In many cases the crystals of a particular mineral grow together in a group rather than as separate,individual crystals. These groups are called mineral aggregates.
The form and appearance of a particular mineral’s aggregate is called its crystal habit.
Sometimes the units in an aggregate are imperfectly formed and distinct crystal faces are absent. When this happens we call the individual parts "grains" rather than crystals. The words "crystalline" and "granular" are used to describe whether the units are well-developed crystals or grains.
Units well-developed as crystals, such as barite.
Units without distinct crystal faces. They are about equal in length, breadth and height, like grains of salt or sand, such as sulfur.
In order to describe crystal habit accurately mineralogists have developed some standard terms. The following descriptive terms are based on the shape of individual mineral units:
- Units long, needlelike, and thicker than a fiber or hair, like strontianite
- Units thin, much longer than wide, and flattened like the blade of a table knife, like barite.
- Units long, parallel or nearly parallel, and stout like a pencil.
- Units long and thin, such as a fiber or hair,such as melanterite.
- Mineral poorly crystallized and dense, without apparent internal structure. It is compact, and the individual grains are microscopic, as in flint.
- Units small and rounded. They look like fish eggs that are stuck together. Some forms of hematite are oolitic.
- Mineral poorly crystallized and porous. It may appear as a dusting of tiny delicate and matted crystals, as in a layer of hexahydrite on another mineral.
- Units flat and platy like the top of a table a thin book. Celestite is an example.>