176 Diamond

Table 9.2 Morphological characteristics of natural diamond crystals showing that they were partially dissolved

There is a great deal of evidence that demonstrates that natural diamond crystals were partially dissolved, and this is summarized in Table 9.2. Special attention should be paid to the superimposed circular ditches (Fig. 9.6). This patterning may be explained by assuming that bubbles, formed by degassing during the uplifting process of the magma, adhered on the crystal surface and resisted dissolution.

The magmas comprising kimberlite and lamproite cylindrical pipes, which act as carriers of diamonds to the Earth’s surface, vary in their contents of volatile components like H2O and CO2, depending on the pipes, which results in the difference in the degree of dissolution rates. Figure 9.7 presents statistically the distribution of the morphology of diamond crystals made on samples from different kimberlite pipes. In some kimberlite pipes, nearly 75% of the crystals are octahedral with slightly rounded corners; in other pipes, the same percentage show strongly rounded forms.

Since natural diamond crystals are affected by the dissolution process to variable degrees, it is necessary to reconstruct the original, as-grown state. There are two methods of achieving this. One is to find crystals that have been only slightly dissolved, and the other is to investigate the texture representing the growth process, which might be recorded in single crystals.

9.3Single crystals and polycrystals

The crystal figures shown in Fig. 9.2 were selected from sketches that appeared in a series of books published between 1913 and 1923, and all represent forms of single or twinned crystals larger than a few millimeters. They are mostly gem-quality diamonds. Other than these forms, there are translucent or opaque

9.3 Single crystals and polycrystals 177

1 mm

Figure 9.6. Superimposed circular ditches (etch patterns).

Figure 9.7. Statistics on diamond morphologies associated with different kimberlite pipes in Siberia. The areas with vertical lines represent an octahedral morphology with only a slight dissolution; those with the circles show crystals bounded by curved faces, which received heavier dissolution. Blank areas correspond to an intermediate type.

diamond crystals, which are mainly used for industrial purposes, either in single crystalline (but full of inclusions) or polycrystalline aggregates of minute crystals. The ratio of gem-quality to industrial-quality stones was, until recently, 1:4, but this has since dropped to 1:1. This is due to changes in the standards required for gem-quality diamonds.

The classification and naming by Dana [9], [10] and Orlov [11] of diamond forms

178 Diamond

Table 9.3 Morphology of diamond

Classification by Dana and Orlov

are summarized in Table 9.3. Although both authors broadly classify natural diamonds into single crystalline and polycrystalline types, the respective meanings are not the same. The method of classification is principally descriptive, and there is no analysis of how the respective forms appear. A classification by the present author will be given at the end of this chapter.

As discussed in Chapter 3, when a crystal grows below /kT *, the crystal will take a polyhedral form bounded by smooth interfaces, and on increasing the driving force the interface will transform into a rough interface, and the morphology changes to hopper, dendritic, and then to polycrystalline aggregate in spherulitic form. (See Fig. 3.21 for a schematic illustration of morphological changes depending on the driving force, assuming a crystal is bounded by {111} faces only.) Crystals grown under a small driving force condition grow as an octahedral single crystal bounded by flat faces, but those formed under higher driving force conditions will appear as polycrystalline aggregates such as spherulites.

Interface roughness varies depending on crystallographic direction (crystal faces). Therefore, on crystals growing under the same driving force condition, the roughness of the interfaces depends on crystallographic directions. In Section 9.4, we will analyze the morphology of diamond crystals, taking this into account.