only with chemical etching.
Zonations are very common and can arise due to purer platinum inside the grains. Other zonations, with minerals in the Pt-Cu and Pt-Pd series, can form edges, lines and other shapes.
Unmixings of iridium and osmiridium occur in the form of delicate skeletons, lamellae and grains.
Inclusions in native platinum are extremely common and can be sperrylite, cooperite, braggite, laurite, stibio-palladinite, chromite and hematite.
Inclusions of native platinum inclusions occur in chromite.
Substitutions are poorly understood. Native platinum substitutes for sperrylite, other rare platinum minerals, and chromite. Iron-rich native platinum grains, where iron has formed goethite through oxidation, can mimic the substitution of platinum by goethite.
Interstitial native platinum can be found between chromite grains
