Metals are a large group of elements characterized by high electrical conductivity, malleability, and a tendency to lose electrons. In nature, they are found either as chemically pure, uncombined elements or, more commonly, locked within chemical compounds. The form and location of a metal depend heavily on its chemical reactivity and the geological processes of its environment. Metals are ubiquitous, ranging from easily collectible surface deposits to elements dissolved in water bodies or arriving from space.
Metals Found in Their Pure State
Only a small number of metals are chemically unreactive enough to exist in nature as native elements, meaning they are not chemically bonded to other substances. Gold is the most well-known native metal, often found as flakes, nuggets, or fine particles in placer deposits or hydrothermal veins. Its resistance to oxidation and corrosion allows it to persist in its elemental form over geologic time. The platinum group metals, including platinum, iridium, and palladium, also commonly occur in their native state, often alloyed together in igneous rocks. Copper is another metal occasionally found in a pure, uncombined state, typically in the oxidized zones of copper deposits.
The Primary Source Mineral Ores
The vast majority of metals, particularly those used in industrial applications, are found chemically bonded with other elements in mineral ores within the Earth’s crust. An ore is defined by the economic viability of extracting the desired metal from it. This means the concentration of the metal compound must be high enough to justify the mining and refining costs.
These metal compounds are classified based on the non-metal element they are bonded to, with oxides and sulfides being the most common types. Oxide ores, where the metal is bonded to oxygen, include Hematite for iron and Bauxite for aluminum. These typically form near the Earth’s surface under conditions rich in oxygen and water.
Sulfide ores contain the metal bonded to sulfur and are the main source for metals like copper, lead, and zinc. Sulfide deposits often form deep within the crust through hydrothermal processes. Other important mineral forms include carbonates, such as Magnesite for magnesium, and silicates, though silicates are less often used as primary ore sources due to extraction difficulties.
The geological processes that concentrate these compounds into valuable deposits include magmatic processes, sedimentation, and hydrothermal activity. The earth’s crust is the primary reservoir for these ores, and the specific chemical form a metal takes depends on its reactivity and the local geological environment. Highly reactive metals like sodium and potassium are virtually always found in compound form, such as in rock salt or other halide ores.
Metals Dissolved in Water Sources
The global oceans represent a massive, though dilute, reservoir of nearly every naturally occurring metal element. Seawater contains high concentrations of dissolved metals like sodium, potassium, calcium, and magnesium, which are present as ions. Magnesium is commercially extracted from seawater due to its high concentration.
A more concentrated source of metals is associated with deep-sea hydrothermal vents, often called “black smokers,” found along mid-ocean ridges. Here, superheated water leaches metals like iron, manganese, copper, and zinc from the surrounding crustal rock. When this hot, metal-rich fluid rapidly mixes with cold deep-ocean water, the dissolved metals precipitate instantly, forming metal sulfides that create the characteristic black plume. This process builds chimney-like structures and seafloor massive sulfide deposits rich in metals, including cobalt and gold.
Extraterrestrial Metal Sources
Metals are continually delivered from space, primarily in the form of meteorites. While most of Earth’s metals are in the core, subsequent bombardment delivered a “late veneer” of metals to the crust. Iron meteorites, which are fragments of larger asteroids, are almost entirely composed of metal, primarily iron alloyed with 5–30% nickel. These alloys often contain trace amounts of siderophile elements, such as gold, platinum, and iridium. Asteroids, particularly M-type asteroids, are thought to be remnants of planetary cores and represent huge potential reserves of these iron-nickel metals.