The mining of valuable minerals is enabled by how near and accessible they are to the surface of the earth and our capacity to locate and harvest them. As with many other types of crystal gems, diamonds were formed under extreme circumstances deep in the earth and carried to the surface from the subcontinental mantle many kilometres below by volcanic action millions of years ago. In the case of kimberlite, pipes of igneous rock are formed. Other 'foreign' nodules of crystalline rocks and minerals were also brought up to the crust, ripped from the walls of the pipe as the magma traversed the lithosphere to the surface. These are known as xenoliths and xenocrysts, xenos meaning foreigner or stranger in Greek. These nodules contain various high-pressure minerals including diamonds.
This image originally appeared on the cover of Nature with the subtitle Origin of diamonds (Vol. 310, 19 July 1984). The following caption appeared on the index page of this issue, below a thumbnail of the cover:
"Lilac-coloured inclusions protruding from the fractured surface of a Kimberley diamond fragment. As reported on page 198, isotopic investigations show that these diamonds originated in old enriched mantle and are thus much older than the host kimberlites."
The diamond fragment is about 1 mm across and if we were writing the caption today, we would probably refer to the inclusions as purple garnets (~200 microns in diameter).
"I made this image in early 1984 on Kodachrome 64 film using a Nikon Multiphot macro-photography system with 19 mm f/2.8 lens, bellows and Nikon F3 camera back set for 1/2 sec exposure."
Sometimes crystals of other minerals like purple garnet [see image] are found embedded inside diamonds themselves – known as diamond inclusions. These represent perfectly preserved geological data: they are entirely sealed within the diamond for vast periods of time, keeping their original crystalline structure and composition intact. In the late twentieth century, it emerged that the comparative study of the specific mineral compositions of the kimberlite (and xenoliths found within it, as well as diamond inclusions) could offer a more accurate determination of the probability of diamondiferous kimberlite. This was of great significance and value to the diamond mining industry but also of academic worth, contributing towards ways of understanding cratons (stable cores of ancient continents evident under the crust), diamond formation and plate tectonics amongst other things.
In particular, garnet geochemistry and analysis pointed to a breakthrough method of determining whether a kimberlite pipe was likely to contain diamonds or not. This milestone methodology was developed in the seventies and eighties by UCT professor Dr John Gurney – now retired. Nick Norman, in The Extraordinary World of Diamonds, describes Gurney’s historic contribution as such:
"Although he would be the first to say that he did it as part of a team, there is no doubt that he was the principal architect of the technology and its application, and was instrumental in collecting hundreds of samples from mines and prospects around the world and having them analysed. Now geochemistry can short-circuit years of field and kimberlite sampling." (Norman, N. 2010: 233)
Gurney's generation of geologists at UCT were instrumental in initiating the collection of discarded kimberlite xenoliths from the mining fields of the Northern Cape and other locations. These xenoliths, jettisoned by the nineteenth century diamond miners who believed them to have a low yield of diamonds compared to the softer kimberlite host, were easily picked up in the veld by the keen geologists. Today, most xenoliths are crushed and milled to dust in the mechanised process of contemporary diamond mining and thus are often much harder to obtain for research. The UCT collection has therefore been essential in the analysis of kimberlite and its precious cargo. The collection also includes beautiful 'slides' – that resemble coloured glassware – made from very thinly sliced xenoliths in which flecks of red garnet, for example, can be seen suspended in larger areas of green peridot and other mineral combinations. Another visual by-product of the research into these mineral specimens is the marvellous images of diamond inclusions, examined and photographed by optical and scanning electron microscope (SEM) techniques.
The act of looking and analysing this visually arresting collection in its multiple forms, allows for new ways of understanding the ancient formation of the earth and the wealth held within its geological layers.