Glossary · Vol. 04 · 2026

A field glossary.

The working vocabulary of mineralogy, defined once. Every starred term across the Index — every hardness, every lustre, every IMA — links back here.

A

Amorphous
No regular atomic lattice — atoms are frozen in random positions, as in a solidified liquid.
Anisotropism
Whether an opaque mineral changes brightness or hue as the stage is rotated under crossed polarisers in reflected light. A key ore-mineral test.
Approved (IMA)
Officially recognised by the IMA Commission on New Minerals, Nomenclature and Classification (CNMNC).

B

Bireflectance
A visible change in reflectance as an opaque mineral is rotated under plane-polarised reflected light. Diagnostic for ore minerals.
Birefringence
The difference between the highest and lowest refractive indices. Drives the interference colours seen under crossed polarisers.

C

Cell angles
The angles (α, β, γ) between unit-cell edges, in degrees. Cubic crystals have 90°/90°/90°; triclinic crystals can take any combination.
Cell parameters
The lengths of the unit-cell edges (a, b, c) in ångströms — the smallest repeating box of atoms that builds the crystal.
Cleavage
The tendency to split along flat planes that match the crystal's atomic structure. Described by quality (perfect, good, poor) and direction.
Colour
The hue of the mineral itself. Often unreliable for identification — many species occur in several colours depending on trace impurities or radiation damage.
Commonly confused with
Other species this mineral is regularly mistaken for in the field. Small tests — hardness, streak, density — usually settle it.
Constituent elements
Chemical elements that make up the mineral's ideal formula, listed by atomic symbol with their share of the total mass.
Crystal system
One of seven geometric families that describe a mineral's atomic symmetry: isometric (cubic), tetragonal, hexagonal, trigonal, orthorhombic, monoclinic, triclinic. Two non-crystalline classes — amorphous and icosahedral — also appear.

D

Density
Mass per unit volume, in grams per cubic centimetre. A useful sorting tool: barite is twice as dense as quartz at the same size.
Discovered
The year the mineral was first found or noted in nature — sometimes only approximate for species known since antiquity. For modern minerals it is usually close to the IMA approval year.
Discredited (IMA)
No longer accepted as a distinct species — superseded, redefined or proven invalid.
Dispersion
How strongly the refractive indices vary across the visible spectrum. Strong dispersion gives gemstones their 'fire'.

E

Epitaxy
Oriented overgrowth of one mineral on the surface of another, controlled by matching atomic spacings between the two.
Etymology & naming
Where the mineral's name comes from — usually a Greek or Latin root, a place, or the person it honours.
Extinction
The orientation, relative to crystal edges, at which a thin section goes dark between crossed polarisers. Used to separate look-alike species.

F

Fracture
The shape of a broken surface that doesn't follow cleavage — conchoidal (curved, like broken glass), uneven, hackly, or splintery.

G

Geological setting
The kinds of rock environments where the mineral typically forms — igneous, metamorphic, hydrothermal, sedimentary, pegmatitic, and so on.
Grandfathered (IMA)
Established as a valid species before the IMA was founded in 1959, accepted by community usage.

H

Hardness
Resistance to being scratched, ranked on the Mohs scale from 1 (talc, marked by a fingernail) to 10 (diamond, scratches everything below it). The classic field test.
Hexagonal
Two equal horizontal axes meeting at 120° and a perpendicular vertical axis of different length — a six-sided prism.

I

Icosahedral
A quasicrystalline arrangement with five-fold symmetry — atomically ordered but never periodic, unlike the seven classical systems.
IMA
International Mineralogical Association — the global body that approves new mineral species and maintains the master list of valid names.
IMA approved
The year the International Mineralogical Association ratified the species. Modern minerals need an IMA number before they are formally recognised.
IMA code
The short symbol assigned by the IMA Commission on New Minerals, Nomenclature and Classification (CNMNC) — a compact identifier used in datasets and crystallographic tables.
IMA status
A mineral's current standing within the IMA classification — approved, grandfathered, pending publication, questionable, or discredited. Tells you how firmly the species is established.
Impurities
Minor or trace elements outside the ideal formula but commonly detected in natural specimens. Often the cause of colour variation.
In the same group
Other minerals that share the same chemical or structural family. They often look related and form in similar environments.
Internal reflections
Coloured glints from inside an opaque mineral, seen when light reaches translucent zones beneath the polished surface.
Isometric
Three equal edges meeting at right angles — the highest-symmetry system, also called cubic.

L

Luminescence
Emission of visible light triggered by an external energy source — UV, heat, friction, or impact. Includes fluorescence and phosphorescence.
Lustre
How the mineral's surface reflects light — from metallic and glassy to silky, pearly, or dull. Often the first clue to identification in the field.

M

Metamict
A once-crystalline mineral whose internal lattice has been destroyed by long-term radiation damage from trace uranium or thorium — still crystal-shaped on the outside, glass-like within.
Monoclinic
Three unequal edges with two right angles and one oblique angle — a tilted box.
Morphology
The overall shape and habit a mineral tends to grow into — cubes, prisms, plates, fibres, druses, masses.

O

Often grow together
Species that frequently occur in the same specimen, formed by the same geological process — what mineralogists call a paragenetic assemblage.
Optical colour
The colour seen through a polarising microscope — often unlike the hand-specimen colour, because section thickness and orientation both matter.
Optical type
Whether the mineral is isotropic (light travels at one speed in all directions, like in glass) or anisotropic — uniaxial or biaxial. Defines which optical tests apply.
Orthorhombic
Three unequal edges all meeting at right angles — a rectangular box.

P

Parting
Splitting along planes of weakness that aren't true cleavage — usually caused by twinning or solid-state pressure.
Pending publication (IMA)
Approved by the CNMNC, awaiting first formal publication.
Pleochroism
A change of colour as a coloured anisotropic mineral is rotated under plane-polarised light. Tourmaline and cordierite are textbook examples.
Principal indices
The refractive indices measured along the crystal's principal optical axes — nω and nε for uniaxial minerals, nα, nβ, and nγ for biaxial.

Q

Questionable (IMA)
Validity not fully established — insufficient data or unresolved review.

R

Ratio a:b:c
Edge-length ratios of the unit cell, normalised so b = 1. Lets you compare related species without quoting absolute ångström values.
Reflectance R%
The percentage of incident light reflected by a polished surface, measured at standard wavelengths. The defining property of ore microscopy.
Refractive index
How much light bends as it enters the mineral. Higher index means stronger bending — diamond's extreme index is why it sparkles.

S

Space group
The full symmetry description of a crystal's repeating atomic pattern — 230 distinct groups, written in Hermann–Mauguin notation (e.g. P6₃/mmc).
Streak
The colour of a mineral's powder, drawn by scraping it across an unglazed porcelain plate. Far more consistent than surface colour.
Surface relief
How sharply a grain stands out against the mounting medium (Canada balsam, n ≈ 1.54) under the petrographic microscope. Low relief means the grain almost dissolves into the slide; very high relief gives crisp, almost three-dimensional outlines.

T

Tenacity
How a mineral resists breaking, bending, or crushing. Brittle snaps cleanly, malleable hammers into sheets, sectile slices like soft metal.
Tetragonal
Three edges at right angles, two of equal length and one different — a square prism.
Translation gliding
Plastic deformation by atomic-plane slip without fracture — the reason some minerals (like halite or galena) can bend slightly under pressure.
Transparency
Whether and how much light passes through a mineral, from transparent (you can see through it) to translucent, then opaque.
Triclinic
Three unequal edges with no right angles between them — the least symmetric system.
Trigonal
Three equal edges meeting at equal but oblique angles — a rhombohedron, with three-fold symmetry.
Tropism
A general label for how an opaque mineral responds to polarised reflected light, summarising bireflectance and anisotropism together.
Twinning
Two or more crystals of the same species grown in a fixed symmetric relationship, sharing a plane or axis. Often visible as re-entrant angles.
Type locality
The place where the mineral was first found and described — by convention, the reference point for the species.
Type-locality form
The crystal habit observed at the type locality — the place where the mineral was first described.

U

Unit cell volume
Volume of the unit cell in cubic ångströms (ų). Reflects how tightly the atoms are packed for a given chemistry.
UV response
How the mineral glows under short- and long-wave ultraviolet lamps. Some species fluoresce in spectacular colours invisible to the naked eye.

V

Varieties
Recognised forms of the same species — set apart by colour, habit, or trace chemistry, but not enough to count as their own mineral. Amethyst and citrine, for instance, are both varieties of quartz.

Z

Z
The number of formula units contained in a single unit cell.