Cleavage .
     (111) - perfect - parallel to the (111) close packed layers .
Crystal habit .
     Predominately octahedral, less commonly dodecahedral, rarely cubic, .
occasionally tetrahedral; twinning on (111) very common. .



Figure 55 -  Crystal habits

of diamond structure    





     The graphite and diamond polymorphs of carbon is a classic illustration of the .
effect that the geometry of the atomic packing has on the chemical and physical
properties of matter.  In its graphite form carbon is one of the softest natural
materials known and it is a good lubricant.  In contrast, in its diamond form, it is the
hardest natural material known and it is an excellent abrasive. The tetrahedral
arrangement of the carbon atoms in diamond is primarily responsible for its
hardness due to the omni-triangulated nature of its structure.  Carbon's extreme
polymorphism is the result of the different conditions under which its crystals are
formed.  It is only when carbon is subjected to extremes of temperature and pressure
that it morphs into the diamond structure.
     Only a few native elements exist in pure form in their natural state on earth (Au,
Ag, Cu, Hg, Pb, Pt, Sn, Fe, C, S, Sb, As, Bi, and the inert gases).  The rest must be
extracted, refined, and crystallized from their ore minerals.  The following section
describes the geometry and symmetry of minerals in terms of their topology.

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Page  37 -  Structure matters - Diamond structure elements

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