Page 104 - Materials Chemistry, Second Edition
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2.3. The Crystalline State
Figure 2.59. Example of cleavage and fracture at the atomic (a, b) and macroscopic (c, d) levels.
In images (c, d), a crystal of NaCl is exposed to a stress along an oblique angle to the cleavage plane,
resulting in both cleavage and fracturing. Images taken with permission from the Journal of Chemical
Education online: http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA2/SMHTM/CLEAVE.HTM.
a fracture refers to chipping a crystal into rough, jagged pieces. Figure 2.59c, d show
photos of cleavage and fracture of a NaCl crystal subjected to stress at oblique
angles to the cleavage plane. Although preferential cracking will occur along the
cleavage plane, smaller fractured pieces will also be formed. In general, as one
increases both the magnitude and obliqueness of the applied stress, the amount of
fracturing will increase, relative to cleavage.
On occasion, the cleavage plane may be easily observed due to a fibrous network
lattice. One example of such a crystal is ulexite, sodium calcium borate of the
chemical formula NaCa(B 5 O 6 )(OH) 6 ·5H 2 O. Upon visual inspection, it is quite
obvious that upon external stress, the crystal will preferentially cleave in directions
parallel to the crystallite fibers (Figure 2.60a). However, more intriguing is the
interesting optical properties exhibited by this crystal, commonly designated as
the “T.V. rock” (Figure 2.60b, c). If the crystal is surrounded by a medium of a
lower refractive index (e.g., air), light is propagated through the individual fibers by
internal reflection. This is analogous to fiber optic cables that will be described a bit
later in this chapter. Figure 2.60b shows that light passing through the crystal will
exhibit concentric circles. This is due to the difference in the effective path lengths