Crystal Form
Crystals of the same mineral can often show differences in shape according to which crystal form is developed. A crystal form comprises all the faces that are required by the symmetry of that mineral. Form can be of two types: (i) Closed – where the space is totally enclosed, or (ii) Open – where the faces do not enclose space (as in two parallel spaces). In the open situation a crystal will develop through a combination of several forms.
The appearance of a crystal due to its arrangement of faces is known as its HABIT. This is often characteristic as some faces will be larger than others.
In the three examples above the same faces have developed but in different habits in each case. The prismatic habit has largely prismatic faces, the pyramidal triangular faces, and the tabular rectangular faces.
Twinning
Twinning occurs when a crystal grows in two directions from a crystallographic plane (growth twinning). It can also develop when two adjacent growing crystals merge together.
In the case of some mineral polymorphs twinning can appear when one is transformed into the other. Deformation twinning is produced when a mineral (such as calcite) is subjected to stress. This stress can be administered artificially with a penknife.
Mineral Aggregates
Most minerals occur as accumulations or aggregates that rarely show perfect crystal shapes. However, the shape of the aggregate can be useful in mineral identification. Malachite will often produce a botryoidal form, while haemetite is often reniform. It is the direction from where the mineral supply comes, as well as the conditions of deposition that influences the shape of aggregates.
Many of the names for these aggregate forms show Greek or Latin roots, e.g. Dendritic: branching; Reniform: kidney-like.
Crystals and Crystal Systems
A crystal forms when a mineral is allowed to grow unhindered by space. It is bounded by regular flat faces that reflect the internal atomic structure of the mineral. This internal arrangement of atoms was discovered through the use of X-rays last century, and is very regular. While some minerals appear in many different forms (quartz for example) the atomic structure is always identical.
Crystals can be described by examination of their elements of symmetry. There are four kinds (i) Planes, (ii) Centres, (iii) Axes (rotational), and (iv) Axes (rotary inversion). Symmetry has been used to define 7 crystal systems, which reflect the seven basic shapes of the primitive Bravais lattices of the atomic structure.
Geological Museum - Trinity College Dublin 