The question of whether oxygen is a mineral requires a precise understanding of scientific classification. While the element oxygen is fundamental to nearly all geological materials, its elemental form, which is what we breathe, does not meet the strict criteria established by mineralogists. The answer lies in oxygen’s physical state and atomic arrangement under natural conditions. Mineralogists use a defined set of five tests to separate true minerals from all other solids, liquids, and gases found in nature.
The Scientific Definition of a Mineral
Mineralogy uses a rigorous set of characteristics to classify a substance. A true mineral must be naturally occurring, meaning it formed through geological processes without human intervention. It must also be inorganic, which excludes materials derived from living organisms.
A mineral must exist as a solid under normal Earth surface conditions. It must also possess a definite chemical composition, which can be expressed by a specific chemical formula.
Finally, a mineral must have an ordered internal atomic structure, where atoms are arranged in a repeating, three-dimensional pattern called a crystal lattice. This crystalline structure distinguishes minerals from amorphous solids like glass. These five criteria act as the necessary filter for classification.
Why Oxygen Fails the Mineral Test
Elemental oxygen (\(\text{O}_2\)), which constitutes about 21% of Earth’s atmosphere, immediately fails the mineral test due to its physical state. In its common form, oxygen is a gas, and a mineral must be a solid under typical surface conditions. This single fact disqualifies atmospheric oxygen from classification.
Even when oxygen freezes into a solid in extremely cold environments, it generally does not qualify as a mineral. The established classification requires a structure that is stable and formed by geological processes on Earth. Furthermore, elemental oxygen, whether gaseous or solid, completely lacks the ordered internal atomic arrangement required of a mineral species.
The simple diatomic molecule (\(\text{O}_2\)) cannot form the complex, repeating crystal lattice that defines a mineral’s internal architecture. The majority of naturally occurring solid oxygen on Earth, such as ice, is also a compound (\(\text{H}_2\text{O}\)), not the elemental form.
Oxygen as a Building Block of True Minerals
While elemental oxygen is not a mineral, oxygen atoms are the single most abundant element in Earth’s crust, making up about 46.3% of its weight. This prevalence means oxygen is incorporated into the vast majority of true minerals.
Oxygen atoms typically exist within these compounds as an anion (\(\text{O}^{2-}\)), bonded to metals and metalloids. The most significant example is the silicate mineral class, which accounts for over 90% of the Earth’s crust.
In silicates, oxygen atoms surround a silicon atom to form the fundamental building block called the silica tetrahedron (\(\text{SiO}_4^{4-}\)). This structure links together in chains, sheets, or three-dimensional frameworks, creating the crystalline mineral. Oxygen also forms the basis of other major mineral groups, such as oxides (e.g., hematite), carbonates, and sulfates.