Glossary

acicular

Needle-shaped.

anion

An atom (or group of atoms) that carries a negative charge.

amorphous

A description of a substance that is without shape; a material that is structurally disordered, e.g. glass.

cations

An atom (or group of atoms) that has lost one or more electrons, to give it a positive charge.

cleavage

Planes of weakness in the crystal structure of a mineral allowing it to fracture along smooth parallel surfaces (cleavage planes). Also planes of weakness in metamorphic rocks, notably slates, caused by the alignment of fine-grained, platy materials. See also slaty cleavage.

cleavage fragments

Broken piece of a crystal with a well-defined, blocky shape and a flat surface.

cleavage planes

A term used in minerals for flat surfaces formed along planes of structural weakness when a crystal is broken. A term used in rocks for a plane along which platy minerals are aligned and splitting tends to occur; most well-developed in slates.

conchoidal fracture

A curved (shell-like) pattern developed on the fractured surface of fine-grained and glassy rocks and some minerals (such as quartz) that lack cleavage. (Pronounced 'con-koi-dal'.)

covalent bonding

A type of bonding in a mineral structure in which most, or all, of the chemical bonds between atoms involve sharing of electrons.

crystal systems

The classification scheme for crystals, based on symmetry elements.

crystalline

A term used to describe solid material composed of crystals, formed when atoms are arranged in a structurally ordered pattern, as occurs when a liquid cools more slowly. Also used to describe the rock texture when crystals have grown together to form an interlocking mass, as occurs in crystallisation from liquid (igneous crystallisation) and solid (metamorphic crystallisation) states.

crystallographic axes

A set of three directions in a crystal (labelled x, y and z) that are parallel to the edges of a unit cell.

dendritic

Descriptive term for a mineral with clusters of crystals in fern-like branches.

density

A measure of how heavy an object is for a given volume.

devitrification

The slow transformation of glass to a crystalline form.

equidimensional

Term used to describe a crystal that has a rather similar appearance in different directions.

ferromagnesian minerals

Silicate minerals rich in iron and magnesium, including olivine, pyroxene, amphibole and biotite mica. See also mafic minerals.

glass

A structurally disordered solid formed by quenching of a liquid.

globular

Descriptive term for a mineral with acicular (needle-like) crystals radiating in all directions from a central point to forma a spherical aggregate.

goniometer

Device for measuring the angle between crystal faces.

grain boundary

The line of contact between adjacent crystals in a rock.

graphite

A polymorph of carbon that consists of sheets of atoms tightly bonded in a hexagonal structure with only weak bonds between sheets. It is stable at low temperatures and pressures.

halite

A mineral form of sodium chloride (NaCl), also known as rock salt. It is an example of purely ionic bonding between Na⁺ cations and Cl⁻ anions.

hardness

The resistance of a material to scratching or indentation.

interstices

The spaces in a close-packed crystal structure that can accommodate small atoms or ions.

ionic bonding

A type of bonding in a mineral structure, generated by the transfer of one or more electrons from one atom to another, creating two ions of opposite charge which are attracted to each other.

ionic substitution

The replacement of ions of one element by those of another in a crystal structure. Ions most likely to substitute for one another have similar sizes and electrical charges.

lattice

An array of objects or points in space that form a repeating pattern in two or three dimensions.

lustre

The surface appearance of a mineral that depends on the way it reflects light. Typical terms used to describe a mineral's lustre include: glassy, resinous, metallic.

massive

Descriptive term for crystals that have grown together in a solid mass, in which individual crystals cannot clearly be seen.

metallic bonding

A type of bonding in a mineral structure involving atoms donating one or more electrons to a 'sea' of free electrons that flows between and around the cations. Such structures are dense and close-packed, yet malleable and ductile.

mirror planes

A plane of symmetry, dividing two halves of an object that are mirror images of each other.

Mohs' scale

A scale of mineral hardness devised by Friedrich Mohs and consisting of ten well-known minerals, ranked in order of hardness from the softest, talc (hardness 1) to the hardest, diamond (hardness 10).

phase boundary

The dividing line between two stability fields on a phase diagram.

phase diagram

A diagram showing the stability fields over which different phases of matter can exist (at equilibrium) and the phase boundaries between them. Usually, such a diagram is plotted for a fixed chemical composition over a range of pressure and temperature, in which case the axes of the graph represent pressure and temperature.

phase transformation

An event involving the conversion of one phase into another phase having the same chemical composition (but with a different crystal structure, if any).

phases

The state in which a particular substance can exist - as gas, liquid or solid. Also used to refer to a mineral with a definite chemical composition.

polymorphs

Substances that have the same chemical composition, but different crystal structures. For example, diamond and graphite are both polymorphs of carbon.

prismatic

Term used to describe a crystal stretched out in one direction.

reflection symmetry

The property where one side of an object is a mirror image of the other (e.g. insects, birds).

rotational symmetry

The property of an object that produces an identical arrangement of crystal faces at regular intervals of rotation.

sphalerite

A mineral form of zinc sulfide, ZnS.

stability field

The range of temperatures and pressures over which a particular phase or phase assemblage is stable.

tabular

Term used to describe a slab-like or platy crystal.

toughness

Term used to describe the resistance of a material to breaking or shattering.

triple point

The point of intersection of three phase boundaries on a phase diagram representing the pressure and temperature conditions at which solid, liquid and gas phases can coexist.

twinning

The term used to describe a type of crystalline defect, in which a crystal contains different regions in which crystal lattices are related in a predictable way (e.g. by reflection in a mirror plane, or rotation about a symmetry axis).

unit cells

The basic building block of a 3-dimensional crystal lattice with three sets of parallel sides and a lattice point at every corner.