handbook of carbon, graphite, diamond and fullerenes

252 Carbon, Graphite, Diamond, and Fullerenes
Diamond Cleavage Planes. Diamond breaks along well-defined
cleavage planes (the cleavage of a crystal being its characteristic of
breaking along given crystallographic planes where the yield strength is
lower due to a high concentration of weaker bonds or a lower total number
of bonds). The dominant cleavage plane is the (111) but many others have
been observed. This cleavage characteristic is the key to the cutting of
gemstones (see Ch. 12). The cleavage energies of the various planes are
reviewed in Sec. 6.0.
2.5 The Polytypes of Diamond
Hexagonal Diamond. Hexagonal diamond is an allotropic form of
carbon which is close to cubic diamond in structure and properties. It is a
polytype of diamond, that is a special form of polymorph where the closepacked
layers ({111} for cubic and {100} for hexagonal) are identical but
have a different stacking sequence. Hexagonal diamond has an ABAB
stacking sequence, so that every second layer is identical as shown in Fig.
11.6.1 71 This two-layer hexagonal sequence (known as 2H diamond) is
different in this respect from the three-layer sequence of cubic diamond
(known as 3C diamond). The crystallographic data of these two polytypes
are listed in Table 11.1.
A
B
C
Note ABAB Sequence
Figure 11.6. Schematic of the structure of hexagonal diamond (lonsdaleite).^