9.2 Growth versus dissolution 171

but they are also extremely valuable information transmitters of the conditions and events that have occurred in the depths of the Earth. There are many cases in which minerals formed in the mantle, deep within the Earth, are transported to the surface through global-scale movement such as plate tectonics, but most transform to other phases, decompose into more than two mineral species, or dissolve entirely. Although diamonds experience severe conditional changes, they do not transform into graphite, the low-pressure stable phase; instead, they survive in a metastable state, and thus can be a unique transmitter of information. In this way, information is recorded in the diamond crystals in the form of morphology, perfection, and homogeneity. It could be said that diamond is a unique letter sent from the depths of the Earth.

From geological and petrological investigations, and thermodynamic stability relations, it is believed that natural diamonds were formed deep under the Earth, under high-pressure, high-temperature conditions, then brought up to the Earth’s surface through rapid ascent of kimberlite and lamproite magmas, and were finally quenched as a metastable phase by adiabatic expansion due to volcanic eruption. From investigations of solid state inclusions in natural diamonds, two kinds of environmental compositions, ultramafic and eclogitic, have been distinguished. In addition, diamond crystals of micrometer size are found in garnet or zircon crystals in ultra-high-pressure metamorphic rocks.

We will explain in the following sections what new information may be obtained from morphology, perfection, and homogeneity in diamond crystals, in addition to the geological, geochemical, petrological, and mineralogical data.

9.2Growth versus dissolution

A total of 364 crystal figures of diamond are reproduced in Goldschmidt’s

Atlas der Kristallformen [1] published in 1913–23. The figures are beautiful sketches of crystals, and they even include surface microtopographs; some of these are shown in Fig. 9.2.

Although there are octahedral, dodecahedral, cubic, tetrahedral, elongated foot- ball-like forms, triangular platy forms, or star-like forms, the characteristic is that the forms are mostly rounded. Whether the rounded forms bounded by curved faces are due to growth or dissolution has been a subject of controversy since ancient times. Similar arguments arose about the origin of triangular depressions (trigons) universally observed on {111} faces with an opposite orientation to the triangle of the {111} faces, and on the origin of center-cross patterns seen in the center of octahedral crystals. The controversies surrounding these problems are summarized in Table 9.1.

The rounded forms of natural diamond crystals are commonly observed in crystals occurring both in alluvial deposits (secondary deposits) and in mother rocks

172 Diamond

Figure 9.2. Sketches of natural diamond crystals extracted from the Atlas der

Kristallformen [1].

(primary deposits), and so cannot be due to simple abrasion in the transporting process by water, but must be due to geological processes experienced during their growth or post-growth history.

Among the controversies summarized in Table 9.1, the argument that the rounded forms are due to growth and represent the as-grown morphology has been completely rejected, and it is now believed that all these forms represent the forms appearing due to the dissolution process.

As-grown crystals of diamond are bounded by flat faces, sharp edges, and corners, similar to ordinary polyhedral crystals. Conclusive evidence to support the dissolution process was obtained by Seal [2], who observed the relation between the rounded surfaces and growth banding revealed in crystals by etching the sections. From the observations showing that the rounded external form cuts the straight growth banding parallel to {111} seen in the interior, it was evident that the original crystal had been larger than its present size, and had changed its form to the present rounded form because the surface became partially dissolved (Fig. 6.1(c)).

As discussed previously, natural diamond crystals formed in the mantle deep within the Earth under high-temperature and high-pressure conditions corresponding to its thermodynamically stable region; they were then brought up to the Earth’s surface by magmatic movement of kimberlite and lamproite, and they eventually survived as a metastable phase due to adiabatic expansion by volcanic eruption. As diamonds ascend to the Earth’s surface, they have to pass through temperature and pressure conditions that can cause instability in diamond at high temperature. In addition, oxidation–reduction conditions vary according to the compositional differences in H2O and CO2 in different magmas, and the degree of dissolution will change depending on the oxygen fugacity, fO2.

Figure 9.3 is a schematic diagram showing how the morphology of the crystal and the surface microtopographs of crystal faces change as dissolution proceeds, starting from an as-grown octahedral crystal. The corner and edges of an

174Diamond

(a)

(b)

Figure 9.3. (a) Change of form as dissolution proceeds, starting from an octahedral

crystal. (b) Rounded crystals.

octahedral crystal are rounded, resulting in a hexa-octahedral form bounded by forty-eight rounded faces consisting of {hhl} faces. Etching also preferentially takes place at outcrops of dislocations and point defects on {111} faces, forming point-bottomed (P-type) and flat-bottomed (F-type) etch pits (trigons) (Fig. 9.4).

Trigons are, in general, oppositely orientated to the triangular form of a {111} face. In etching experiments in the laboratory using an oxidizing agent such as KNO3, it was only possible to produce etch pits with the same orientation as the triangular form of the {111} face. This led to arguments about whether the origin of trigons was due to growth or dissolution. It was shown experimentally that etch pits with the same orientation as naturally observed trigons were produced when diamonds were heated in powders of kimberlite. Later, it was demonstrated by Kanda et al. [8] that the orientations of etch pits of diamonds were reversed depending on the oxygen fugacity fO2 and the temperature T (Fig. 9.5).