• Alex Savin
• March 11, 2022
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Origin of diamonds

Diamonds have always occupied really a unique place in history. They present a dazzling piece of eternity shedding its divine light through centuries. They have always presented a sort of an enigma to humankind and something everybody yearned for. In this article, we will reveal to you the secret of the origin of diamonds.

How diamonds were formed

In mineralogy, diamond (from the ancient Greek αδάμας – adámas “unbreakable”) is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions. Diamond is renowned as a material with superlative physical qualities, most of which originate from the strong covalent bonding between its atoms. In particular, diamond has the highest hardness and thermal conductivity of any bulk material. Those properties determine the major industrial application of diamond in cutting and polishing tools.

The formation of natural diamond requires very specific conditions—exposure of carbon-bearing materials to high pressure, ranging approximately between 45 and 60 kilobars (4.5 and 6 GPa), but at a comparatively low-temperature range between approximately 900–1300 °C. These conditions are met in two places on Earth; in the lithospheric mantle below relatively stable continental plates, and at the site of a meteorite strike.

When did diamonds form?

Diamonds were formed at least 990 million years ago, although some are estimated to be as many as 4.25 billion years old, thereby pre-dating life on this planet. Diamonds are formed at pressures of 45-60 kbar. A kilobar is a metric unit for measuring high pressure. This corresponds to a depth of 125-200 kilometers below the Earth’s surface where the pressure is around fifty thousand times that of atmospheric pressure at the Earth’s surface. Some diamonds form at depths of 300-400 kilometers, or even deeper, but these diamonds are particularly rare. Diamonds are formed at temperatures between 900°C and 1,300°C.

Syngenetic mineral inclusions in diamonds provide the best means of determining their origin. Among such inclusions, the peridotitic paragenesis of olivine, orthopyroxene, and chrome-pyrope garnet is on average far more abundant than the eclogitic paragenesis of pyrope–almandine garnet and omphacitic clinopyroxene1–3. Diamonds of peridotitic paragenesis from the ~ 100-Myr-old Kimberley and Finsch kimberlites in southern Africa were previously shown to have originated in 150–200-km-thick lithosphere beneath the Kaapvaal craton ~3,300 Myr ago4,5. At a few localities, such as the Premier kimberlite in southern Africa and the Argyle lamproite in northwestern Australia, diamonds of eclogitic paragenesis predominate6–8, allowing recovery of sufficient material for complementary analysis, the results of which are reported here. Eclogitic garnet and clinopyroxene inclusions in Premier and Argyle diamonds yield Sm-Nd isochron ages of 1,150 and 1,580 Myr respectively, compared with host diatreme emplacement ages9,10 of 1,100–1,200 Myr. Eclogitic inclusion ages and precursor isotopic signatures indicate a second genetically distinct origin of diamonds, apparently related in time and space to kimberlite or lamproite magmatism.

Сosmic origin of diamonds

A number of diamond-bearing Canyon Diablo specimens have been investigated in order to determine whether the diamonds were produced under high gravitational pressures in a meteorite parent body, or upon impact with the earth, as proposed by Urey and Nininger. respectively. Metallographic studies confirm Nininger’s observation that the diamond-bearing fragments, and only these, were reheated strongly after the formation of the Widmanstätten pattern. The metal phase appears to have been reheated to ~950°C for 1–5 sec, followed by cooling in less than 2 min. The rapid cooling rate implies that the process took place after the meteorite fragments had attained their present, small size.

Possible origins of diamonds in other meteorites are discussed. It is shown that in the presence of free iron, formation under high gravitational pressures could take place only under very extraordinary circumstances. Instead, it is considered much more likely that all meteoritic diamonds were produced by catastrophic events: either upon impact with the earth or during the breakup of the meteorite parent bodies. This mode of origin obviates the need for postulating meteorite parent bodies of lunar or planetary size, with interior pressures of 3 × 104 atm or greater.

The laboratory formation of diamond is discussed in terms of metal carbide dissociation equilibria, and several features of the General Electric process arc are explained in this manner. Some comments are offered on Moissan’s experiments.

Most natural diamonds are thought to have formed within the upper mantle, before transport to the surface by kimberlites or lamproites, in a magnesian, calcium-poor, garnet-bearing harzburgite or dunite. Serpentinites, formed by hydration of oceanic lithosphere, are sufficiently poor in calcium and have high enough Mg/(Mg + Fe) values that, following subduction and prograde metamorphism to ~1, 100 °C, 55–60 kbar, they would equilibrate as such harzburgites and dunites. Diamonds may be the high-pressure equivalent of graphite produced, or introduced, during serpentinization. The apparent restriction of diamonds and low-Ca garnet xenocrysts to cratons may result from the fact that subducted metaserpentinites, lodged within the mantle beneath the cratons, have been effectively isolated from participation in subsequent tectonic and magmatic events that occurred outside the craton margins.

The mystery of the origin of diamonds

Ureilites show high carbon contents comparable with those of CM chondrites1,2. One of the major questions about ureilites is why they contain large amounts of noble gases concentrated in carbon-rich veins 3,4. Diamond is shown to be one of the noble-gas carriers, while graphite is gas-free 5. We synthesized diamonds by chemical vapor deposition (CVD)6−8 from gaseous mixtures of H2 and CH4 including Ar and examined the Ar trapped in diamond by mass spectrometry employing the stepwise heating technique. The partial pressure of 36Ar in the gaseous mixture during the synthesis of diamond was 5.3×10−6 atm. The content of 36Ar was about as great as 8×10−6 cm3 STP g−1 which was extracted at 2,000°C. Meanwhile, the 36Ar concentration in graphite was much less than 5% of that in diamonds. These results suggest that diamonds in ureilites may have been directly formed from the solar nebula.

Diamonds as a symbol of unbreakable love

People’s infatuation with diamonds is completely understandable if one takes into consideration their complex structure and complicated and long process of their creation. All that adds to their value but above all is their beauty. Diamonds are forever, it is said and they are eternal so it is no wonder why they are given as a token of never-ending love.