What is a crystal?
A crystal is a chemical compound in which the external shape reflects the internal atomic structure. Any given mineral may form crystals belonging to only one of seven crystal systems. The full definition of each system requires a knowledge of three-dimensional geometry but a rough description is as follows:
- Cubic or isometric system: Crystals belonging to this system are highly symmetrical and tend to be equidimentional e.g. pyrite and garnet.
- Tetragonal systeml: Crystals belonging to this system tend to be based on a four-sided pyramid and/or prism, e.g. zircon.
- Hexagonal system: Crystals belonging to this system tnd to be based on a six-sided prism and/or pyramid. e.g. calcite.
- Trigonal systeml: Crystals belonging to this system tend to be based on a three-sided prism and/or pyramid, e.g. calcite, tourmaline.
- Orthorhombic; Crystals belonging to this system tend to be based on a matchbox shape, e.g. barite.
- Monoclinic: Crystals belonging to this system tend to be based on a matchbox shape that has been tilted in one direction, e.g. gypsum.
- Triclinic system: Crystals belonging to this system have very little symmetry, e.g. kyanite.
The term crystal should not be used for the following:
- Crystal fragments. Sometimes fragments of crystals are sold as “crystals” in order to distinguish a fragment composed of one mineral grain from a fragment composed of a number of grains.
- Cleavage fragments. Calcite can easily be cleaved into rhombohedra and fluorite into octahedra. So common are the cleavage fragments of fluorite that some writers have claimed this to be the normal form of fluorite.
- Amorphous substances such as amber or natural glass which, since they do not have a regular atomic strucre, cannot for crystals.
- Masses formed of many grains of a single material. These include:
- Substances formed from grains or fibres such as botryoidal malachite. The latter is formed of sub-parallel fibres of malachite the ends pf which produce rounded surfaces.
- Concretions which are typically formed when material is added round a central core..
- Pseudomorphs which may have the outer form of a crystals but the original mmiral has been relaced by another mineral, e.g.devil's dice in which pytite cubes have been replaced by iron oxide minerals.
Other uses of "crystal"
Rock crystal is another name for clear colourless quartz.
Crystal as in "crystal vase" will mean lead glass.