Crystalline Carbon's Pedigree

Many things are not understandable to us not because
 our concepts are weak but because these things do not
 belong to the circle of our concepts.
Kozma Proutkov

Once my teacher Zbigniew Bartoszynski, professor of the Mineralogy Department with Lvov University, said with a note of irritation: “Diamonds will soon be found next door”. This happened in 1980 and the case in point was the Kumdy-Kol deposit in northern Kazakhstan, where diamonds were found in the rock of nonvolcanic origin. This discovery struck another blow against the accepted views that the only source of natural diamonds can be kimberlite, an igneous rock. 

Back at that time, the geology of diamonds looked like the classical physics of the 19th century, when most phenomena observed could be explained using one prevailing paradigm. However, the famous Michelson-Morley experiment, which showed that the speed of light did not depend on the speed of the source of light, opened up revolutionary prospects in physics.

Something similar has occurred to the theory of diamond origin, though conservative thinking – a psychological aspect of knowledge – is still at work here. “Entities should not be multiplied unnecessarily” (lex parsimoniae) – this principle of the medieval scholiast, later called Ockham’s razor, surreptitiously lures us towards the sofa with soft, worn-out slippers next to it and prevents us from going barefoot along a stony path.

Nevertheless, discoveries of new deposits have made kimberlite one of the many sources of diamonds, whose total number is not yet known. Geologists, though unwillingly, are beginning to accept the new reality: diamonds are polygenous, that is, have different origin. Today, we know a lot about diamond polygenesis but this knowledge has not been systemized yet, therefore, we will mainly resort to facts. 

To begin with, let us remember that diamonds form under high pressure and temperature (minimum 45 kilobar and 1200°С). Such conditions occur 120 km deep, in the Earth’s upper mantle. (Recently super-deep diamonds have been found that crystallized at up to 600 km depths, which corresponds to the lower mantle.) 

It should be noted that no primary or placer deposit will produce two similar crystals. If for no other reason, they cannot be completely identical because they could not have occupied the same space at the same moment in time. Their nonequivalence follows necessarily from the local and temporal alterations in their crystallization conditions as well as from other factors. 

Evidently, diamond polygenesis is a very broad concept, and it is next to impossible to give a comprehensive idea of it. We believe it would be reasonable to divide all natural diamonds by an agreed criterion and study polygenesis within the resulting groups. The matter is that a selection of diamonds homogeneous in terms of a certain criterion can vary a lot if other criteria are applied.

By analogy, let us consider mankind as a totality of individuals: it can be split in homogeneous groups based on gender, race, nationality, religious beliefs, etc. Similarly, diamonds can be broken into groups depending on their crystallization conditions, composition of their parent rock, and source of carbon.

The property that gives the most comprehensive portrayal of diamonds’ original polygenesis is, apparently, the type of the primary source.

Kept in placers

Diamond polygenesis in terms of the types of primary sources can be seen most clearly in placers – accumulations of comparatively small fragments of massive rocks destroyed by temperature, water, and wind variations. Minerals resistant to mechanical wear, physical force, and chemical action can stay in placers millions of years, being many times re-deposited in younger deposits. 

Diamond is unique in this respect – it is a true Wandering Jew of the mineral world thanks to its hardness and rigidity towards environmental changes. The most ancient of the diamond placers known, the Witwatersrand placer in South Africa is 2.9 billion years old. Much more common are Proterozoic (from 2.5 billion years) and younger placers, up to the most widespread Quarternary deposits (from 1.8—1.6 million years).

The younger a placer, the higher the chance it will contain diamonds coming from primary deposits of different age and genesis. Also, it should be borne in mind that placers can form right above the primary deposit as a result of rock weathering (eluvial placers).

DIAMOND PLACERS CAN BE OF TWO TYPES:

1) alluvial placers whose primary sources are known – they are rich and quite large kimberlite bodies; as a rule, these placers are not older than the late Paleozoic;
2) placers with unidentified primary deposits, which can have diamonds originating from different sources of different age including kimberlite; such placers, possibly very ancient, may have been redeposited several times to produce increasingly young sediments down to and including up-to-date

Diamonds have been mined at placer deposits since time immemorial. The majority of the best known “historical” stones were found at the River Krishna deposits, in the vicinity of the commercial town of Golkond, India. In 1725 diamonds from Brazil came out on the market; most famous Brazilian gems were mined in the late 19th century. In Australia, the first diamonds were discovered in 1961, in the south-east of the continent. Also, they were mined on the island of Borneo (Kalimantan), in Sumatra, in the Shandun Province (China), and in some other locations.

Kimberlite carrier

And yet the true story of diamonds connected with kimberlite as the main kind of primary deposits dates back to 1866, when fifteen-year-old Erasmus found a glittering pebble on his father’s farm near the town of Kimberley (Cape Province, South Africa). In five years the first kimberlite pipe was discovered there, and in the following 15 years a great many other kimberlite bodies were found in South Africa, and twice as many diamonds were mined there as in two thousand years in India. Today, kimberlites have been discovered on all the continents except Antarctic though it is likely that they are found there too but are hidden under thick ice. 

Kimberlite forms in the Earth’s upper mantle at a minimum depth of 100 km. Kimberlite bodies have volcanic origin, therefore, are mostly shaped as volcanic pipes; the so-called dikes – long, narrow cross-cutting masses whose length ranges from several centimeters to several dozen meters – occur less often.

Kimberlite carries up to the surface a variety of interior minerals such as red, orange, and violet garnet (pyrope); black picroilmenite; yellowish-green olivine; phlogopite looking similar to mica; and bright green pyroxene (chrome-diopside). All this stone rainbow is alien to kimberlite, which was caught by the magmatic melt in the place of its birth and as it was moving through the lithosphere. 

For our hero, the diamond, kimberlite only serves as a carrier. And though kimberlite bodies are the major type of diamond primary deposits, only one-tenth of them contains diamonds and only very few of these are of commercial interest. On the Siberian platform, for instance, only mid-Paleozoic (late Devonian) kimberlites are diamondiferous while most of the later Mesozoic kimberlites contain no diamonds. 

Kimberlite is a definite type of igneous rock but the diamonds embedded in it demonstrate a vivid example of polygenesis. This is firstly attributable to the diamond’s nature that is “alien” to kimberlite. The latter catches diamonds in the mantle when passing through various diamondiferous rocks. Fragments of these rocks in the form of diamondiferous xenolyths are an invaluable source of information about the conditions, time, and location of diamond formation.

Mantle diamondiferous rocks fall into two main groups: peridotites and eclogites, which is attributable to the global differentiation of the Earth’s matter.
The groups differ in their content of silicon oxide: as compared to the latter, the former is undersaturated with earth silicon SiO2, which is primarily needed to form silicon minerals like pyropes, pyroxenes, and olivines. Diamonds associated with these types of rocks (type P – pridotite, type E – eclogite) also differ in their properties, in particular, in the set of mineral inclusions corresponding to their composition of parent rocks.
On the whole, type P is a more homogenous group than type E. The two groups differ not only because they originate from different kinds of matrices but also because they have different sources of carbon and growth conditions reflected in their interior structure and crystal distribution by size.
Type P is characterized with a narrow range of carbon isotopic composition, which corresponds to the mantle carbon reservoir; layer-by-layer growth and octahedron as the prevailing shape. Diamonds of this type are normally large (over 0.5 mm) and feature a perfect crystal structure.
In contrast, type E features a broad range of carbon isotopic composition, which corresponds to the crust carbon: from marine carbonate rock enriched with a heavy isotope to organic carbon. The morphology of these diamonds is surprisingly varied; there are many crystals with a defected crystal structure, crystal-jams, etc. They are usually microcrystals occurring in rocks in great numbers; sometimes though large high-quality crystals can be found.

Composition of parent rocks is reflected in the diamond’s mineral inclusions. Based on their nature, diamonds fall into peridotite (type P) and eclogite (type E). Judging from the carbon isotopic composition, the source of carbon for the diamonds of the former type was the Earth’s mantle and for diamonds of the latter type, the crust. 

Different kimberlites show different ratios between the two types of diamonds. According to N.V. Sobolev’s estimate, commercial deposits of the Siberian platform contain over 96 % peridotite diamonds. However, 40% diamonds of the Dianga pipe, Yakutia, belong to type E, and nearly all diamonds produced from Venezuelan kimberlites (Guaniamo region) are eclogite. 

Note that practically all kimberlite diamonds have been affected to a degree by magma erosion. As a result of the kimberlite melt’s aggressive attack, octahedrons turn into rounded dodecahedroids, and multiple corrosion figures appear on the crystals’ surface. Though this process makes marked changes in the way crystals look, it is unrelated to polygenesis. 

Together with magma

The following type of rock forming commercial diamond deposits is lamproite. Even though this igneous rock is not a twin to kimberlite, they are closely related.

Differing from kimberlite in its chemical composition and mineralogy, lamproite also forms volcanic pipes, a small portion of which contains diamonds. The diamonds themselves are in many ways similar to the kimberlite ones. Thus, diamonds from the Australian Argyle pipe – the largest lamproite deposit – are totally analogous to the kimberlite diamonds in terms of morphology, the only difference being pronounced predominance of E type with a lighter isotopic composition of carbon. 

Other lamproite bodies (for example, the Majhgawan pipe in Central India and dikes of the Ingashinsky field in the Eastern Sayan mountains, Russia) contain a larger share of rounded diamonds of dodecahedron habitus in comparison with kimberlites. This form of crystals is the result of dissolution of the originally octahedron diamonds. 

Rounded crystals often occur in placer operations of different age all over the world (Brazil, India, Urals, Eastern Sayan, northeast end of the Siberian platform), including numerous placers of the Proterozoic age. Though primary deposits of these diamonds are unknown, there is good reason to believe that these were lamproites. Subsequently, ancient placers must have been appreciably eroded, and the diamonds were re-deposited in younger placers including the modern ones. 

It should be noted here that lamroites are not the only newcomers to the family of diamond-containing matrices. By now diamonds have been discovered in many other non-kimberlite rocks. They are lamprophyres of Canada, komatiites of French Guiana, shonkinites of Uzbekistan, and others.

The primary nature of some other diamond-bearing rocks that have undergone marked changes (for example, diamondiferous phyllites of Brazil) still remains a mystery. One way or the other, the circle of exotic igneous rocks containing diamonds has become wider, which suggests that the number of its members will increase even further. 

However, kimberlite is not the only one that has been driven back…

The Kokchetav surprise 

The story of the extraordinary Kumdy-Kol deposit mentioned above dates back to 1967. It was then that exploration of young titan-zirconium placers of the Kokchetav Massif brought Simferopol geologists to the discovery of small yellow cubic-shaped diamonds. This promoted prospecting works but the diamond-bearing rocks themselves were discovered only in the late 1970s.

And then the geologists were given a big surprise. As it turned out, primary deposits of the Kokchetav diamonds were not igneous but metamorphic rocks, i.e. sedimentary or plutonic rocks that had undergone dramatic changes in the earth’s crust and mantle under the action of chemically active agents, high temperature and pressure. 

When and how did these remarkable crystals form in metamorphic rocks? 

The Kokchetav Massif is a complicated geological structure including the ancient (2.2—2.3 billion years) gneiss foundation superposed by younger deposits. According to the hypothesis by N.V. Sobolev and V.S. Shatsky, in the early Cambrian (530—540 billion years) a block of the earth’s crust submerged to the minimum depth of 125 km as a result tectonic processes. The rocks were exposed to high pressure (40 kbar minimum) and temperature (up to 900°С).

As a consequence, diamonds were formed from the core carbon contained in the rocks, which is confirmed by their “lighter” isotopic composition. Later, the block of diamondiferous rocks came back to the surface. 

The morphology of the Kokchetav diamonds appeared to be extremely various, and the crystals could be placed both between the grains building up the rock and inside the minerals themselves: garnets, phlogopites, quartzes, etc.

The singular nature of metamorphic diamonds and their high content in the rock attracted considerable interest. Geological prospecting showed that diamond reserves amounted to 3 billion carats and the content of diamonds was super high: up to hundreds of carats per a ton of rock! The only “but” was that the crystals were rather small (20 µm on the average), which restricted their use to engineering applications. Moreover, the mining technology was very complicated so the deposit was declared unprofitable. 

Possibly, crystals from graphitized peridotites from the Ospinsky Massif in the Eastern Sayan Mountains found as early as in the 1930s (the first primary diamonds discovered in the USSR) had the same origin as the Kokchetav diamonds. Sadly, this collection was lost during the Second World War. 

An important point is that metamorphogenic diamonds can not always bear the lift from the earth’s depths up to the surface. As a result, we get the so-called graphite paramorphs, i.e. graphites having all morphological properties of diamonds. These unusual structures have been discovered in various regions of the planet such as Spain, Morocco, and Urals, to name just a few. 

Messengers from heaven

Apart from the diamond-bearing rocks and diamonds described above, there are some other, more exotic types, even though having no commercial significance. The most extraordinary must be the so-called impact diamonds. Impact processes accompany high-speed collisions of planets and sufficiently large space objects (meteorites or comets) and result in crater formation on the planets’ surface. 

Today, the Earth counts over 150 impact craters whose diameter ranges from 100 meters to 200 kilometers and more. One of the largest craters known – the Chicxulub astrobleme buried underneath the Yucatan Peninsular in Mexico – is 170 km in diameter and 65 million years old. Supposedly, it was then, in between the Cretaceous and Paleogene periods, that massive extinction of organisms, including the dinosaurs, occurred. 

The age of another largest impact crater, the Popigay astrobleme near the Taimyr Peninsular, 100 km in diameter, is 35 million years. The planetary cataclysm that led to the formation of this crater is implicated in causing another notable extinction of biota in the late Eocene.

A relatively young impact crater is the Puchezh-Katun structure in the vicinity of Nizhniy Novgorod (80 km in diameter, 183 million years old). Generally, the older the crater, the more difficult it is to prove its cosmogenic origin – though for a number of such structures this has been done; for instance, the Vredefort Dome in South Africa (1.97 billion years) and the Sudbury Basin in Canada (1.84 billion years).

Profound transformation of the target substance occurring during impact processes – impact metamorphosis – is mainly brought about by the action of shock waves generating from a high-velocity collision. These processes feature extreme parameters such as impulse pressures up to or exceeding 3,000 hPa (hectopascal) and residual post-impact temperatures up to 30,000 degrees! Under these conditions carbon substances (graphite, coal) give birth to specific impact diamonds.

First diamonds of this kind (their nature was established afterwards) were discovered in 1888 in the Novyi Urei meteorite by the Russian explorers M. V. Erofeev and P. A. Lachinov. Later, impact diamonds were found in the debris of the iron meteorite of the Arizona Crater. 

The impact diamonds embedded in the rock of the meteorite crater itself (the Popigay astrobleme) were first found by our compatriot geologist L. Masaitis. Since then impact crystals have been discovered in some other craters as well as in the sedimentary rocks outside the craters, where diamonds find their way as a result of a rock outburst at the time of collision. 

Impact diamonds, which are paramorphs of the original carbon substance, inherit some of their properties including the morphology of graphite particles and the carbon isotopic composition. 

As for their structure, they are polycrystalline close-grained aggregates whose crystallites’ size ranges from several nanometers to several microns. The grains are not large either: for instance, at the Popigay astrobleme their size varies from 0.1 to 0.5 mm, very occasionally reaching 10 mm (Vishnevskiy et al., 1997).

An important feature of impact diamonds is the presence of the lonsdaleite phase, a peculiar hexagonal diamonds’ modification characteristic exclusively of impact-metamorphic processes. The latter is unstable and transforms at high temperatures into a cubic modification, which, according to some evidence, can be much stronger than the ordinary one. 

An impact event like the Popigay collision generates a huge amount of energy. The so-called yakutites, the coarse grains of impact diamonds up to 1 cm in size, were discovered 500 km away from the crater. In order to travel this distance, ejecta had to have the initial velocity of 2.2—2.4 km/sec at the trajectory angle of 40—60°. And some of the matter evaporated during the collision supposedly jetted up at 14.6 km/sec! As a result, minute substance including impact diamonds could be carried by air flows a great distance away (Vishnevskiy et al., 1997).

The number of diamonds originating from impact structures varies a great deal. As for the mining prospects, separate blocks of the Popigay astrobleme, for instance, could be a source of high-quality industrial diamonds. However, remoteness and high cost make their mining unprofitable for the time being.

Porous and fibrous

The diamond-bearing primary sources described above have been studied to a greater or lesser extent. At the same time, placers are home to some exotic crystals whose origin remains a mystery. 

Firstly, here belong the abovementioned carbonado, mined alongside ordinary diamonds from Brazilian placer operations in the first half oh the 19th c. In addition to Brazil, carbonado occurs in West Africa; this areal must be attributable to forecontinent Gondvana, in which Africa and South America constituted one whole before they broke up in the Mesozoic. 

“Carbon” diamonds are irregularly-shaped polycrystalline structures, often rounded, of black, brown, reddish, and grey colors. Carbonado is saturated with alien silicate and oxide minerals, which bloom out with time to produce porous diamonds. Carbonado’s specific shape suggests that they all are fragments (debris) of a single large body. 

Hypotheses about these diamonds’ origin are many. A block of the earth’s crust with a massive high-carbon body of the schungite type (ancient metamorphosed fossil coal) may have submerged into the mantle in the early Proterozoic, where it was exposed to the depth factors and subsequently surfaced. This, already “diamond”, body was eroded and its fragments settled in placers. 

Carbonado has been reportedly found beyond the mentioned areal; however, similarity relies on only one criterion – the polycrystalline structure – which in not nearly enough for trustworthy diagnostics. 

No less exotic diamonds have been found at placers in the northeast of the Siberian platform (the interfluve of the Anabar and Lena rivers). These are quite large octahedron and dodecahedron crystals abounding in black inclusions. They feature a light carbon isotopic composition, which is indicative of its core origin. The diamonds are often rounded, which indirectly points to the ancient (Precambrian) age of their primary source.

This list of the diamonds’ extraordinary varieties is far from complete. We believe, however, that what has been said is enough to estimate their natural diversity both in terms of mineralogy and origin. 

The carbon isotopic composition of a high percentage of diamonds suggests that they could have originated from core rocks submerged in the mantle as a result of subduction, slow movement of the thin oceanic crust under the thick continental lithosphere.

As for the diamonds whose carbon is of the mantle origin, there is yet no clear understanding of their crystallization mechanisms. Today, there are speculations about possible crystallization of natural diamonds in metastable conditions (in particular, concerning the Kokchetav Massif) – that is, in conditions when the crystals can remain unchanged for a long time, without transforming into graphite. The problem with this hypothesis is that it requires too much additional evidence, which makes it practically unreal. 

This survey, incomprehensive as it is, shows that geologists have to go beyond the kimberlite paradigm when dealing with diamonds. Even though we have nothing so far but the polygenesis idea, research logics will inevitably bring us to a new understanding of the origin and new methods of searching for crystalline carbon, which never ceases to surprise us.

Reference
Afanasiev V. P., Efimova E. S., Zinchuk N. N., Koptil V. I. Atlas morfologii almazov Rossii (Atlas of Russian Diamonds’ Morphology). – Novosibirsk: SB RAS Publishers, United Institute of Geology, Geophysics and Mineralogy, 2000. – 293 p.
Vishnevskiy S. A., Afanasiev V. P., Argunov K. P., Palchik N. A. Impaktnyie almazy: ih osobennosti, proishozhdeniye i znacheniye (Impact Diamonds: their Features, Origin, and Meaning). Novosibirsk: SB RAS Publishers, United Institute of Geology, Geophysics and Mineralogy, 1997. – 54 p. (Rus., Eng.)
Orlov Yu. L. Mineralogiya almaza (Diamond Mineralogy). 2nd edition. Moscow: Nauka, 1984. – 264 p.
Chepurov A. I., Fedorov I. I., Sonin V. M. Eksperimentalnoye modelirovaniye protsessov almazoobrazovaniya (Experimental Modeling of Diamond Formation). – Novosibirsk: SB RAS Publishers, United Institute of Geology, Geophysics and Mineralogy, 1997. – 197 p.

Photos are courtesy of K. P. Argunov’s archive (State Archive of the Sakha Republic)