Engineering geology

Crystal Shape, or Habit

Mineralogists use the term crystal shape, or habit, to refer to the common or characteristic shape of individual crystals or aggregates of crystals. Some minerals tend to grow equally in all three dimensions, whereas others tend to be elongated in one direction or flattened if growth in one dimension is suppressed. A few minerals have crystals that exhibit regular polygons that are helpful in their identification. For example, magnetite crystals sometimes occur as octahedrons, garnets often form dodecahedrons, and halite and fluorite crystals tend to grow as cubes or near-cubes.
Minerals tend to have one common crystal shape, but a few, such as the pyrite samples shown in Figure 1, have two or more characteristic crystal shapes.

Figure 1 – Common crystal shapes of pyrite

In addition, some mineral samples consist of numerous intergrown crystals that exhibit characteristic shapes that are useful for identification. Terms commonly used to describe these and other crystal habits include equant (equidimensional), bladed, fibrous, tabular, prismatic, platy, and blocky. Some of these habits are pictured in Figure 2.

Figure 2 – Some common crystal habits A. Thin, rounded crystals that break into fibers. B. Elongated crystals that are flattened in one direction. C. Minerals that have stripes or bands of different color or texture. D. Groups of crystals that are cube shaped.

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