Minerals are the fundamental building blocks of the rocks that form Earth’s crust and mantle. Scientists currently recognize over 5,000 distinct mineral species. This diversity necessitates a structured classification system to organize and study their unique properties. Researchers group minerals based on their inherent chemical and structural characteristics, which helps predict their geological behavior and occurrence.
The Scientific Definition of a Mineral
A substance must meet five specific requirements to be classified as a mineral. First, it must be naturally occurring, forming through geologic processes rather than being a synthetic product. It must also be a solid under normal Earth surface conditions, excluding liquids and gases.
The substance must be inorganic, meaning it did not originate from a living organism. A mineral must also possess a definite chemical composition, represented by a specific chemical formula. Finally, it must exhibit an ordered internal atomic structure, a repeating three-dimensional pattern referred to as crystalline. This internal order distinguishes a true mineral from an amorphous solid like glass.
Primary Classification by Chemical Composition
The primary method for classifying minerals organizes them into major classes based on the dominant anion or anionic group they contain. This chemical grouping is highly predictive, as the anion dictates many of the mineral’s fundamental chemical and physical properties. The Silicates are the most abundant class, making up roughly 90% of Earth’s crust, defined by the presence of the silicon-oxygen tetrahedron (SiO4 4-).
The Oxides contain the simple oxygen anion (O2-) bonded with a metal, such as in the iron ore hematite (Fe2O3). The Sulfides are defined by the sulfur anion (S2-), a grouping that includes important ore minerals like pyrite (FeS2) and galena (PbS). The Carbonates are characterized by the complex carbonate group (CO3 2-), as seen in the mineral calcite (CaCO3).
The Sulfates contain the anionic group (SO4 2-), a class that includes gypsum (CaSO4 · 2H2O). Minerals in the Halide class feature a halogen element, such as chlorine or fluorine, bonded with a metal, with halite (NaCl) being a familiar example. The final major grouping is the Native Elements, which are composed of a single element, such as gold (Au) or copper (Cu).
Subdividing Classes by Internal Atomic Structure
Within the major chemical groups, a second level of classification focuses on the internal arrangement of atoms. This structural classification is based on how basic chemical units, such as the SiO4 tetrahedra in silicates, are linked together. The arrangement of these structural units determines the mineral’s crystalline habit and its mechanical properties.
In Silicates, the SiO4 tetrahedra can link up in five distinct ways, creating different subclasses. The simplest are the nesosilicates, which have isolated tetrahedra bonded to surrounding metal ions, such as in the mineral olivine. As the tetrahedra begin to share oxygen atoms, they form chain structures, known as inosilicates, which can be either single chains (pyroxenes) or double chains (amphiboles).
Further sharing of oxygen atoms leads to the formation of two-dimensional sheet structures, classifying them as phyllosilicates, which includes the micas and clay minerals. The maximum degree of linkage creates the tectosilicates, or framework silicates, where every oxygen atom is shared between two tetrahedra. This detailed structural subdivision explains why minerals with similar chemical formulas can exhibit dramatically different physical properties.