Crystal habit / mode of occurence
Crystal habit - is the most commonly found shape or form that a mineral adopts.
Mode of occurrence - reflects the environment of a mineral's formation (see Notes below).
If crystals are visible, use crystal habit terms, otherwise use mode of occurrence terms.
Crystal habit terms:
Prismatic - forming prismatic crystals, equant to elongated;
Tabular - plate-like;
Acicular - needle-like;
Fibrous - fibre-like.
Mode of occurrence terms:
Cryptocrystalline - individual crystals too fine to distinguish;
Massive - with no distinguishing crystal form;
Granular - composed of mineral grains;
Concretionary - outer surface rounded or spherical.
Notes: When minerals are free to grow without constraint, they form 3 dimensional solid forms which are bounded by surfaces (crystal faces) arranged in a regular and repetitive way. The shape of a crystal is determined by the arrangement of atoms in its structure and this structure is known as the crystal lattice. All minerals occur as crystals, although in many cases the crystals are very fine grained and only recognisable through a microscope. Often minerals are unable to grow without constraint because of neighbouring crystals or other space limitations. Under these circumstances particular crystal faces may be advantaged while the growth of others is suppressed, and the apparent form of the crystal may be very different to its unconstrained growth form. This is referred to as its mode of occurrence. The mode of occurrence is seldom diagnostic, although it can be a useful guide.