The Foundations of Mineral Classification
For a substance to be officially recognized as a mineral, it must meet several criteria, including being a naturally occurring, inorganic solid with a defined chemical composition and an ordered internal atomic arrangement. The two most crucial factors for classification are this specific chemical composition and the internal crystalline structure.
The Importance of Chemical Composition
Since the mid-19th century, chemical composition has served as the highest-level distinguishing factor for mineral classification. The dominant anion or anionic group (a negatively charged group of atoms) dictates the class to which a mineral belongs, largely because minerals with the same anion tend to have similar properties and often occur in similar geological environments. For example, all carbonate minerals share the ($CO_3$)$^{2-}$ anionic group and effervesce in acid.
The Role of Crystal Structure
While chemical makeup provides the broad category, the internal atomic structure determines a mineral's specific physical properties and helps to refine its classification. Advanced techniques like X-ray diffraction, pioneered by scientists like W. Lawrence Bragg, enable the determination of this internal arrangement. This is especially important for complex groups like the silicates, where the arrangement of silicon-oxygen tetrahedra is the basis for further subdivision.
Major Classification Systems: Dana and Strunz
Two principal systems are used by mineralogists today: the Dana system and the more modern Strunz system.
- The Dana System: Developed by American mineralogist James Dwight Dana in 1848, this system organizes minerals into several main classes based on their chemical composition, primarily by the dominant anion. The core classes include Native Elements, Silicates, Oxides, Sulfides, Sulfates, Halides, Carbonates, and Phosphates.
- The Strunz System: Created in 1941 by German mineralogist Karl Hugo Strunz and later refined with Ernest H. Nickel, this is the system currently used by the International Mineralogical Association (IMA). It is more detailed than Dana's, using both chemical composition and crystal structure for its hierarchical classification, which consists of ten main classes.
The Primary Mineral Classes
- Native Elements: Minerals composed of a single element, such as Gold (Au), Silver (Ag), and Diamond (C).
- Sulfides: Compounds of sulfur combined with a metal. Examples include Pyrite ($FeS_2$) and Galena (PbS).
- Oxides and Hydroxides: Formed from a metal combined with oxygen. Examples include Hematite ($Fe_2O_3$) and Magnetite ($Fe_3O_4$).
- Halides: Compounds where a halogen element (Cl, F, Br, I) is the dominant anion. Halite (NaCl) is a well-known example.
- Carbonates: Characterized by the carbonate anionic group ($CO_3$)$^{2-}$. Calcite ($CaCO_3$) is a common carbonate mineral.
- Sulfates: Minerals containing the sulfate group ($SO_4$)$^{2-}$. Gypsum ($CaSO_4$·2$H_2O$) is a familiar sulfate.
- Phosphates: Composed of the phosphate group ($PO_4$)$^{3-}$ and metal cations. Apatite is a common phosphate mineral.
The Silicate Group: A Structural Hierarchy
As the largest and most abundant group, comprising over 90% of the Earth's crust, silicates are further sub-classified based on how the silicon-oxygen tetrahedra are linked.
- Nesosilicates (Isolated Tetrahedra): Examples include the Garnet and Olivine groups.
- Sorosilicates (Double Tetrahedra): Minerals like Epidote fall into this category.
- Cyclosilicates (Ring Silicates): This group includes Tourmaline and Beryl.
- Inosilicates (Chain Silicates): Features either single or double chains of tetrahedra, such as those found in Pyroxenes and Amphiboles.
- Phyllosilicates (Sheet Silicates): Micas and clay minerals have a sheet-like structure.
- Tectosilicates (Framework Silicates): Minerals like Quartz and Feldspar, where all oxygen atoms are shared.
The Role of Physical Properties in Identification
While chemical and structural properties are the basis for scientific classification, physical properties are essential for field identification.
- Hardness: A mineral's resistance to scratching, measured using the Mohs hardness scale.
- Cleavage and Fracture: How a mineral breaks, either along smooth planes (cleavage) or irregular surfaces (fracture).
- Luster: The appearance of the mineral's surface in reflected light (e.g., metallic, glassy, pearly).
- Color and Streak: While color can be unreliable due to impurities, the streak (color of the powdered mineral) is more consistent.
Classification of Minerals: A Comparison
To understand the different approaches to mineral organization, here is a comparison of classification by chemical composition versus physical properties.
| Criterion | Chemical Classification (e.g., Dana/Strunz) | Physical Properties (for identification) |
|---|---|---|
| Primary Basis | Dominant anion/anionic group and internal crystal structure | Observable characteristics like color, hardness, and luster |
| Use | Standard scientific framework for cataloging minerals | Practical tool for field and lab identification |
| Reliability | Highly reliable and consistent for defining a species | Can be less reliable, as properties may vary within a single species (e.g., Quartz color) |
| Examples | Silicates, Oxides, Halides | Mohs hardness scale (Talc to Diamond) |
Conclusion
In summary, the question of how are minerals classified is answered primarily by their chemical composition and internal atomic arrangement, which form the basis of the internationally recognized Dana and Strunz systems. These systems categorize minerals into distinct groups, such as native elements, sulfides, and the complex silicate group. Although observable physical characteristics like color and hardness are useful for practical identification in the field, they are a reflection of these more fundamental chemical and structural properties. For a more detailed look at the scientific methodology behind this, Britannica provides an in-depth resource.
Mineral classification methods: A quick guide
- Chemical Composition: Minerals are grouped primarily by their dominant anion or anionic group, which is the basis for major classification systems like Dana and Strunz.
- Crystal Structure: The internal, ordered arrangement of atoms determines a mineral's specific category, especially for the large silicate group.
- Silicate Sub-classes: Silicates are subdivided into subclasses (e.g., nesosilicates, tectosilicates) based on the unique linkage of their silicon-oxygen tetrahedra.
- Physical Properties: Observable traits like hardness, cleavage, and luster are used to identify minerals in the field but are less reliable for formal classification than chemical makeup.
- Primary Systems: The Dana and Strunz classifications are the most widely used systems for organizing minerals based on their chemical and structural properties.
- Non-silicate Groups: Native elements, sulfides, oxides, halides, carbonates, sulfates, and phosphates are among the primary mineral classes.
FAQs
Question: Why is chemical composition the primary basis for mineral classification? Answer: Chemical composition, specifically the dominant anion, is used because it has the most significant effect on a mineral's properties. Minerals with the same dominant anion tend to share similar characteristics and form in comparable geological settings.
Question: What are the seven crystal systems? Answer: The seven crystal systems are based on the geometric arrangement of atoms in a mineral's crystalline structure. They are triclinic, monoclinic, orthorhombic, tetragonal, hexagonal, rhombohedral, and cubic.
Question: What is the difference between cleavage and fracture? Answer: Cleavage is the tendency of a mineral to break along specific, flat, parallel planes of weakness within its crystal structure. Fracture, in contrast, is the way a mineral breaks when it does not follow these specific planes.
Question: Are physical properties like color reliable for mineral classification? Answer: No, physical properties are generally not reliable enough for formal classification. Color can be influenced by impurities and weathering. However, they are still valuable for initial identification in the field.
Question: What is the Strunz classification system? Answer: The Strunz system is a modern mineral classification system that categorizes minerals into ten main classes based on their chemical composition and internal crystal structure. It is endorsed by the International Mineralogical Association.
Question: Why do silicates have their own sub-classification system? Answer: Silicates are the largest and most abundant mineral group. Their internal structure, specifically how the silicon-oxygen tetrahedra are linked, is complex and varies significantly, necessitating a more detailed, structural sub-classification.
Question: What is a native element mineral? Answer: A native element is a mineral that occurs in nature in its pure, uncombined form. Examples include native gold, copper, and sulfur.
Citations
["Mineral - Classification, Properties, Types | Britannica", "https://www.britannica.com/science/mineral-chemical-compound/Classification-of-minerals"]
