Crude platinum, as recovered, contains from 70 to 90 per cent. of the metal, and, as mentioned above, is really an alloy of platinum with one or more of the allied metals, the chief impurities consisting principally of iron and copper.
The table on the next page gives the analyses of typical samples of crude platinum from the Urals, California, British Columbia, and other places.
Platinum also occurs in combination with arsenic in the mineral Sperrylite (PtAs2), in the form of minute octahedral crystals. The colour of this mineral is tin white, its lustre is metallic and brilliant, its hardness varies from 6 to 7, and its specific gravity is 10·6. The mineral is brittle and breaks with a conchoidal fracture. It is very rare, and is interesting as being the only mineral of platinum known besides the native metal. It occurs associated with sulphide minerals of magmatic origin in gabbros and diabases, notably in the nickeliferous pyrites of Sudbury, Canada, and in the copper ores of the Rambler Mine, Laramie, Wyoming.
It is probable that the palladium, which is also found in these deposits, is similarly present in the form of an arsenide, but such a mineral has not yet been definitely proved to exist.
The following is an analysis of a sample of sperrylite: platinum, 54·47 per cent.; rhodium, 0·76 per cent.; palladium, trace; arsenic, 42·23 per cent.; antimony, 0·54 per cent.[[2]] p. 69.
Properties of the Platinum Metals
Platinum.—The colour is white with a greyish tinge. When pure it is very malleable and ductile. Its coefficient of expansion is less than that of all other metals. Platinum fuses at about 1750° C., but the presence of impurities lowers the melting-point. Its specific gravity is 21·5, and its hardness is from 4 to 5. Its electric conductivity is low, being 13·4 at 0° C.[[3]] p. 398.
| Composition of Native Platinum and Osmiridium | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Locality. | Pt. | Fe. | Pd. | Rh. | Ir. | Os. | Cu. | Os-Ir. | Remarks. |
| Platinum | Per cent. | Per cent. | Per cent. | Per cent. | Per cent. | Per cent. | Per cent. | Per cent. | |
| Urals 1[[B]] | 76·22 | 17·13 | 1·87 | 2·50 | tr. | 0·36 | 0·50 | Magnetic grs. | |
| Urals 2 | 73·58 | 12·98 | 0·30 | 1·15 | 2·35 | tr. | 5·20 | 2·30 | Magnetic grs. |
| Urals 3 | 81·34 | 11·48 | 0·32 | 2·14 | 2·42 | tr. | 1·13 | 0·57 | Non-magnetic. |
| Urals 4 | 78·94 | 11·04 | 0·28 | 0·86 | 4·97 | 0·70 | 1·96 | Non-magnetic. | |
| Urals 5 | 86·50 | 8·32 | 1·10 | 1·15 | tr. | 0·45 | 1·40 | Non-magnetic. | |
| Borneo | 82·60 | 10·67 | 0·30 | 0·66 | 0·13 | 3·80 | Gold, 0·20 per cent. | ||
| Brazil | 72·62 | tr. | 21·82 | 0·88 | Sand, 0·42 per cent. | ||||
| Granite Cr., B.C. | 68·19 | 7·87 | 8·26 | 3·10 | 1·21 | 3·09 | 14·62 | Gangue, 1·69; non-magnetic. | |
| Granite Cr., B.C. | 78·43 | 9·78 | 0·09 | 1·70 | 1·04 | 3·89 | 3·77 | Gangue, 1·27; magnetic. | |
| Chocó, Col. | 86·20 | 7·80 | 0·50 | 1·40 | 0·85 | 0·60 | 0·85 | Sand, 0·95. | |
| Chocó, Col. | 84·30 | 5·31 | 1·06 | 3·45 | 1·46 | 1·03 | 0·74 | Gold, 1·0; sand, 0·61. | |
| California | 85·50 | 6·75 | 0·60 | 1·00 | 1·05 | 1·40 | 1·10 | Gold, 0·8; sand, 2·95. | |
| California | 79·85 | 4·45 | 1·95 | 0·65 | 4·20 | 0·75 | 4·95 | Gold, 0·55; sand, 2·69. | |
| Oregon | 51·45 | 4·30 | 0·15 | 0·65 | 0·40 | 2·15 | 37·30 | Sand, 3; gold, 0·85. | |
| Fifield, N.S.W. | 75·80 | 10·15 | tr. | 1·30 | 1·30 | 0·41 | 9·30 | Gold, nil; sand, 1·12. | |
| “Australia” | 61·40 | 4·55 | 1·80 | 1·85 | 1·10 | 1·10 | 26·00 | Gold, 1·2; sand, 1·4. | |
| Currumbin, Q. | 26·12 | 27·17 | 2·51 | 40·02 | Sand, 1·33. | ||||
| Osmiridium | |||||||||
| Urals | 10·08 | tr. | tr. | 1·51 | 55·24 | 27·23 | tr. | Ru, 5·85 (Nevyanskite). | |
| Urals | 0·14 | 0·63 | 1·65 | 43·94 | 48·85 | 0·11 | Ru, 4·58 (Nevyanskite). | ||
| California | 2·60 | 53·50 | 43·40 | Ru, 0·50 (Nevyanskite). | |||||
| “Australia” | 3·04 | 58·13 | 33·46 | 0·15 | Ru, 5·22 (Nevyanskite). | ||||
| Currumbin, Q. | 3·00 | 62·00 | 33·00 | Sand, 2·00 (Nevyanskite). | |||||
| /\ | |||||||||
| Tasmania | 0·37 | 0·30 | 0·21 | 33·80 | 57·09 | tr. | Ru, 8·19; Au, ·04 (Siserskite). | ||
| Platiniridium | |||||||||
| Condado, Brazil | 55·44 | 4·14 | 1·49 | 6·86 | 27·79 | tr. | 3·30 | ||
| Native Iridium | |||||||||
| Urals | 19·64 | 0·89 | 76·80 | 1·78 | |||||
| Reference—Pt., platinum; Fe., iron; Pd., palladium; Rh., rhodium; Ir., iridium; Os., osmium; Cu., copper; Os-Ir., osmiridium; Ru., ruthenium. | |||||||||
[B]. Average percentage of Urals platinum is a little under 80.[[4]] p. 558.
Platinum is not acted upon by either nitric, sulphuric or hydrochloric acid, but is soluble in aqua regia (1 part of nitric to 2 parts of hydrochloric acid), or in other mixtures liberating chlorine, with the formation of platinum tetrachloride (PtCl4). It is not acted upon by air or moisture, even at high temperatures. It is corroded by caustic alkalis, sulphides, sulphates, phosphates and arsenides, if heated in contact with them. It will not amalgamate with mercury unless sodium is present, in this respect differing from gold and silver.