Superdeep Super Freaks
The birthplace of the world’s biggest diamonds has been queried for many years.
A recent study published in Science in December 2016 shed some light on their unexpected origins. Diamond Girl explores deeper into these freaks of nature…
Evan Smith, a diamond geologist at the Gemological Institute of America (GIA) has led research into some of the world’s biggest and rarest diamonds, revealing their surprising origins.
Even though we find our diamonds on the Earth’s surface, diamonds grow within the mantle and were bought up millions of years ago via deep-rooted volcanoes. This research not only reveals more about diamonds but is highly valuable in understanding the structure of the inner Earth.
Until recently it was believed that all diamonds grow below the Earths crust at depths of 150-200 kilometres. At these depths the pressures are between 45,000 – 65,000 times atmospheric pressure (45-65 Kbar) and temperatures are between 900 – 1300 degrees centigrade.
It has been previously suspected that the larger, more rare diamonds have a different origin to most, because these diamonds have a number of qualities that differ from the norm.
Getting your hands on samples of such rare specimens to study is a difficult, if not impossible task, however, Smith managed to obtain off-cuts of these enormous stones that were discarded during the fashioning process - his contacts at GIA certainly paid-off!
His research has revealed and confirmed some startling facts about these newly nicknamed ‘CLIPPIR’ diamonds.
‘CLIPPIR’ refers to the common traits of these big and beautiful stones. All of the diamonds in question are ‘Cullinan-like’, which refers to their typical Large sizes, lack of Inclusions, Pure crystal-structure and IRregular external shapes.
Exceptional quality, large diamonds such as the 3106.75ct Cullinan diamond are ‘Type IIa’ diamonds, which refers to their nitrogen-free pure carbon crystal-structures - accounting for less than 2% of all natural diamonds. These Type IIa diamonds can also to grow to impressive sizes and almost all of the gem-quality diamonds over 100ct are amongst this rare type.
What allows these diamonds to grow to such spectacular sizes? And how is it that unlike the other 98% of diamonds, which contain nitrogen, these diamonds avoided this impurity when crystallising?
Smith et al. is answering these questions and proving that these larger-than-life diamonds have deeper-seated roots than originally thought possible… The key lies in the study of the inclusions trapped within the diamonds.
What is an inclusion?
An inclusion is a physical presence within a gemstone such as a solid, liquid or gas that has been encapsulated and trapped inside a gemstone during its growth. These inclusions reflect the environment where the diamond grew.
A diamond is one of the most chemically resistant materials, and so it can be likened to a little time capsule that may contain perfectly preserved samples of minerals within them.
"You really couldn't ask for a better vessel to store something in. Diamond is the ultimate Tupperware," says Smith.
In 2014, research led by Graham Pearson at The University of Alberta discovered that some diamonds are from more extreme depths of 410 – 660 kilometres below the Earths surface, which is three to four times deeper than the majority of diamonds. Diamonds from these extreme depths are referred to as ‘Superdeep’ or ‘Ultradeep’ diamonds.
This discovery was made thanks to a miniscule inclusion found in a small diamond from Juína, Brazil. The inclusion was just 40 microns (0.0040cm) in diameter and invisible to the naked eye, however the researchers realised it was an inclusion never before seen in diamond. In fact, it was never before seen originating from this planet.
The inclusion was a water-rich mineral known as ringwoodite and was discovered in a stroke of luck whilst looking for another feature.
Previously, ringwoodite was only known to exist within extraterrestrial meteorite rock so to find it on Earth was a great discovery in itself. This specimen is the only known sample of terrestrial ringwoodite to this day.
So what does this mean?
For ringwoodite to have been found within a diamond, the diamond itself must have also formed alongside this mineral.
Ringwoodite forms at extreme temperatures and pressures and so would require depths of 520 – 600km to be able to form. It can exist up to 410km before converting to another mineral known as olivine.
This also supports evidence for mantle convection as the diamonds must have moved to depths of 150 – 450km in order to have been bought up by the igneous rock kimberlite via volcanic eruptions.
Type IIa Superdeep Diamonds.
Evan Smith and his team have exposed that not only do the large Type IIa diamonds have superdeep origins but they also grow within liquid metal. This challenges previous beliefs that the mantle was a uniform mix of oxygen-rich rocks, but instead may be scattered with pockets of ‘metal pools’.
How do they know?
Smith discovered nickel and iron alloys surrounded by an invisible layer of liquid methane, sulphur and hydrogen. The inclusions within these samples have never been seen before in diamond.
The existence of these metallic inclusions tell scientists that the diamond must have grown within the metal, which must have been in liquid form to allow the carbon atoms to aggregate, bond and crystallise. This would have also been an oxygen-deprived area otherwise different compounds would have formed.
This theory is entirely reasonable, as liquid iron and nickel are known to be excellent materials for dissolving carbon. In fact, many synthetic diamonds are grown by high pressure high temperature (HPHT) methods which use a mixture of iron, nickel and colbalt as a catalyst and as a metallic flux to permit the growth of synthetic diamond crystals.
The metal pools and diamond formation have been calculated to be within the transition zone (between the upper and lower mantles of the earth) at depths of 410–660 kilometres.
To support the above research, Smith has tested a further 53 Type IIa diamonds, of which 72% of them contained these same inclusions. The remaining 28% also contained a variety of garnet known as majorite garnet. This silica based mineral only forms at extreme pressures and temperatures, supporting the evidence that these diamonds must have grown at depths of 360–750km.
So what does this mean?
These ‘metallic pools’ explain why these Type IIa diamonds remained free of impurities, such as nitrogen, keeping them free of colour. This also contributes to explaining their size as individual carbon atoms are able to move freely within the liquid metal until they reach and bond to growing diamond crystals, thereby permitting them to grow larger.
These groundbreaking findings within our purest type IIa not only reveal their unique origin, but also provides us window to an inaccessible part of our planet.
Many ask me why diamonds are considered so valuable… much of the above confirms that the world’s ultimate gemstone is not just a pretty face but a precious enabler for a greater understanding of the inner workings of our world.
Science AAAS: Earth’s rarest diamonds formed in pockets of liquid metal.
NPR.org The Two-Way: Big Diamonds Bring Scientists A Message From Superdeep Earth.
Carnegie Institution for Science: Biggest and best diamonds formed in deep mantle metallic liquid
Nature News & Comment: Tiny diamond impurity reveals water riches of deep Earth
International Business Times UK: Diamonds: Huge, rare precious stones form 750km underground in Earth's mantle.
Bec Crew. ScienceAlert: Scientists have figured out where the rarest diamonds on the planet form