In the alluvial deposits the associated minerals are chiefly those of great density and hardness, such as platinum, osmiridium and other metals of the platinum group, tinstone, chromic, magnetic and brown iron ores, diamond, ruby and sapphire, zircon, topaz, garnet, &c. which represent the more durable original constituents of the rocks whose distintegration has furnished the detritus.
In the Gira river the valuable alloy osmiridium has been discovered.
It may be prepared from osmiridium by fusing the alloy with zinc, the zinc being afterwards removed by distillation.
As an alternative the osmiridium is fused with zinc, the regulus treated with hydrochloric acid, and then heated with barium nitrate and barium peroxide.
The metal may be obtained from the residues obtained in the separation of osmium from osmiridium.