The Basis for Mineral Classification
Minerals are naturally occurring, inorganic solids with a distinct chemical composition and a characteristic crystal structure. The most widely accepted classification system is based on the mineral's chemical composition, specifically the dominant anion or anionic group. This method is highly effective because minerals with the same anion or anionic group tend to share similar properties and often form under similar geological conditions.
Classification by Anionic Group
Here's a breakdown of the major mineral classes according to their chemical composition:
- Native Elements: These are minerals that exist in a pure, uncombined elemental state. They include metals, semimetals, and nonmetals. Examples include gold (Au), copper (Cu), sulfur (S), and diamond (C).
- Silicates: By far the largest and most important class, silicates contain silicon and oxygen as their primary components, often bonded with metal cations. They make up approximately 90% of the Earth's crust and are further divided into sub-classes based on how the silica tetrahedra are linked.
- Oxides: This group is formed by the combination of a metal with oxygen. Oxide minerals are significant sources of iron, aluminum, and manganese, and examples include hematite ($Fe_2O_3$) and corundum ($Al_2O_3$).
- Sulfides: These minerals consist of a metal cation bonded with a sulfide anion ($S^{2-}$), often serving as important sources of base metals like copper, zinc, and lead. Pyrite ($FeS_2$) and galena ($PbS$) are classic examples.
- Sulfates: Containing the sulfate anionic group ($SO_4^{2-}$), these minerals commonly form in evaporite environments or as secondary minerals in ore deposits. Gypsum ($CaSO_4 \cdot 2H_2O$) and barite ($BaSO_4$) are well-known examples.
- Carbonates: These minerals contain the carbonate anion ($CO_3^{2-}$) and are significant components of sedimentary rocks like limestone. Calcite ($CaCO_3$) and dolomite ($CaMg(CO_3)_2$) are part of this class.
- Halides: Halide minerals are salts that form from the evaporation of salt water. They contain halogen elements such as fluorine (F), chlorine (Cl), or bromine (Br), combined with metal elements. Halite (NaCl), or table salt, is a common example.
- Phosphates: Composed of the phosphate anionic group ($PO_4^{3-}$), this class includes minerals essential for biological and agricultural purposes. Apatite is a notable phosphate mineral found in bones and teeth.
- Organic Minerals: These are rare, natural organic compounds that have been formed through geological processes. They typically have complex structures and often occur in association with biological materials like guano or fossilized wood.
Comparison of Major Mineral Classes
This table highlights the key differences between the major mineral classes.
| Classification | Anionic Group | Common Occurrence | Economic Importance | Example Minerals |
|---|---|---|---|---|
| Silicates | $(SiO_4)^{4-}$ tetrahedron | Igneous, metamorphic, and sedimentary rocks | Essential rock-forming minerals | Quartz, Feldspar, Mica |
| Native Elements | None (pure element) | Hydrothermal veins, magmatic | Precious metals, gems, industrial use | Gold, Diamond, Sulfur |
| Oxides | $O^{2-}$ | Weathering zones, igneous rocks | Major ores for iron and aluminum | Hematite, Corundum |
| Sulfides | $S^{2-}$ | Hydrothermal veins, volcanic | Primary source for many metals | Pyrite, Galena, Sphalerite |
| Carbonates | $(CO_3)^{2-}$ | Sedimentary rocks, marine environments | Building materials, metal ores | Calcite, Dolomite |
| Halides | $Cl^{-}$, $F^{-}$, etc. | Evaporite deposits | Sources of salt and chemicals | Halite, Fluorite |
Classification by Crystal Structure
Beyond chemical composition, minerals are also categorized based on their internal atomic arrangement, or crystal structure. There are seven crystal systems—cubic, hexagonal, tetragonal, orthorhombic, monoclinic, triclinic, and rhombohedral—that describe the geometry of a mineral's crystal lattice. Within the silicate class, this structural classification is particularly important and includes subclasses like tectosilicates (frameworks) and phyllosilicates (sheets). For example, the same chemical formula for calcium carbonate ($CaCO_3$) can result in two different minerals, calcite and aragonite, due to differences in their crystal structure, a phenomenon known as polymorphism.
Other Classification Methods
While chemical composition and crystal structure form the bedrock of modern mineralogy, other less common classification methods exist. Some approaches consider the formation process (e.g., primary vs. secondary minerals), while others group minerals based on physical properties like hardness or luster. For example, a geologist might group metallic minerals (containing a metal) separately from non-metallic minerals (lacking metals) for economic or resource-related purposes. This highlights that context and purpose can influence the specific classification approach used.
Conclusion
In summary, the most authoritative answer to what can minerals be classified as is based on their dominant anionic group, which provides a chemically consistent and reliable method for categorization. This system, supplemented by classification based on crystal structure, helps mineralogists understand the vast diversity of minerals found in nature. From the common rock-forming silicates to rare organic compounds, the systematic organization of minerals is key to unlocking the secrets of the Earth’s geological processes and the materials that comprise our planet.
The Role of Outbound Linking
For more in-depth exploration of the chemical basis of mineral classification, the online mineralogical database Mindat provides comprehensive data on every known mineral species.
