Platinum, Native

a group of platinum minerals that are disordered natural solid solutions of iron, copper, nickel, iridium, rhodium, palladium, tin, osmium, ruthenium, gold, silver, bismuth, and lead in platinum.

Native platinum usually contains two or three major (mineral-forming) metals and various quantities of metal impurities. The main component of the minerals is platinum. In its crystal structure, native platinum is a metal solvent; its structure is transmitted to native platinum minerals. The atoms of the secondary mineral-forming and impurity elements are distributed statistically in the transmitted platinum structure as though they were dissolved in it.

The following minerals of native platinum have crystal structures of this type: polyxene (2.5–11.9 percent Fe by weight) and ferroplatinum (12.0–28.1 percent Fe), which are solid solutions of iron in platinum; platiniridium (10.4–37.5 percent Ir), a solution of iridium in platinum; palladic platinum (19.4–40.0 percent Pd), a solution of palladium in platinum; and stannopalladic platinum (16–23 percent Sn and 17.2–20.9 percent Pd), a solution of tin and palladium in platinum. The maximum contents of impurities in minerals of native platinum are 8.8 percent Ir, 6.8 percent Rh, 6 percent Pd, 3.3 percent Cu, and 2.3 percent Ru in polyxene; 14.3 percent Ni, 14 percent Cu, 12.9 percent Pd, 7.5 percent Ir, 5.8 percent Rh, and 3 percent Bi in ferroplatinum; 11 percent Os, 4 percent Pd, and 2.5 percent Ru in platiniridium; 3 percent Au in palladic platinum; and 2.5 percent Bi in stannopalladic platinum. Polyxene and ferroplatinum containing more than 4 percent Rh are called rhodic platinum, ferroplatinum with more than 7 percent Cu is called cuproplatinum, and ferroplatinum with more than 3 percent Ni is called nickel-platinum. Ferroplatinum and polyxene are the most common minerals of native platinum.

Minerals of native platinum crystallize in a cubic habit, with a crystal structure of the copper type; the lattice is face-centered cubic. The minerals are opaque steel-gray and silver-white substances with a yellow hue in palladic platinum and a bronze hue in cuproplatinum. A metallic sheen is especially pronounced in the case of platiniridium. Deposits of these minerals (nodules, concretions, and crystals) are often coated with a black oxide film that is thin and brittle. Most deposits of polyxene and ferroplatinum, and some deposits of cuproplatinum, have magnetic properties. Almost all minerals of native platinum are malleable, except for the somewhat brittle platiniridium. Their hardness on Mohs’ scale is 3.5–5.5; it is lowest for minerals containing copper and nickel and highest for iridium-containing minerals. The density of native platinum minerals ranges from 13,100 to 21,500 kg/m³; the least dense are ferroplatinum (13,100–16,000 kg/m³) and stannopalladic platinum, and the most dense is pure natural platinum.

Minerals of native platinum are good conductors of electricity. Deposits of native platinum minerals in the form of irregular nodules and sparse rectangular, cubic, octahedral, and cubic-octahedral crystals are common. Twinned concretions of crystals are rarely encountered, and rounded and angular nuggets (nodular aggregates) are found extremely rarely. The size of the nodules and crystals ranges from tenths of a micron or a few microns to a few millimeters (very rarely to several centimeters), and the size of the nuggets ranges up to 20 cm, with a weight of a few grams to several kilograms. The largest nuggets in the USSR were found in dunites of the Nizhnii Tagil massif in the Central Urals (the largest nugget found weighed 427.5 g) and in alluvial platinum deposits in the area (the largest nugget found weighed 9,439 g). The nuggets consist not only of minerals of native platinum (ferroplatinum, polyxene, and platiniridium) but also of inclusions of iridium and osmium minerals. Large platinum nuggets (weighing hundreds and thousands of grams) are retained by the state.

Minerals of native platinum are endogenous, and their formation is associated with late magmatic and metamorphic stages of the formation of magmatic deposits and the hydrothermal stage of the formation of postmagmatic deposits (in pegmatites, skarns, and hydrothermal veins). These minerals are concentrated to the greatest degree in deposits of platinum ores.

One of the rarest minerals of native platinum, metallic platinum, has been identified among the decomposition products of natural solid solutions of platinum in iridium contained in platinum ores that are genetically related to forsteritic dunites.

Minerals of native platinum are a major natural source of platinum group metals.

L. V. RAZIN