guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Cleavage and fracture
3.2 Cleavage, fracture, deformation and recrystallization phenomena
Cleavage
Many minerals split preferentially along one or more directions. This is called cleavage. Through
the application of mechanical force - whether related to geological processes or to thin section
preparation - crystallographically defined planar fractures can be generated in mineral grains. The
abundance and quality of cleavage are mineral-specific, but also depend on the level of mechanical
stress the mineral grains were subjected to. Cleavage follows crystallographic planes that are
characterized by weak bonding and thus reflects a specific crystal structure. Examples are sheet
silicates with their prominent basal cleavage and chain silicates showing two prismatic cleavage
sets, but no basal cleavage (Fig. 3-11 A-C). It is quite common that cleavage planes have an orientation
parallel to dominant crystal faces (Fig. 3-11 A-D,F). However, this cannot be generalized
(Fig. 3-11 E).
In mineral cross-sections cleavage is recognised as – in the ideal case – straight traces of planes
dissecting the grain. If the cleavage planes are oriented parallel to the viewing direction they
appear as thin dark lines. With increasing tilt of the cleavage planes the lines broaden and the
traces appear increasingly blurry. If the cleavage orientation is at a small angle to thin section
plane, cleavage traces may not be observed.
Cleavage, and also the absence of cleavage (e.g., quartz, staurolite), is an important morphological
property for mineral identification. The number of cleavage sets and the specific angles
between them can be distinctive for mineral species and also entire mineral groups (pyroxenes,
amphiboles, etc.). It should be noted that the often cited cleavage angles that are critical for specific
Raith, Raase & Reinhardt – February 2012
Figure 3-11. Cleavage
A: Augite− Section orthogonal to the c axis. The {110} cleavage planes form angles of 87˚ and 93˚. B: Hornblende−
Section orthogonal to the c axis. The {110} cleavage planes form angles of 56˚ and 124˚. C: Biotite− Section
orthogonal to the perfect cleavage (001).
D: Kyanite− Section approximately orthogonal to c shows the typical pattern of very good cleavage (100) and distinct
cleavage (010). E: Sillimanite− The section orthogonal to c shows the good cleavage (010). F: Andalusite− Section
orthogonal to the c axis. The good {110} cleavage planes intersect at almost right angles.
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