Definition and Classification of Minerals
- A mineral is a solid substance with a specific chemical composition and crystal structure.
- Minerals occur naturally in pure form and are distinct from rocks.
- Some minerals can be biogenic or organic compounds.
- Mineraloids are natural solid substances without a definite crystalline structure.
- Different crystal structures of the same chemical compound are considered different mineral species.
- The International Mineralogical Association (IMA) is the standard body for mineral species definition and nomenclature.
- The IMA recognizes 5,955 official mineral species as of July 2023.
- Small amounts of impurities can vary the chemical composition of a named mineral species.
- Specific varieties of a mineral species may have their own names.
- Minerals with variable proportions of chemical elements may form mineral groups.
- To be considered a distinct mineral, it must occur naturally through geological processes.
- Minerals must be solid in their natural occurrence, except for native mercury.
- A mineral must have a well-defined crystallographic structure.
- It must have a fairly well-defined chemical composition, although some substances with variable composition may be considered single mineral species.
- Stability, crystal size, and controversies regarding amorphous substances are factors considered by the IMA.
- Mineral species are commonly named after a person or discovery location.
- Some mineral names are based on chemical composition or physical properties.
- Most mineral names end in -ite, with exceptions for well-established names like galena and diamond.
- The IMA has approved 5,955 mineral species.
- Names based on chemical composition or physical properties are common in mineral naming.
- The IMA excludes biogenic crystalline substances from the definition of minerals.
- Some geologists and mineralogists argue that organisms can form minerals that cannot be formed inorganically.
- Biominerals, created through metabolic activities of organisms, are considered minerals by some.
- Advances in genetics and spectroscopy may provide new insights into the relationship between microorganisms and minerals.
- The Working Group on Environmental Mineralogy and Geochemistry focuses on the biogeochemical aspects of minerals.
Minerals in Different Environments
- Minerals can be found on nearly every rock, soil, and particle surface.
- They exist in the hydrosphere, atmosphere, and biosphere.
- Microorganisms contribute to the formation of minerals through biogeochemical cycles.
- These microorganisms can precipitate metals from solution, forming ore deposits.
- They can also catalyze the dissolution of minerals.
Rocks, Ores, and Gems
- Schist is a metamorphic rock characterized by platy minerals.
- Rocks are aggregates of one or more minerals.
- Some rocks are primarily composed of one mineral, such as limestone or quartzite.
- Granite is defined by proportions of quartz, alkali feldspar, and plagioclase feldspar.
- The major rock-forming minerals include quartz, feldspars, micas, amphiboles, pyroxenes, olivines, and calcite.
- Commercially valuable minerals other than gemstones, metal ores, or mineral fuels are known as industrial minerals.
- Muscovite, a white mica, can be used for windows, as a filler, or as an insulator.
- Ores are minerals with a high concentration of a certain element, typically a metal.
- Examples of ores include cinnabar, sphalerite, cassiterite, and colemanite.
- Gems are minerals with ornamental value, distinguished by their beauty, durability, and rarity.
Crystal Structure and Chemical Composition
- The abundance and diversity of minerals are controlled by their chemistry.
- Eight elements, including oxygen and silicon, account for most of the key components of minerals.
- The minerals that form are those that are most stable at the temperature and pressure of formation.
- Complex thermodynamic calculations are required to predict the minerals present in a rock of a particular composition.
- Chemical substitution and coordination polyhedra explain the variability in mineral composition.
- Coordination polyhedra are geometric representations of how a cation is surrounded by an anion.
- The base unit of silicate minerals is the silica tetrahedron, with one Si surrounded by four O.
- Silicon typically has a coordination number of 4, except in high-pressure minerals where it is in six-fold coordination with oxygen.
- The substitution of Si by Al allows for a variety of minerals due to the need to balance charges.
- Bigger cations have bigger coordination numbers compared to oxygen.
- Aluminosilicates like kyanite, andalusite, and sillimanite differ in the coordination number of Al.
- Changes in pressure and temperature can cause these minerals to transition from one another.
- Changes in temperature, pressure, and composition can alter the mineralogy of a rock sample.
- Changes in composition can be caused by processes like weathering or hydrothermal alteration.
- Changes in temperature and pressure occur during tectonic or magmatic movement.
- Thermodynamic conditions can lead to mineral reactions and the formation of new minerals.
- Two rocks can have similar bulk rock chemistry but different mineralogy due to mineralogical alteration.
Physical Properties and Crystal Habit
- Classifying minerals involves considering physical properties.
- Physical properties used for classification include crystal structure, hardness, lustre, color, and specific gravity.
- Other tests like fluorescence, magnetism, and reactivity to acids can also be used.
- Optical, chemical, or X-ray diffraction analysis may be required for complex identification.
- Some physical properties are sufficient for full identification, while others require more detailed analysis.
- Crystal structure is based on the orderly arrangement of atoms in a mineral's internal structure.
- Crystal structure can be determined by X-ray diffraction.
- Minerals are described by their symmetry content and belong to 32 point groups.
- Polymorphs are minerals with the same chemical formula but different crystal structures.
- Differences in crystal structure and chemistry influence other physical properties of minerals.
- Various types of twins exist, including contact twins, reticulated twins, geniculated
Mineral Data Sources