104 ECONOMIC GEOLOGY 



ferro-magnesian rock known as peridotite. Many platinum 

 placers have been traced back directly to the decomposition of 

 such a rock. However, all peridotite does not bear platinum. 



Methods of Extraction. (1) By placer mining. Platinum is 

 obtained by panning the lower gravels of placers or by hydraulic- 

 ing and dredging the entire gravels of the larger placers for the 

 gold. (2) The wet method. The ores of platinum are treated 

 with hot aqua regia which dissolves all of the platinum and part 

 of the iridium. After evaporating the excess acid the platinum 

 is precipitated by ammonium chloride as an ammonium chloro- 

 platinate (NH 4 )2PtCl 6 . Ammonium chloride and chlorine are 

 volatile upon ignition and the platinum is left behind as a spongy 

 metal. (3) Recovery from waste solutions. The waste solution 

 is boiled to expel any excess of nitric acid; it is then filtered to 

 remove any platinum sponge that may have been left. Barium 

 chloride is added to precipitate any sulphuric acid that may be 

 present. The platinum salts now in solution are reduced to the 

 elemental state by concentrated hydrochloric acid and zinc. 

 Electrolysis may be substituted in place of zinc. 



Uses of Platinum. According to Pliny, platinum was known 

 to the ancients, for it occurred in many alluvial beds associated 

 with gold, and remained with the yellow metal after the washing 

 of the gold. In 1735 it was recognized in Columbia, S. A. In 

 1740 it was exported from Jamaica to Europe. Near the middle 

 of the eighteenth century, the Spanish government forbade its 

 further extraction and ordered all the platinum thrown into the 

 seas to prevent its use as an adulterant of gold. In 1819 platinum 

 was discovered in serpentine rocks on the Siberian side of the 

 Urals. Until 1823 the world's supply of platinum came solely 

 from South America. Since 1824 Russia has been practically the 

 only producer. Platinum was used by the ancients as an adul- 

 terant of gold. It is used in many forms of chemical apparatus 

 in which a high melting-point is necessary. It is the only avail- 

 able metal which will withstand the continuous heat of baking, 

 and for this reason, used extensively as pins to hold artificial 

 teeth together. It is also used for filling teeth; platinized paper 

 for photographic purposes; jewelry, and in coinage when alloyed 

 with 2 per cent, of iridium, especially in Russia, where it was first 

 introduced in 1824, on account of its malleability, its unadulter- 

 ability, and its intrinsic value. Platinum is also used in the 

 manufacture of contact points of telegraph keys; for stills or 



