Iron is a chemical element and a metal, not a type of rock itself. Iron rarely exists in its pure, metallic state on Earth’s surface, instead forming compounds with other elements, most commonly oxygen. The question of what rock iron is found in is best answered by looking at the minerals and geological formations where these iron-bearing compounds are concentrated. These compounds are the building blocks of the rocks that serve as the world’s source of this fundamental metal.
Iron: An Element, Not a Rock
Understanding where iron is found requires distinguishing between an element, a mineral, and a rock. Iron (Fe) is a chemical element with the atomic number 26, a fundamental substance that cannot be broken down further by chemical means. A mineral is a naturally occurring solid with a defined chemical composition and an ordered atomic structure. A rock, by contrast, is an aggregate of one or more minerals.
Iron is the fourth most abundant element in the Earth’s crust by mass, accounting for about five percent of the crustal material. Nearly all of this iron is chemically bound to other elements, primarily oxygen. The high reactivity of iron means that any pure metal exposed to the atmosphere and water will quickly oxidize, a process known as rusting, to form iron oxides.
Because of this constant oxidation process, iron is not found as a native metal on the surface. Instead, it is incorporated into various minerals, which form iron-rich rocks. Economically extracting iron involves mining these rocks that contain a high concentration of specific iron-bearing minerals.
The Key Iron-Containing Minerals
The primary sources for commercial iron are minerals known as iron ores, which are typically iron oxides. The two most important ore minerals are hematite and magnetite, both compounds of iron and oxygen. These minerals are distinguished by their chemical formulas, iron content, and magnetic properties.
Hematite (\(\text{Fe}_2\text{O}_3\)) is the most frequently mined iron ore, containing a theoretical maximum of 70% iron by weight in its pure form. It is named from the Greek word for blood because it leaves a characteristic reddish streak. Hematite is non-magnetic, which influences how it is processed and separated from the surrounding rock material.
Magnetite (\(\text{Fe}_3\text{O}_4\)) is the other major iron source, possessing a slightly higher theoretical iron content at 72.4% iron by weight. This mineral is unique for its strong magnetic properties, making it easy to separate from non-magnetic rock material using magnetic separation techniques. Magnetite ore often requires more processing than hematite because the mineral is typically less concentrated in the mined rock.
Less common but commercially used iron minerals include goethite, a hydrated iron oxide, and limonite, a mixture of various hydrated iron oxide-hydroxide minerals. These minerals represent the concentrated form of iron necessary for large-scale steel production.
Understanding Banded Iron Formations
The vast majority of the world’s mineable iron reserves come from Banded Iron Formations (BIFs). These sedimentary rock formations are characterized by distinctive layering, showing alternating bands of iron-rich minerals and iron-poor chert, a type of microcrystalline quartz. Individual BIFs can be hundreds of meters thick and extend for hundreds of kilometers.
The deposition of BIFs occurred primarily between 3.8 and 1.8 billion years ago, during the Precambrian Eon. This time frame coincides with the Great Oxidation Event (GOE), a major shift in Earth’s atmospheric chemistry. Before the GOE, the oceans contained enormous amounts of dissolved, soluble iron.
Primitive photosynthetic organisms, such as cyanobacteria, began producing free oxygen. This oxygen dissolved into the ocean water and immediately reacted with the dissolved iron, causing it to precipitate as insoluble iron oxides. These oxides then settled onto the ocean floor. The alternating bands represent cyclical changes in the amount of oxygen and dissolved iron available in the ancient oceans. Once the reservoir of dissolved iron was exhausted, the formation of BIFs ceased, marking the transition to an oxygen-rich environment.
Iron Found Outside of Ore Deposits
While iron is overwhelmingly found in oxide minerals within sedimentary rocks, a small portion exists in its elemental, metallic state in other specific contexts. The largest reservoir of metallic iron on Earth is inaccessible, comprising the planet’s core, which is thought to be a massive alloy of iron and nickel. The movement of this molten iron-nickel alloy in the outer core is what generates Earth’s magnetic field.
Outside of the core, elemental iron is most readily found in iron meteorites, which are fragments of asteroids that have fallen to Earth. These meteorites are primarily composed of iron-nickel alloys, showing the metal’s natural state in the vacuum of space. The iron-nickel minerals in these extraterrestrial rocks are called kamacite and taenite.
On the Earth’s surface, rare occurrences of native iron, sometimes called telluric iron, can be found in certain volcanic rocks. These deposits, such as those found on Disko Island in Greenland, formed when molten basaltic magma came into contact with carbon-rich sedimentary rocks. This contact created highly reducing chemical conditions that stripped the oxygen from the iron oxides, allowing droplets of pure metallic iron to form within the cooling lava.