Introduction
Mineralogy studies minerals — the naturally occurring, inorganic, crystalline building blocks of rocks — and provides the compositional foundation needed before rocks themselves (Chapter 5) can be properly classified and understood.
1. Crystal systems and basic crystallography
A mineral is defined by a specific, ordered internal atomic arrangement (its crystal structure), which is classified into one of seven crystal systems (cubic, tetragonal, orthorhombic, hexagonal, trigonal, monoclinic, and triclinic) based on the symmetry and relative lengths/angles of the crystal’s defining axes. This internal atomic order — rather than external shape alone, which can be modified by growth conditions — is what fundamentally defines a mineral and, ultimately, controls the physical properties described in point 2.
2. Physical properties of minerals
Minerals are identified in the field and laboratory using a consistent set of physical properties: hardness (resistance to scratching, measured on the relative Mohs scale from talc at 1 to diamond at 10), cleavage (the tendency to break along smooth, flat planes determined by weaknesses in the crystal structure) versus fracture (irregular breakage where no such planes exist), luster (how a mineral’s surface reflects light — metallic or non-metallic, further subdivided), color (often variable and unreliable alone, since impurities can alter it), streak (the color of a mineral’s powder, obtained by rubbing it on an unglazed porcelain plate — often more diagnostic than the mineral’s surface color), and specific gravity (density relative to water). Reliable identification generally requires combining several of these properties rather than relying on any single one.
3. Optical properties and identification concepts
Under a polarizing microscope (using thin, translucent slices of rock or mineral), minerals display characteristic optical properties — including how they interact with polarised light (Physics Chapter III) — such as birefringence (splitting light into two rays travelling at different speeds) and characteristic interference colors, allowing precise mineral identification even when physical hand-specimen properties are ambiguous or when minerals are too fine-grained to identify by eye alone.
4. Silicate structures and major mineral groups
Silicates — built from a basic silicon-oxygen tetrahedron (SiO₄) — are the most abundant mineral group in the Earth’s crust, and are classified by how these tetrahedra link together: isolated tetrahedra (e.g., olivine), single chains (pyroxenes), double chains (amphiboles), sheets (micas), and fully interconnected three-dimensional frameworks (quartz, feldspars). This linkage pattern directly controls a silicate mineral’s cleavage behaviour (e.g., sheet silicates like mica cleave into thin, flat sheets, directly reflecting their sheet-like atomic structure) and overall physical properties.
5. Common rock-forming and economic minerals
Rock-forming minerals (quartz, feldspars, micas, pyroxenes, amphiboles, olivine, calcite) make up the vast majority of common rocks and are essential to recognise for rock classification (Chapter 5). Economic minerals are valued specifically for their industrial, metallurgical, or commercial use (such as metal ores, or minerals mined for specific chemical or physical properties) — this distinction is one of use and value rather than a fundamentally different physical or chemical nature, and a single mineral can sometimes fall into both categories depending on context.
Key Takeaways
- A mineral’s crystal system reflects its fundamental internal atomic ordering, distinct from, though it also influences, its external shape.
- Combining several physical properties (hardness, cleavage/fracture, luster, streak, specific gravity) is more reliable for identification than relying on color or any single property alone.
- Optical properties under a polarizing microscope allow precise identification of minerals too fine-grained or ambiguous for hand-specimen analysis.
- Silicate minerals are classified by how their basic SiO₄ tetrahedra link together, and this linkage pattern directly explains their cleavage and other physical properties.
- Rock-forming and economic minerals are distinguished by use and abundance rather than fundamentally different composition, and the two categories can overlap.