194 Diamond

Table 9.4 Morphology of diamond

Sunagawa’s classification

(b)octahedral growth on cuboid seed

9.4.8Type II crystals showing irregular forms

Natural diamonds grew in the mantle deep under the Earth and were

brought up to the surface by the rapidly ascending movement of kimberlite or lamproite magma. During this process, crystals were partially dissolved and also experienced plastic deformation due to stress associated with the rapid ascending movement. The evidence supporting this is recorded in the form of tangled dislocations observed in single crystals, the occurrence of slip lines, and bent crystals.

Type I diamond has a high content of nitrogen, which occurs in the form of precipitated nitrogen platelets or aggregates. In contrast to the fact that Type I crystals may be regarded as a C–N alloy, Type II diamond contains nitrogen at less than part per million order, and can be regarded as a purer carbon crystal. As can be seen from the large differences in critical shear stress that causes plastic deformation between pure aluminum and duralumin, an Al alloy containing less than 2% Cu, Type II (corresponding to pure metal) is plastically weaker than Type I (corresponding to an alloy), and thus it is expected that Type II diamond will be plastically more deformed and will eventually fracture before Type I crystals under the same applied shear stress.

External forms of diamond crystals are used as a criterion in sorting rough stones of diamond. Rounded crystals, octahedrons, and cuboids are all grouped as Type I crystals, and are further subdivided by color and forms on a fashioning basis. In contrast, crystals showing irregular or platy forms without distinct crystal faces are all classified as Type II. This empirical classification has been found to be essentially correct on checking using the transmittance of ultra-violet rays. Namely, Type I crystals exhibit rounded, polyhedral forms, and Type II crystals are characterized by irregular and platy forms, not bounded by crystallographic faces. The following reasons have been suggested as explanations [19].

(1)Irregular forms of Type II crystals may represent broken forms of what were originally polyhedral forms in the Earth and became irregular either (i) by the uplifting process or (ii) due to shock received during or after mining operations.

(2)The remarkable anisotropy in the dissolution in the magma may have contributed to the irregularity.

9.4 Morphology of single crystals 195

Figure 9.20. Type II diamond showing irregular forms.

(3)The growth of diamond in interstices of solid particles may be an explanation.

(4)Polyhedral (principally octahedral) crystals of Type I may have appeared due to selective adsorption of impurities to suppress the growth rate of {111}, and therefore pure Type II grew without such an effect, thus resulting in irregular forms.

The last explanation, i.e. the development of {111} due to impurity adsorption, can be easily refuted on the basis of the anisotropy involved in diamond structure (PBC analysis). The likelihood of (2) and (3) being true is also remote.

As seen from Fig. 9.20, Type II crystals show irregular forms and their surfaces show minute undulation, indicating that the surface was etched after the irregular forms appeared. Under a polarization microscope, Type II crystals show a characteristic tatami-mat pattern [20] formed by crossing slip lines parallel to {111} or they universally show strain birefringence. X-ray topographs of Type II crystals consist of irregular areas showing contrast images, and those are entirely out of contrast, which indicates that the crystal is bent. The X-ray topographic characteristics are very different from those seen in Type I crystals. All this indicates that Type II crystals are plastically more heavily deformed than Type I crystals. It is anticipated that the probability of Type II crystals taking irregular forms is much higher than for Type I crystals, if deformation proceeds further and crystals are broken. Judging from the observation that Type II crystals show surface etching, we may conclude that the morphological characteristics of irregular or platy forms