The classification of minerals is a fundamental concept in geology and mineralogy, providing a standardized way to organize and understand the vast array of mineral species found in the Earth's crust. While physical properties like hardness and color are useful for identification, the formal scientific classification is rooted in a mineral's inherent chemical makeup and its ordered atomic arrangement. These two internal characteristics are the key determinants of all external properties. Historically, different systems have been used, but modern mineralogy relies on comprehensive chemical-structural principles, most notably represented by the Strunz classification.
The Primary Basis: Chemical Composition and Crystal Structure
At the heart of mineral classification are two fundamental principles: chemical composition and crystal structure. Every recognized mineral species possesses a specific chemical formula and a distinct internal atomic arrangement.
Classification by Chemical Composition
The most important and broadest level of mineral classification is based on the dominant anion or anionic group within its chemical formula. This approach groups minerals with similar chemical properties together, as the anionic group often dictates many of the mineral's shared characteristics.
Common anionic classes include:
- Silicates (SiO4)4-: This is by far the largest and most important mineral class, comprising over 90% of the Earth's crust. These minerals are built from silicon-oxygen tetrahedra that link together in various complex ways, forming everything from the single chains of pyroxenes to the three-dimensional frameworks of quartz.
- Native Elements: Composed of a single chemical element, these minerals occur in an uncombined state in nature. Examples include gold (Au), copper (Cu), and carbon (as diamond or graphite).
- Oxides: These minerals feature oxygen bonded with one or more metal cations. This class includes economically important minerals like hematite (Fe2O3) and magnetite (Fe3O4).
- Sulfides: Consisting of a metal cation combined with sulfur, this group contains many significant metal ores. Examples are galena (PbS) and pyrite (FeS2).
- Carbonates (CO3)2-: These minerals contain the carbonate anionic group. Calcite (CaCO3) is a classic example and effervesces when exposed to acid.
- Halides: These are minerals with a halogen element (like chlorine or fluorine) as the dominant anion. Halite (NaCl), or rock salt, is a well-known halide.
- Sulfates (SO4)2-: These contain the sulfate anionic group. Gypsum (CaSO4·2H2O) is a common example.
Classification by Crystal Structure
Beyond chemical composition, a mineral's internal atomic arrangement, or crystal structure, provides a further layer of classification. This is particularly crucial for complex classes like the silicates, where the arrangement of the silicon-oxygen tetrahedra is used to define subclasses. The specific crystal structure dictates a mineral's symmetry and is the ultimate cause of its physical properties. A chemical compound can even form different minerals if its crystal structure changes, a phenomenon known as polymorphism. For instance, diamond and graphite are both composed of pure carbon but have vastly different properties due to their distinct crystal structures. Minerals are organized into seven main crystal systems based on their symmetry: cubic, hexagonal, tetragonal, trigonal, orthorhombic, monoclinic, and triclinic.
Major Mineral Classification Systems
Two prominent systems guide mineral classification: the historical Dana system and the modern Strunz system, which incorporates both chemistry and structure.
The Dana Classification System
Developed in the mid-19th century by James Dwight Dana, this system was one of the first comprehensive attempts to organize minerals based on their chemical composition. It groups minerals into classes based on their dominant anionic group, with further organization by structure. The Dana system was revolutionary for its time and remains a widely referenced text in mineralogy.
The Strunz Classification System
The Strunz system, and its modern revision the Nickel-Strunz system, is the internationally recognized standard used by the International Mineralogical Association. It refines the Dana system by placing a stronger emphasis on the crystallographic structure alongside chemical composition. This hierarchical approach allows for a more precise and comprehensive classification of mineral species.
| Comparison of Dana and Strunz Classification Systems | Feature | Dana Classification System | Nickel-Strunz Classification System |
|---|---|---|---|
| Basis for Grouping | Primarily based on chemical composition (dominant anion). | Combines chemical composition with detailed crystal structure. | |
| Hierarchy | Divides minerals into nine major classes with subclasses based on composition and structure. | Follows a more complex, multi-level hierarchy using classes, divisions, families, and groups. | |
| Silicate Subdivisions | Initially used subgroups for silicates, primarily focused on composition ratios. | Subdivides silicates based on the polymerization of the silicate tetrahedra (e.g., nesosilicates, tectosilicates). | |
| Status | A historically significant and widely referenced system. | The modern standard for mineral classification, used by the IMA. | |
| Level of Detail | Offers a clear, broad-level classification useful for many purposes. | Provides a more granular and chemically-accurate classification of all mineral species. |
Physical Properties as a Tool for Identification
While chemical composition and crystal structure are the definitive basis for classification, physical properties are the observable clues geologists use for identification. These properties are a direct manifestation of the mineral's internal makeup.
- Hardness: A mineral's resistance to scratching, measured on the Mohs hardness scale. The scale ranges from talc (1) to diamond (10) and is determined by the strength of atomic bonds.
- Cleavage and Fracture: Cleavage is the tendency of a mineral to break along specific, flat planes of weakness in its crystal structure, while fracture is irregular breakage.
- Luster: Describes how a mineral reflects light, categorized as metallic or nonmetallic (e.g., vitreous, pearly).
- Color and Streak: Color is often unreliable for identification due to impurities, but streak (the color of the mineral in powdered form) is a more consistent property.
- Specific Gravity: The ratio of a mineral's density to the density of water. Minerals with higher atomic mass elements generally have higher specific gravity.
In conclusion, the fundamental basis on which minerals are classified lies in their unique chemical composition and highly ordered internal crystal structure. While physical properties like hardness and luster serve as practical identification tools, the modern Strunz system provides the most accurate and universally accepted framework by systematically organizing minerals according to their chemical and structural relationships. This rigorous scientific approach is essential for mineralogists to understand the formation, properties, and relationships between the thousands of known mineral species on Earth and beyond. For more detailed information on specific mineral groups, authoritative sources like the Mineralogy Database are invaluable.
