9.4.5Twins

188Diamond

be initiated under higher driving force conditions than /kT **, and the driving force gradually diminishes to lower than /kT *, which leads to the formation of a single crystal in appearance, with a central core portion formed by dendritic growth, surrounded by mantle sections formed by layer growth. If magma containing polyhedral crystals grown under /kT * is uplifted, the driving force increases above /kT **, and dendritic growth will occur on the surface of preformed single crystals. As a result, a crystal appears with a clear core, surrounded by mantles with fibrous texture formed by dendritic growth. Depending on the growth history, variations of this type may occur repeatedly, resulting in more complicated textures; for a discussion, see Section 3.10. Conditional changes of this type are recorded as internal textures in diamond single crystals.

There are two types of internal textures seen in natural diamond crystals that show conditional changes. Although Dana [9], [10] and Orlov [11] distinguished between single crystals and polycrystalline aggregates, they did not put crystals that had experienced the two conditions into different categories.

The first and the most typical type is called coated stone, which has a clear, gemquality internal portion, coated by a mantle portion with fibrous texture. In appearance, this type of diamond is opaque and of industrial quality, but if a window is made the clear, gem-quality interior is discernible. Coated stones have the following growth history.

If magma containing polyhedral single crystals of diamond formed under a driving force condition below /kT * is uplifted and the diamonds are placed under a higher driving force condition, dendritic growth takes place on the substrate of a polyhedral crystal, thus forming a mantle portion surrounding a clear single crystalline core. Crystals showing similar textures to coated stones are universally observed among various phenocrysts of rock-forming minerals in volcanic rocks, and diamond is not an exception.

There is a group of natural diamonds called cuboids, which exhibit a cubic or cuboid morphology. These occur frequently in a particular locality, such as in the Republic of the Congo, and are mostly of industrial quality. Cuboids are not bounded by crystallographically flat {100} faces, but instead show exclusively rugged surfaces. According to X-ray topographic investigations [16], [17], cuboid diamonds are characterized by a columnar (thick fibrous) texture developing from the center in the 111 and 100 directions, as schematically shown in Fig. 9.16. Sometimes a central clear core portion is detected; sometimes it is not. Therefore, the formation of cuboids is essentially the same as that of the mantle portion of coated stones, and the difference between the two types is simply the difference in the thickness of the mantle portion.

In the second type (layers), the cuboid takes the role of a seed, as opposed to the above case, in which a single crystalline diamond grows under the condition below

9.4 Morphology of single crystals 189

Figure 9.16. Schematic illustration of the internal texture of a cuboid, based on an

X-ray topograph [16], [17].

/kT *. In recent investigations on cut stones, an example showing, for the first time, the presence of a seed in the crystal growth of natural diamonds has been discovered [18]. It is also the first example to show the relation between diamonds formed by ultra-high-pressure metamorphic rocks and those formed in ultramafic magma.

9.4.7Origin of seed crystals

Gem-quality diamond is usually cut along a line parallel to the (100) direction of an octahedral crystal to obtain two cut stones. Two such samples, A and B, shown in Fig. 9.17, were compared to see if it is possible to identify whether the two stones came from the same rough or not. X-ray topography indicated that sample A contains a small number of dislocation bundles radiating from the center to {111}, and the Burgers vector is exclusively along 110 . Growth banding that is straight and parallel to {111} is observed in sample A. These features are commonly observed in gem-quality octahedral crystals. In contrast, in sample B, a square core portion is observed at the center, and dislocation bundles are generated principally from the surface of the core, and the Burgers vector is along 100 . The nature of the dislocations is entirely different in the two samples, and it is clear that samples A and B came from different rough stones. A further sample, C, was then compared to A and B. X-ray topographs of sample B and C match perfectly, proving that the two stones came from the same rough (Fig. 9.18).

190 Diamond

Figure 9.17. Three cut stones were compared to ascertain whether they originated from the same rough stone. (a) Sample A; (b) sample B; (c) sample C. Samples A and B were seen to come from different rough stones, whereas it was confirmed that samples B and C came from the same rough [18].

The two stones B and C show hitherto unknown features [18], as follows. There is a core portion with a square outline in cross-section and cuboid form in three dimensions, and all dislocation bundles with Burgers vector 100 generate from the surface of the core portion (Fig. 9.19). This implies that the core portion was formed somewhere else; it was then trapped in a different environmental phase and acted as a seed under the new conditions, after which the major part of the crystal was formed. This was the first piece of evidence to prove the presence of seed crystals in the growth of natural diamond.

The size and morphology of the core portion provide us with useful information that allows us to deduce where the diamond crystal that acted as the seed was formed. The core portion is cuboid in form, indicating that it was formed under a driving condition above the /kT ** of the {100} face. In contrast to this, the growth of the major portion of the crystal, which grew around the seed, took place under conditions below the /kT * of the {111} face. The dislocation directions are

100 , which are distinctly different from the directions of 110 generally observed in gem-quality diamonds. However, on X-ray topographs and cathodoluminescence tomographs of the samples, it is observed that, starting from the seed, micro-facets of {111} appear, which gradually change to larger {111} faces.

There are three types of rocks that are known to fulfil the conditions for diamond growth, and, in fact, diamonds are found in all of them.

(1)Ultra-high-pressure metamorphic rocks. Diamonds occur sporadically in crystals of garnet or zircon in various ultra-high-pressure metamorphic rocks formed in deep subduction zones. Crystals are of micrometer size, and the morphology is mostly spherulitic or cuboid, but octahedral is also

9.4 Morphology of single crystals 191

Figure 9.18. X-ray topographs of the three cut stones shown in Fig. 9.17. (a) Sample A;

(b) sample B; (c) sample C. X-ray topographs taken by T. Yasuda.

9.4 Morphology of single crystals 193

found. The content of diamonds is extraordinary high, up to 2%, which is the characteristic feature of this mode of occurrence. Considering that the original source of ultra-high-pressure metamorphic rocks was oceanic sediments subducted deep into the Earth, the high content of diamond is understandable. This suggests that these diamonds grew in silicate liquid droplets formed by the partial melting of mother rocks under the condition above /kT ** for diamond growth, since carbon content can be very high due to subducted carbon of organic origin.

(2)Eclogite. This is found as xenolith in ultramafic rocks (see (3) below), and consists mainly of garnet. The origin has not been clarified, and so it is not known yet whether it is of ultra-high-pressure metamorphic origin or of magmatic origin. According to Orlov [11], the proportion of polycrystalline versus single crystalline diamond in eclogite is much higher than in ultramafic rocks.

(3)Ultramafic rocks. Diamonds are found and are considered to have been grown in ultramafic magma (which has a high content of Mg and Fe, and

a low SiO2 content). The content of diamonds is extremely low,

0.2 g/tonne; the proportion of single crystals within this group is high. It is assumed that diamonds were grown under conditions below /kT *.

There is a clear difference in syngenetic inclusions in diamond crystals between

(2) and (3), indicating a difference in the chemical environments of diamond formation.

Many of the diamonds in types (2) and (3) were trapped in the uplifting process of kimberlite and lamproite magma and were brought up to the Earth’s surface, whereas it is thought that those in type (1) have been brought up by a reverse subduction movement.

Considering the sites of diamond formation and the subsequent uplifting movement of magma, we may suggest the following scenario from the discovery of cuboid seeds of type (2) (growth of diamond under conditions below /kT * on a cuboid seed). Namely, diamond crystals of micrometer size formed by ultra-high- pressure metamorphism due to subduction are further transported to the depths of the earth and incorporated into ultramafic magma, at which time they acted as seeds, and growth of a diamond proceeded below /kT *. It is surprising to see within such a small diamond crystal a record of the vast movement and history of the Earth’s hidden depths.

Although Dana [9], [10] and Orlov [11] classified diamond crystals broadly into two types, single and polycrystalline, we can also construct a classification (given in Table 9.4) based on the preceding analysis. By this classification we are able to correlate the morphology of diamonds with their growth conditions and growth histories.