The Foundations of Mineral Classification
The organized study of minerals, known as mineralogy, relies on classification systems that help scientists manage the thousands of known mineral species. Modern classification is built upon the principles of chemistry and crystallography, moving beyond simpler historical methods. Two prominent systems, the Dana classification and the Strunz classification, have been instrumental in this field. While the older Dana system primarily groups minerals by their chemical composition, the more recent Strunz system, endorsed by the International Mineralogical Association (IMA), offers a more nuanced approach by incorporating both chemical and structural properties. The defining characteristic for grouping is the dominant anion or anionic group present in the mineral's chemical formula, as this anion largely dictates the mineral's shared physical and chemical traits.
The Major Mineral Classes
Native Elements
These minerals occur in nature in a pure, uncombined form, consisting of a single chemical element. They are subdivided into metals, semi-metals, and non-metals.
- Metals: The gold group (gold, silver, copper), the platinum group (platinum), and the iron group (iron, nickel-iron). These are typically soft, malleable, and good conductors of electricity.
- Semi-metals: Includes arsenic, antimony, and bismuth, which are more brittle and poorer conductors.
- Non-metals: Such as carbon (as diamond or graphite) and sulfur, which have vastly different properties from one another.
Silicates
As the largest and most important class, silicates make up about 90% of the Earth's crust and are based on the silicon-oxygen ($SiO_4$) tetrahedron as their fundamental building block. Their subclasses are based on the degree of polymerization, or how these tetrahedra link together.
- Nesosilicates (Isolated Tetrahedra): The tetrahedra are isolated from one another, with metal cations linking them. Examples include olivine and garnet.
- Sorosilicates (Paired Tetrahedra): Two tetrahedra are linked by a single oxygen atom. Examples include epidote.
- Cyclosilicates (Rings): Tetrahedra are linked to form rings. Examples include tourmaline and beryl.
- Inosilicates (Chains): Tetrahedra form single or double chains. Single-chain examples are pyroxenes, while double-chain examples are amphiboles.
- Phyllosilicates (Sheets): Tetrahedra link to form extensive sheets, resulting in minerals with excellent cleavage. Micas and clays are key examples.
- Tectosilicates (Frameworks): All four oxygen atoms of each tetrahedron are shared, creating a three-dimensional framework. This class includes quartz and feldspars.
Oxides and Hydroxides
These minerals combine oxygen or a hydroxyl group ($OH^-$) with a metal. They can range from very hard gemstones like corundum (sapphire, ruby) to soft, economically significant iron ores like hematite. Examples include magnetite and goethite.
Sulfides
Characterized by the sulfide anion ($S^{2-}$), these minerals are often important ores of metals such as lead, zinc, and copper. They tend to have metallic luster and high densities. Notable examples are pyrite (fool's gold), galena, and sphalerite.
Carbonates
These minerals contain the carbonate anion ($CO_3^{2-}$) and are common in sedimentary and metamorphic rocks, often formed in marine environments. Calcite ($CaCO_3$) is a prime example, forming limestone and marble. The carbonate group, which also includes dolomite, is known for effervescing in acid.
Halides
This group includes minerals with a dominant halide anion (a halogen such as fluorine, chlorine, bromine, or iodine). They are typically soft and soluble in water. The most common example is halite ($NaCl$), or rock salt. Fluorite is another well-known halide.
Sulfates
Composed of the sulfate anion ($SO_4^{2-}$), these minerals often form in evaporite deposits where water has evaporated. Gypsum, a hydrated calcium sulfate, is a soft and common sulfate used in construction.
Phosphates
These minerals contain the phosphate anion ($PO_4^{3-}$). While not as common as silicates or carbonates, they can be brightly colored and are often formed through weathering processes. Apatite is a key example.
Comparison of Major Classification Systems
| Feature | Dana Classification | Strunz Classification |
|---|---|---|
| Basis | Primarily chemical composition. | Chemical composition combined with crystal structure. |
| Classes | Originally nine main classes, now expanded. | Ten major classes with numerous subdivisions. |
| System Detail | Offers a traditional approach that is still widely recognized. | Uses a more complex alphanumeric system for greater precision. |
| Adoption | Predominantly used in English-speaking countries, especially the US. | The universal standard endorsed by the International Mineralogical Association (IMA). |
| Example Case | Classifies Quartz (SiO2) as a silicate. | Classifies Quartz (SiO2) as an oxide. |
Conclusion: A Systematic Approach to Mineralogy
In conclusion, the detailed classification of minerals is a fundamental component of geology, enabling the systematic study of Earth's vast array of inorganic materials. By grouping minerals based on their dominant anionic group and crystal structure, systems like Strunz and Dana provide a logical framework for understanding mineral properties and identifying species. From the complex arrangements of silicates to the pure state of native elements, this organizational method offers profound insights into geological processes, formation environments, and the economic potential of mineral resources. The ongoing evolution of mineral classification, guided by scientific research, ensures our understanding of mineral diversity remains precise and up-to-date. For further reading, an excellent resource on mineralogy is available on the Mineralogical Society of America website.
Diagnostic Properties of Mineral Classes
Beyond chemical and structural criteria, geologists use several physical properties to classify and identify minerals in the field.
- Hardness: Resistance to scratching, measured on the Mohs scale. For example, gypsum (2) is much softer than quartz (7).
- Cleavage and Fracture: The way a mineral breaks. Cleavage describes breaking along smooth, flat planes, while fracture describes irregular breaks. Carbonates often show distinct cleavage.
- Luster: The appearance of a mineral's surface in reflected light, described as metallic, vitreous (glassy), pearly, or dull. Sulfides often have a metallic luster.
- Specific Gravity: The ratio of a mineral's density to the density of water. It tends to be higher in metallic and oxide minerals.
- Color and Streak: The color of the mineral and the color of its powder when scraped across an unglazed porcelain plate. Hematite, for example, has a reddish-brown streak, regardless of its external color.
- Reaction to Acid: Particularly useful for carbonates, which effervesce when exposed to dilute hydrochloric acid, releasing carbon dioxide.
- Crystal Habit: The characteristic external shape of a mineral's crystals.
The Role of Polymorphism
Polymorphism is a phenomenon where a single chemical compound can exist in more than one crystal structure. A classic example is calcium carbonate ($CaCO_3$), which occurs naturally as both hexagonal calcite and orthorhombic aragonite. Though chemically identical, their different atomic arrangements result in distinct physical properties, leading them to be classified as different minerals. This highlights why both chemistry and crystal structure are crucial for accurate classification.
A Note on Mineraloids
Some naturally occurring, inorganic substances lack the ordered internal atomic structure of true minerals and are thus called mineraloids. Examples include opal, obsidian, and amber. Although not formally classified as minerals, their study is an important part of mineralogy.
Economic Importance
The detailed classification of minerals has significant economic implications. Different mineral classes concentrate different elements, making some more valuable as ores or industrial minerals. Sulfides, for example, are a primary source of many metals. Understanding the conditions under which these minerals form, as revealed by their classification, is essential for successful resource extraction. The organization of mineralogy, therefore, is not merely an academic exercise but a practical science with wide-reaching applications.
