FORMATION OF DIAMONDS
The reality about the places that diamonds are found is that our planet might host enormous quantities of diamonds below its rocky surface. However, it is only the diamonds that are found at the surface of the earth that are commercially interesting for us. The rarity of diamonds derives mostly from a combination of the unique conditions beneath cratons that allow diamonds to survive on their journey to the surface, and also the violent means by which they travel. To put rarity of diamonds, their formation and challenges for mining them into perspective, we need to address that diamonds arrive at the surface of the earth through volcanic eruptions that bring high-pressure magma from the lower layers of the earth to the surface. It is only when these eruptions pass through the diamond stability field, which enjoys the relative protection of a shielding craton that we find quantities of diamonds that can be mined economically. Human beings have never witnessed such an eruption and the last one is believed to have taken place more than 25 million years ago! Kimberlite eruptions are caused by a differential in pressure that forces hot magma up through weaker areas of rock. The heat of the magma can often combine with carbon dioxide that reverts into a gaseous state and forces the kimberlite up with even more force. As the hot magma approaches the surface, the pressure exerted on it by surrounding rocks gets weaker and weaker. This often causes the kimberlite to expand, leading to the recognizable carrot-shaped profile of most diamond mines, which lends itself well to modern open-pit mining. The kimberlites often encounter ground water close to the surface. This water is quickly vaporized and the resulting gases add to the power of the kimberlites and cause them to violently explode upon reaching the surface. The term “kimberlite” comes from the Kimberley area of South Africa where they were first discovered in the diamond rush of the late 1800s. Kimberley diamonds were originally found in weathered kimberlite, colored yellow by a mineral called limonite, which led to it being called yellow ground. Later digging uncovered serpentinized kimberlite high in olivine, which gives the rock its noticeable blue/green color, which miners aptly named blue ground.
Diamonds within kimberlites are just a tiny fraction of their total mineral composition. Some mines can economically produce diamonds at a rate of just 20 carats per hundred metric tonnes of rock. This equates to just four grams of diamond per 100 million grams of rock. The largest capacity dump trucks used in construction typically carry around 350 tons of rock, now imagine a massive dump truck filled with rocks yielding just 15 grams, or about one cubic centimeter of diamonds, of which only a fraction are gem quality. Although research has shown that diamonds take approximately one billion years to form in nature with some dating back as far as 3.5 billion years, kimberlites themselves are a relatively recent phenomenon in geological terms. Most kimberlite eruptions are believed to have taken place during the Mesozoic era, between 70 and 150 million years ago. Some kimberlites in South Africa are believed to be as old as 1.6 billion years, though such an old kimberlite is extremely rare. It is possible that older kimberlite eruptions have occurred but that hundreds of millions of years of weathering and erosion have hidden them from sight beneath the earth. Although most diamond mines form out of kimberlites, this is not the only way for it to happen. Lamproites are a similar type of rock formation which develop from volcanic explosions. Lamproites were long thought to be kimberlites, however research in the 1940s identified important differences from their kimberlite cousins. Diamond bearing lamproites have been discovered in Western Australia, the US, and India. The Argyle Mine is the first economic diamond deposit known to occur in a lamproite, with the nearby Ellendale Mine (now closed) the world’s only other known example.
