Page 104 - Materials Chemistry, Second Edition
P. 104

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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
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