Platinum is a precious metal renowned for its rarity, often occurring at a concentration of only about 5 parts per billion in the Earth’s crust, making it significantly scarcer than gold. Its value stems from its limited supply and unique chemical and physical stability, making it useful for industrial applications and jewelry. While many are familiar with the bright, polished sheen of refined platinum, its raw, unrefined state looks markedly different. Understanding raw platinum requires examining the metal as it naturally occurs within geological formations. This natural form, often mixed with other elements, is the material miners must identify and extract before refining begins.
Identifying Raw Platinum
Raw platinum, often called native platinum, typically ranges in color from a dull steel-gray to a silvery-white, sometimes appearing darker due to impurities. The metal possesses a distinct metallic luster, allowing it to reflect light even in its unpolished, natural state. When found in secondary deposits, the metal rarely forms large, distinct crystals but instead appears in small, irregular shapes.
The most common natural forms are fine grains, small flakes, or tiny, rounded nuggets. These forms occur because the platinum has been weathered out of its original host rock and transported by water, causing the edges to become worn and smooth. Though less common, raw platinum can sometimes be found as larger nuggets, which are usually alloys containing small amounts of other platinum-group metals or iron. These nuggets appear dense and compact, lacking the crystalline structure expected of a mineral.
Geological Habitats
Raw platinum occurs in two primary geological settings: hard-rock formations and loose sedimentary deposits. Primary deposits contain the metal embedded within its original host rock, typically a mafic or ultramafic igneous rock. These rocks, such as dunites and pyroxenites, form deep within the Earth from magma rich in iron and magnesium.
In primary deposits, platinum is often disseminated in thin, layered horizons that formed as the magma slowly cooled. Secondary, or alluvial, deposits form when the host rock erodes, and the liberated metal is carried by rivers and streams. Due to its high density, the platinum grains collect in placer deposits, often alongside other heavy minerals like gold, in riverbeds. This concentration process led to the discovery of historically mined platinum in regions like the Ural Mountains and Colombia.
Key Physical Properties
One distinguishing characteristic of raw platinum is its exceptional density, which is often the quickest field test to separate it from similar-looking minerals. Pure platinum has a density of 21.45 grams per cubic centimeter, making it significantly heavier than gold (19.3 g/cm³) and silver (10.5 g/cm³). However, native platinum in nature is rarely pure and often contains iron, which lowers its density to a range between 14 and 19 g/cm³.
The metal also exhibits remarkable malleability and ductility, meaning it can be hammered into thin sheets or drawn into fine wire without fracturing. This property is a key diagnostic feature, as a raw platinum specimen will simply deform or flatten when struck, rather than shattering like many other metallic minerals. Furthermore, platinum is categorized as a noble metal due to its chemical inertness and strong resistance to corrosion. Unlike most metals, it will not oxidize or tarnish, and it remains unaffected by single acids; it only dissolves when exposed to aqua regia, a highly corrosive mixture of nitric and hydrochloric acids.