At the October meeting, member Tony Thorp gave a talk on “Diamonds”, in particular synthetic diamonds, but excluding gems. The contribution which diamond research and particularly that of Loring Coes made to geology is little acknowledged; but his synthesis of some forty metamorphic and other minerals enabled geologists for the first time to establish the P/T phase diagrams so essential in working out the provenance of metamorphic rocks.
Anyone interested in gems and the story of how De Beers operated the most successful cartel in marketing history for over a century, should consult “The Death of the Diamond” by Edward Jay Epstein. It is a good read, and one of the few books on diamonds unsullied by output from De Beers and Ayers publicity departments. It is now somewhat out of date, so supplement it by searching for ”Element Six”, or go to www.e6.com (De Beers have always been coy about using their own name!) Diamond possesses extraordinary mechanical, electrical;, optical, chemical and thermal properties which have made it a critical strategic material, particularly in times of war.
After Lavoisier and Smithson Tennant showed that diamond was indeed 100% carbon, like common graphite, researchers have endeavoured to convert the one into the other.
The discovery, in the 1870s, of primary diamonds at Kimberley in South Africa concentrated research into high temperatures and high pressures. James Hannay, Henri Moissan, Crookes, Parsons and others all failed, often in dangerous experiments ending in explosions, to produce diamond.
Before the turn of the century, the limit to achievable pressures in the laboratory was only some 2-3000 atmospheres. It was Percy Bridgman, regarded as the “Father of high pressure chemistry” and a Nobel laureate, who opened the way to higher pressures by inventing the “Bridgman Seal”, a simple device, based on the “de Bange” breach mechanism (invented in 1877 and still in use for the field pieces firing the salute on the queen’s birthday). With this device he defined the high pressure phases of dozens of materials, famously discovering ice VI, stable at 95 °C. However, although he squeezed carbon to over 100 000 atmospheres, he never produced diamond.
With World War 11, diamond became a critical strategic material and in 1941 GE, Norton Abrasives and Carborundum sponsored Bridgman in his efforts, however he was co-opted onto the Manhattan project and research was interrupted. (The behaviour of uranium and plutonium at high pressures was critically important at that time!)
With the breakup of the Bridgman team, Norton continued research on their own and their Loring Coes (of coesite fame) developed a simple pressure assembly comprising an Alundum (proprietary alumina-based ceramic) cylinder with a quarter inch bore and a steel reinforcing belt to keep it in compression. In the bore were top and bottom carbide pistons which could be used as electrodes and could apply pressure to a sample. If necessary, the sample could be surrounded by a carbon cylinder as a heating element.
With this device Coes proceeded to produce a whole series of minerals which were associated with diamond in kimberlites and eclogites, including garnets, pyroxenes, staurolite, kyanite, zircon, idocrase, tourmaline, beryl, sphene and topaz. He also squeezed quartz to 35000 atmospheres at 800 °C, producing coesite, which was unknown in nature at the time.Although Coes enabled the whole field of metamorphic mineralogy to be put on a quantitative basis, he never achieved diamond synthesis.
This was left to a team set up at GE as project “Superpressure”. The GE team included Francis Bundy, Herb Strong and Tracy Hall. The team worked very much as individuals and developed a series of different geometries of carbide anvils, retaining belts and gaskets. Some four years into the project, with no results from hundreds of experiments, most of which had a duration of a few hours, top management were considering whether to continue or not. This was a moment of truth and in a last ditch effort, Herb Strong set his apparatus up with a sample of “Steco” (a case hardening compound containing carbon and iron). He aimed at 50 000 atmospheres and 1250 °C and left it for 16 hours, overnight. Next day, he opened it up and there were apparently no diamonds, but he was interested in how much carbon was dissolved in the iron and sent a blob of fused material to metallurgy, for polishing and examination. A week later, metallurgy came back saying they could not polish the sample as it gauged up their polishing wheel! Eventually, two small diamonds were separated from the sample. It seemed they had succeeded. The next day, Dec 16th 1954, Tracy Hall set up his own apparatus in which the sample was contained between tantalum discs in the press and set to 100 000 atmospheres and 1600 °C and ran for 38 minutes. When opened, he saw dozens of diamonds stuck to the tantalum disc!
Strong’s result could not be repeated, but Hall’s was reproducible. Diamonds had been synthesised for the first time in a commercially reproducible procedure.
Meanwhile, out on a limb, in Sweden, the eccentric inventor, Balzar von Platen, working with the Swedish electrical firm ASEA developed a complex, expensive device comprising a cube packed with carbon, iron and a thermite mix, surrounded by carbide and steel wedges and miles of piano wire, in which he probably managed to produce the world’s first synthetic diamond in February 1953; but it was covered by a cloak of secrecy and the process was not revealed until after GE had announced their success. It is a mystery how this came about. It was either just commercial secrecy or possibly that they only discovered their small diamonds after the GE announcement.
So much is history, patents have run out and diamond presses are now commercially available for modest amounts. Diamonds are cheap and of higher quality than naturally occurring stones.
Even the amateur geologist can afford diamond hones, cutting and lapping discs for polishing specimens and making thin sections. The industry has moved on and synthetic diamonds are now produced by several processes, including crystallising from plasma (CVD) and explosive processes. A once rare material is in everyday use and we are all the better for it.