Mineralogy is the study of minerals. Minerals make up most of the rocks and sediments comprising the Earth and its landscapes. Mineralogists have identified about 4,000 minerals. Fewer than 50 are considered common rock-forming minerals of the Earth’s crust. A mineral is a naturally occurring, solid, crystalline material, formed by geologic processes, that has a definable chemical composition. Most are inorganic. The atoms, ions, or molecules in a mineral are ordered into a geometric arrangement.
A crystal is a single, continuous (uninterrupted) piece of crystalline solid, typically bounded by flat surfaces, called crystal faces, that grow naturally as the mineral forms. The angle between two adjacent crystal faces of one specimen is identical to the angle between the corresponding faces of another specimen. X-ray diffraction is used to discover the specific arrangement of atoms in crystals. A three-dimensional geometry of points representing this pattern is a crystal lattice. A crystal structure has symmetry, meaning that the shape of one part of the structure is the mirror image of the shape of a neighboring part. Two different minerals that have the same composition (such as graphite and diamond) but have different crystal structures are called polymorphs.
New crystals can form in several ways. Solidification (freezing) of a melt happens when a liquid cools and turns into a solid. Precipitation from a water solution takes place when dissolved ions bond together to form crystals that settle out of the water or grow out from the walls of a container. Solid-state diffusion results from the movement of atoms or ions through a solid to arrange into a new crystal structure. Biomineralization takes place when minerals grow at the interface between the physical and biological components of the Earth System (metabolic processes of some living organisms can cause minerals to precipitate). Precipitation directly from a gas can occur around volcanic vents or around geysers. The first step in forming a crystal involves the chance formation of a seed, an extremely small crystal. Once the seed exists, other atoms in the surrounding material attach themselves to the face of the seed. As the crystal grows, crystal faces move outward but maintain the same orientation. As crystals grow, they develop their particular crystal shape, based on the geometry of their internal structure. If a mineral’s growth is unimpeded so that it displays well-formed crystal faces, then it is a euhedral crystal. Minerals without well-formed crystal faces are anhedral grains. A geode is a mineral-lined cavity in rock. A mineral can be destroyed by melting, dissolution, or some other chemical reaction.
Some of the physical properties most commonly used in basic mineral identification are as follows:
- Certain minerals always have the same color but many display a range of colors. Color variation in some minerals are due to the presence of impurities.
- The streak of a mineral refers to the color of a powder produced by pulverizing the mineral. The color of a mineral powder tends to be more consistent than the color of a whole crystal.
- Luster refers to the way a mineral surface scatters light.
- Hardness is a measure of the relative ability of a mineral to resist scratching, and it therefore represents the resistance of bonds in the crystal structure to being broken. The Mohs hardness scale lists mineral in sequence of relative hardness. Diamond, the hardest mineral known, can scratch anything.
- Specific gravity represents the density of a mineral.
- The crystal habit of a mineral refers to the shape of a single crystal with well-formed crystal faces, or to the character of an aggregate of many well-formed crystals that grew together as a group. The habit depends on the internal arrangement of atoms in the crystal.
- Some minerals have special properties that readily distinguish them from other minerals (such as magnetism or reactivity with acid).
- Different minerals fracture (break) in different ways, depending on the internal arrangement of atoms in the minerals. If a mineral breaks to form distinct planar surfaces that have a specific orientation in relation to the crystal structure, then we say that the mineral has cleavage. Cleavage forms in directions where the bonds holding atoms together in the crystal are weaker. Some minerals have one direction of cleavage, others have two or three directions of cleavage, and some have no cleavage at all.
Minerals can be classified by specifying the principal anion (negative atom) or anionic group (negative molecule) within the mineral. Mineralogists distinguish several principal classes of minerals. Some of the major ones are silicates, sulfides, oxides, halides, carbonates, native metals, and sulfates. Silicates make up over 95% of the continental crust and almost 100% of the oceanic crust. The silicon-oxygen tetrahedron, a silicon atom surrounded by four oxygen atoms, serves as the fundamental building block of silicate minerals. The silicate groups are isolated tetrahedra, single chains, double chains, sheet silicates, and framework silicates.
A gemstone is a mineral that has special value because it is rare and people consider it beautiful. A gem, or jewel, is a finished stone ready to be set in jewelry. Precious stones are particularly rare and expensive; all precious stones are transparent crystals, though most have some color. Semiprecious stones are less rare and less expensive; these also include opaque or translucent minerals. The gems found in jewelry have been faceted; the facets are not natural crystal faces or cleavage surfaces. The fire of a jewel comes from the way it reflects light internally.
—January 2021
—April 2023
Lance Jones 