Cleavage

Cleavage can be an excellent diagnostic property. It helps, for example, distinguish some amphiboles from other similar minerals.

The term cleavage refers to the way a mineral cleaves, or breaks, in prefered directions. Cleavage directions represent planes of weak bonding in the mineral's atomic structure. Because mineral structures are repetitive, a single cleavage often appears as multiple cracks, all parallel. If there is only one direction of atomic weakness, then the mineral may exhibit only one cleavage. In thin section, a single cleavage will appear as a few (or many) several parallel cracks, if the mineral is viewed in an appropriate orientation. Biotite, for example, often shows a single cleavage (set of parallel cracks) but biotite's cleavage will not be visible if a grain is viewed in a specific orientation (see section on biotite, below).

Minerals may exhibit zero, one, two, or more cleavages. However, some cleavages develop more easily than others. Just because a mineral has a direction of atomic weakness does not mean it will crack in that direction. For example: in principle feldspars have two prominent cleavages (often seen in hand specimen), but the cleavages may not be visible in thin sections.

If a mineral has two cleavages, then the angle between the cleavages (which is controlled by the atomic arrangement), called the cleavage angle, is often a useful property for mineral identification. Complications arise because many minerals show different cleavages, and angles between cleavages, depending on grain orientation. Consequently it is necessary to look at many grains to make sure you are identifying the characteristics of a mineral's cleavage correctly. To determine the true cleavage angle, it is necessary to look at multiple grains in a thin section and find the maximum angle. Diopside, for example, has a cleavage angle of near 90o, but the angle seen will only be 90o for grains in a specific orientation. See the photos of diopside, below.

Quartz

The large grain in the center of these photos is quartz. Quartz has no cleavage but often, as can be seen here, develops fractures. The key to distinguishing fractures from cleavage (in thin section) is that fractures have random orientations and do not form "en echelon" - meaning they do not form sets of parallel cracks..

Sillimanite is also present in this view -- in the lower left of the photo. The blocky sillimanite crystals have higher relief than the quartz and some grains show one direction of cleavage.

Minor brown biotite is also present.

   

Biotite

The large greenish brown to tan grains are biotite. Biotite shows no cleavage when the view looks down on a flake. However, in other orientations biotite shows one excellent cleavage, best demonstrated by the diagonally oriented rectangular grain in the lower left corner of this photo. Note also the clear (PP) blocky mineral above and to the left of the biotite (and also below the biotite). It is orthopyroxene, and shows fractures, not cleavage.

   

Diopside

The large grain at the center of this view is diopside. One diagnostic characteristic of diopside and other pyroxenes is that two cleavages, nearly perpendicular, are visible in basal (end) views. However, when viewed from other perspectives, only one cleavage may show. The pinkish mineral (PP) surrounding the diopside is calcite, stained for identification. In XP light, the calcite can be seen to be twinned.

   

Augite

The large grain at the center of these photos is augite, a clinopyroxene. It only shows one cleavage due to grain orientation. However, in some of the other smaller augite grains it is possible to see -- or at least to imagine -- two cleavage directions. One grain of orthopyroxene is present on the left edge of the photo -- it has higher relief than the augite and is somewhat fractured. A grain of spinel (green in PP) is also present.

   

Hornblende

Here we see one large, and several small grains of green hornblende. Hornblende is an amphibole. Like pyroxene, amphiboles are characterized by two cleavages, visible in some views. For amphiboles, the angle between the cleavages is close to 60o in perfectly oriented end views. (In pyroxenes, the angle is close to 90o.) In views that are not perfectly oriented, the apparent cleavage angle is smaller, depending on grain orientation.