Diamonds are Forever? Probably. – David Cliff

 

The rather tongue-in-cheek nature of the title belies the fact that diamond can be reduced to carbon dioxide if a high enough heat source is applied to it so that it readily oxidises. However, there is a serious side to the stability of diamond under the ‘right’ conditions. Diamond is a metastable allotrope of carbon and is the hardest naturally-occurring substance on earth. It also has the highest thermal conductivity of any bulk material. With a specific gravity of 3.52, this makes it dense relative to the host rock and thus enables relatively simple recovery techniques to be used in the mining industry. Optical dispersion gives diamond its desirable characteristics for the jewellery industry, although the biggest proportion of all production lies in the industrial applications; abrasive tools and cutting applications. 80% of the 130 million carats mined annually are used in industry and 4.5 billion carats are produced synthetically. The highest value diamonds are marked up some six fold at retail from the US$9 billion annual mine production after passing through the cutting, polishing and marketing stages.

Diamonds have been recovered for jewellery over some 6000 years, primarily in India, but also more recently in Brazil, entirely from alluvial sources. Primary sources of diamond were unknown until the accidental discovery of the diamond-mineralised volcanic pipe at Kimberley in 1871 by the De Beers brothers on the farm Vooruitzight. The statistics for the Big Hole, as it became known are impressive; 50000 miners on the claims by 1872, the open pit reached 215 metres depth by 1914 and the underground mine ceased at a final depth of 1097 metres after more than 14.5 million carats (3000kg) produced from 22 million tonnes of rock.

The sale of large diamonds at auction still captures headlines on a regular basis.

Most of the talk centred on the geology of diamond, as well as means of exploration, evaluation and recovery in the mining process. An excellent synthesis of diamond geology, including the latest research has been produced by Steven Shirey and James Shigley (Carnegie Institute and GIA Carlsbad Az. respectively) at; https://www.gia.edu/gems-gemology/WN13-advances-diamond-geology-shirey.

Natural diamonds are an insight into the deep earth and are known to be much older than the host magmas that transported them to surface; rocks generally grouped as kimberlites (occasionally lamproites or lamprophyres), which are derived from deep in the earth’s mantle and relatively ‘undersaturated’ or silica-poor compared with crustal origin magmas. Clifford’s Rule has been invoked since the mid-sixties to account for diamond occurrence, although there are several notable exceptions. This states that diamondiferous magmas are to be found in rocks older than 2.5 billion years in the cores of continental massifs known as cratons. The concept holds where diamond is stable in the mantle at a depth of at least 110km (equivalent to 4GPa pressure) and a temperature of at least 900oC and not more than 1400oC. The ‘keels’ of ‘cold’ cratonic cores that have existed since 2.5 billion years provide the fertile source rock. The mechanism that brings diamonds to the surface is mantle upwelling deep in the earth that transports the diamond in olivine or garnet rich magmas (peridotitic or eclogitic types) from the upper mantle that lies beneath the deepest crust.

While diamond it transported to surface by kimberlite (and lamproite) eruptions (rare events in geological time), the speed of transportation and cooling history are equally important. Slow-ascending magmas or slow cooling would cause diamond to revert to graphite. Rough diamonds frequently show signs of etching caused by this resorption process. The violent nature of the eruptions not only help preserve diamond as ‘xenocrysts’, but also contribute to the champagne flute shape of the kimberlite pipes. To a large extent, Clifford’s Rule accounts for the endowment of diamondiferous kimberlites in the old cratonic rocks of southern Africa, the Canadian (Laurentian) Shield, the Baltic Shield and Siberia, but does not fully explain why Brazil, with more than 1300 kimberlite pipes and an abundant alluvial endowment, does not currently possess a single economic hard-rock deposit.

In the exploration context, the statistics are telling. There are more than 10 000 kimberlite pipes worldwide. Of these, about 1000 are diamondiferous with some 100 classed as economic. ‘World class’ diamond pipes number no more than 15-20, depending on how ‘world class’ is defined. The odds of discovery are very low, but the rewards can be very high. Diamond can be found in alluvial deposits, of course, and these sediments can be traced back to source. Diamond’s rarity makes this difficult and exploration relies very heavily on other, more common, mantle-derived minerals that exist in the diamond stability field; chrome garnet, chrome diopside (a magnesium and calcium silicate), chrome spinel (a form of chromite with high Cr: Cr/Fe ratio) and ilmenite (titanium oxide). Various geophysical methods have been employed to locate kimberlite bodies including; airborne magnetics and electromagnetic applications and ground-based gravity and electrical conductivity methods. All of these methods can be expensive, especially where there is glacial deposit cover (over Canada and Siberia, for example) or thick sands such as the Kalahari of Botswana.

Evaluation is also difficult and expensive, even when the host kimberlite is found and sampled for diamond. With grades expressed as carats per tonne – or even carats per hundred tonne – there is a heavy reliance on the lognormal distribution of microdiamonds (largest dimension less than 0.85mm) released by caustic fusion in the bulk samples recovered by drilling in order to predict the likely macro stone size. This carries risks, as population outliers (big stones) can be shattered in the rock crushing process. Grades and stone quality can vary markedly even within a pipe, as there are often several magmatic pulses involved in the pipe’s history, with markedly varying diamond grades.

On the positive side, once a pipe is deemed economic by bulk sampling, mining is usually by conventional open pit or even underground, sometimes in a block cave mining method (https://www.youtube.com/watch?v=MVDAw56s5dU ). Processing can be achieved by exploiting the rock density difference between kimberlite (about 2.5g/cc and diamond at 3.52g/cc) using a heavy medium slurry, followed by x-ray sorting and stone picking in sealed glove boxes using trained personnel.

All naturally-occurring diamonds carry inclusions (the stones with the least are most valuable) and some attempts are being made to identify diamonds that are conflict free to back up the Kimberley Process, set up in 2000 by the major producers to certify the origin of the individual stone. It is possible to date the diamond from the inclusions using isotope ratio methods, as well as the host magmas utilising the diamond stability field minerals such as pyrope garnets. These studies have led to the origin of the diamond before it becomes part of the mantle keel. As so often, subduction of oceanic plate and sub-oceanic mantle is the key to understanding. Hydrated carbonated oceanic crust is constantly being subducted into the deep mantle so that the interaction of these fluids with mantle silicate reduces the fluids and the carbonate. Diamond is formed and is uplifted to reside in the cratonic root accumulations. Reaction barriers created by pressure and temperature determine whether the diamond remains in the stability zone.

Some notable exceptions of diamond occurrence do not apparently follow the rules, although none are economic as bedrock sources, despite Copeton and Bingara in New South Wales having produced more than 500 000 carats from alluvials. It is believed that ‘cold slab’ subduction has, in these cases, been reversed and the diamonds in them have stayed in the stability field. Other, stranger, occurrences have been reported from metamorphic rocks in Norway, and lamproites from Linhorka (Czech Republic) have been reported. Pervasive microdiamonds in Australia might have been derived from space dust, according to some theories. Certainly, some recent research into cosmic sources for diamond have produced some intriguing concepts; http://www.astronomy.com/news/2011/08/a-planet-made-of-diamond and http://science.sciencemag.org/content/286/5437/25.1.

In the concept of the original carbon cycle, there is some truth in the view that diamonds could be ‘forever’ – at least within the realms of human timescales!

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