An introduction to minerals and rocks under the microscope
An introduction to minerals and rocks under the microscope

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An introduction to minerals and rocks under the microscope

1.3.1 Crystal shape

Well-developed crystals show a number of flat faces and a distinct shape. The shape of the crystal, and the precise arrangement of its crystal faces, relate to its internal structure, and are expressions of the regular way the atoms are arranged.

Many terms are used to describe the shapes of different crystals. These can be broadly grouped into three categories:

  • prismatic (the crystal is stretched out in one direction; Figure 5a)
  • tabular (the crystal is squashed along one direction, so appears slab-like or platy; Figure 5b and c)
  • equidimensional (the crystal has a rather similar appearance in different directions, e.g. cubes, octahedra, and 'rounded' crystals with many faces of similar size; Figure 5d and e).
Figure 5 Some examples of crystal shapes: (a) prismatic (quartz) (6 cm long); (b) tabular (mica) (9 cm across); (c) tabular (barite) (field of view 10 cm across); (d) equidimensional (garnet, MS X) (4 cm); (e) equidimensional (pyrite, MS I) (2 cm).

Crystals of the same mineral tend to show the same general crystal shape. Quartz, for example, is almost always prismatic, rather than tabular or equidimensional. However, the exact shape of crystals of the same mineral can vary, depending on the conditions at the time of growth. Two crystals of the same mineral may differ in the relative sizes of specified crystal faces, or some faces may not be present. Although the relative sizes of specific crystal faces often vary, the angles between such faces are always fixed as they are defined by the crystal structure. This consistency of angles may be verified by measuring the angle between crystal faces with, for example, a protractor (Figure 6) or a more accurate device called a goniometer.

Figure 6 Idealised quartz crystal showing that the prism faces are 60° apart. The prism faces may vary in size, but the angles between them are all exactly 60°.
  • Some minerals have a crystal shape that is best described as 'acicular' (i.e. needle-like). In which of the three general categories of crystal shape mentioned above do acicular crystals lie?

  • Acicular crystals belong to the prismatic category as they are stretched out in one direction.

Note that there is often a distinction between the shape of individual crystals and the form they may take when many crystals are assembled into an aggregate. For example, some minerals have acicular crystals that radiate in all directions away from a central point, forming a globular (i.e. spherical) aggregate. Aggregates may also be fibrous, columnar, dendritic (clusters of crystals in fern-like branches), and so on. When crystals grow together in a solid mass, in which individual crystals cannot clearly be seen, aggregates are described as massive.

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