Laboratory Manual for Introductory Geology 4e

Fracture occurs when there are no zones of particularly weak bonding within a
mineral. When such a mineral breaks, either irregular (irregular fracture) or curved
(conchoidal fracture) surfaces form (FIG. 3.5). Conchoidal fracture surfaces are common
in thick glass and in minerals whose bond strength is nearly equal in all directions
(e.g., quartz).
FIGURE 3.5 Types of fracture in minerals.
Irregular
fracture
Crystal
face
(a) Irregular fracture in garnet.
(b) Conchoidal fracture in quartz.
Cleavage occurs when the bonds holding atoms together are weaker in some
directions than in others. The mineral breaks along these zones of weakness, producing
flat, smooth surfaces. Some minerals have a single zone of weakness, but
others may have two, three, four, or six (FIG. 3.6). If there is more than one zone
of weakness, a mineral cleaves in more than one direction. In describing cleavage,
we note how many directions there are and the angles between them. Two
different minerals might each have two directions of cleavage, but those directions
might be different. For example, amphiboles and pyroxenes (two important
groups of minerals) are similar in most other properties and each have two directions
of cleavage, but amphiboles cleave at 568 and 1248 whereas pyroxenes cleave
at 908 (see Fig. 3.6).
Note in Figure 3.6 that there may be many cleavage surfaces, but several are parallel
to one another, as shown for halite. All of these parallel surfaces define a single
cleavage direction. To observe a mineral’s cleavage, hold it up to the light and rotate
it. Parallel cleavage surfaces reflect light at the same time, making different cleavage
directions easy to see.
Both crystal faces and cleavage surfaces are smooth, flat planes and might be
mistaken for each other. If you can see many small, parallel faces, these are cleavage
faces because crystal faces are not repeated. In addition, breakage occurs after
a crystal has grown, so cleavage or fracture surfaces generally look less tarnished
or altered than crystal faces. Exercise 3.5 will help you recognize the difference
between cleavage and fracture.
3.4 PHYSICAL PROPERTIES OF MINERALS
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