Steel is not a mineral.
The Scientific Definition of a Mineral
A substance must meet five criteria to be classified as a mineral. The first requirement is that the substance must be naturally occurring, meaning it forms through geologic processes without human intervention. This excludes anything manufactured in a factory or laboratory setting.
The material must also be inorganic, which specifies that it cannot be composed of compounds derived from living organisms, like wood or sugar. Furthermore, a mineral must exist as a solid under normal conditions, which means liquid water is not a mineral, but naturally formed ice is.
A mineral possesses a definite chemical composition, represented by a specific chemical formula or a narrow range of compositional variation. For example, quartz is always silicon dioxide (\(\text{SiO}_2\)). Finally, it must have an ordered internal structure, meaning its atoms are arranged in a regular, repeating, three-dimensional pattern known as a crystalline structure.
Steel’s Identity as an Alloy
Steel is accurately defined as a metallic alloy, a substance made by melting and mixing two or more elements, at least one of which is a metal. Its composition is primarily iron, with a carefully controlled amount of carbon, typically ranging from 0.02% to 2.14% by weight.
The production of steel begins with iron ore, a mineral such as hematite (\(\text{Fe}_2\text{O}_3\)). The ore is subjected to high-temperature smelting, often in a blast furnace, to reduce the iron oxide into metallic iron.
This initial product, known as pig iron, contains too much carbon and other impurities. It is further refined in vessels like a Basic Oxygen Furnace, where pure oxygen is blown through the molten metal to burn off excess carbon and other elements.
Manufacturers then add specific elements like manganese, chromium, or nickel to the molten iron-carbon mixture to create different grades of steel, such as stainless or tool steel. This human-controlled process of alloying gives steel its superior strength and versatility over pure iron.
Why Steel Fails Mineral Classification
Steel is not a mineral because it fails the “naturally occurring” criterion. Steel does not form spontaneously in the Earth’s crust; it is the result of complex industrial processes that refine iron ore and mix it with carbon and other elements.
Steel also fails the requirement of having a definite chemical composition, as its formula is highly variable and depends on the desired properties. Mild steel, for example, is nearly 99% iron with less than 0.25% carbon, while a common stainless steel contains only about 70% iron with a minimum of 18% chromium and 8% nickel. This wide range of compositions means steel cannot be represented by a single chemical formula like a true mineral.
While steel is a solid and inorganic, the process of alloying and heat treatment often results in a complex blend of phases, such as ferrite, cementite, and martensite. Although these phases can have ordered atomic arrangements, the overall structure of the finished steel is not the single, consistent crystalline structure required for mineral classification.