Platinum

Platinum forms in ultramafic igneous rocks, particularly in layered intrusions where dense platinum-group minerals settle out of cooling magma. The Bushveld Complex in South Africa contains the world's largest reserves in thin, laterally extensive layers called 'reefs' (the Merensky Reef and UG2 Reef).

Native platinum occurs as small grains, nuggets, and rarely as cubic or octahedral crystals within dunite, chromitite, and other ultramafic rocks. It's almost always alloyed with other platinum-group elements (palladium, rhodium, iridium, osmium, ruthenium) and often contains iron.

Placer deposits (river gravels) were the original source of platinum. Indigenous peoples in Colombia's Chocó region worked platinum centuries before Europeans encountered it. Spanish conquistadors initially considered it a nuisance metal contaminating their gold, calling it 'platina' (little silver). They couldn't melt it with their technology and sometimes threw it back in the river. It wasn't until the 18th century that European scientists recognized platinum as a distinct, extraordinarily valuable element.

Native platinum is identified by its silvery-white color, very high specific gravity (21.45, the densest commonly encountered mineral), malleability, and resistance to acids. It doesn't tarnish in air and is only dissolved by aqua regia (a mixture of hydrochloric and nitric acids).

Distinguish from silver (lighter SG of 10.5, tarnishes), white gold (an alloy, not found in nature), and palladium (slightly lighter SG, very similar appearance). Platinum's extreme density is the most practical field test: it feels remarkably heavy for its size.