02 Minerals
Learn what defines a mineral, how to identify one from its properties, how minerals form, and how their chemical composition shapes their classification.
What Defines a ?
A is a naturally occurring, inorganic solid with an orderly internal crystal structure and a characteristic chemical composition, sometimes with a limited range of variation. Minerals are the building blocks of rocks: granite, for example, consists mostly of quartz, feldspar, and mica.
Within a , atoms follow a repeating arrangement. This internal structure, together with the ’s composition, helps determine its properties. A visible crystal is a grain that grew with recognizable faces. Many grains do not show crystal faces because they grew in crowded spaces.
Takeaway: A is defined by its natural origin, inorganic composition, solid state, crystal structure, and characteristic chemistry—not simply by how it looks.
Identifying Minerals by Their Properties
Geologists identify minerals by combining observations; color alone is not reliable because impurities can change a ’s color and different minerals can share the same color.
describes how a surface reflects light, such as with a metallic, glassy, or dull appearance.
is the color of a ’s powder, usually tested on an unglazed porcelain plate. It may differ from the visible color.
is resistance to scratching. The Mohs scale ranks ten reference minerals from talc at 1 to diamond at 10. It is a relative scale, so its steps are not equal intervals. A can scratch materials softer than itself.
is breaking along repeated, relatively flat planes set by the crystal structure. Mica, for example, splits into thin sheets.
is breakage that does not follow planes. Quartz commonly breaks with curved, shell-like surfaces.
describes the typical external shape or growth pattern of crystals, when visible.
Density, heft, magnetism, and reaction with dilute acid can provide additional clues for some minerals.
A practical identification combines these clues. Quartz and calcite may both appear clear, but quartz has a Mohs of 7 and lacks . Calcite is softer, with a Mohs of 3, has prominent , and fizzes in dilute hydrochloric acid. Use appropriate safety precautions for chemical tests; no single property usually proves an identification.
Takeaway: Compare several properties, especially , , , and how the breaks, rather than relying on color.
How Minerals Form
Minerals form when atoms assemble into crystals under suitable chemical and physical conditions. Several processes can produce them:
Crystallization from magma or lava: As molten rock cools, minerals crystallize. Slow cooling underground can allow larger crystals to grow, while rapid cooling at the surface generally produces smaller crystals.
Precipitation from water: Dissolved substances can form crystals as water evaporates or its temperature or chemistry changes. Halite can form as salty water evaporates, and calcite can precipitate from water in caves.
Growth from hot, -rich fluids: Heated water moving through cracks can deposit minerals in veins as it cools or reacts with surrounding rock.
Recrystallization and alteration: Heat, pressure, and chemically active fluids can transform existing minerals or produce new ones without fully melting the rock. Weathering at Earth’s surface can also alter minerals, forming products such as clay minerals.
The same can form through different processes. Quartz, for example, can crystallize from magma, grow from fluids, or occur in rocks that have been altered or recycled. Its setting and neighboring minerals can help reveal its history.
Takeaway: A ’s formation depends on its environment, and one may form in more than one way.
Classes and Silicate Structures
Minerals are commonly grouped by chemical composition, especially by their principal anion or anion group. Nonsilicate classes include:
Native elements, such as gold and copper
Sulfides, such as pyrite
Oxides, such as hematite
Halides, such as halite
Carbonates, such as calcite
Sulfates, such as gypsum
Phosphates, such as apatite
are the largest and most important group of rock-forming minerals. Their basic structural unit is a silicon atom surrounded by four oxygen atoms in a tetrahedron. These units can remain isolated or link into chains, sheets, and three-dimensional frameworks. Olivine contains isolated tetrahedra; pyroxenes form chains; micas form sheets; and feldspars and quartz have framework structures. The way the tetrahedra connect helps explain differences in structure and properties.
Takeaway: classification reflects chemistry, while the way atoms are arranged and connected helps explain structural differences.