Diamond-Surplus Weblog

September 5, 2008

About Diamonds: A Lesson in Geology

The Origin and formation of diamonds

Ever wondered how diamonds were formed? Well, I guess not many of us really give a thought about it when we shop for a gorgeous rock (yes it is essentially just a piece of rock, albeit a shiny one, haha!) in hope to see that priceless expression on the face of our loved ones.

Since I am in the educational mode, I figured it’ll be worthwhile to know how diamonds are formed! Here’s an article I found that would help expand your geological knowledge.

Enjoy!

Since the early 1980s, a vast amount of new scientific information has become available on the origin of diamonds.

Much of this information has been obtained through the study of ‘inclusions’- natural material found within diamonds.

Diamonds were formed at least 990 million years ago, although some are estimated to be as many as 4.25 billion years old, thereby pre-dating life on this planet.

Diamonds are formed at pressures of 45-60 kbar.  A kilobar is a metric unit for measuring high pressure. This corresponds to a depth of 125-200 kilometres below the Earth’s surface where the pressure is around fifty thousand times that of atmospheric pressure at the Earth’s surface.

Some diamonds form at depths of 300-400 kilometres, or even deeper, but these diamonds are particularly rare.

Diamonds are formed at temperatures between 900°C and 1,300°C.

Formation of Kimberlite pipe

In the Earth’s upper mantle, the pressure of magma, or molten rock, cracks the surrounding rock around 125km below the Earth’s surface. The magma contains dissolved carbon dioxide, which begins to bubble and expand, either due to heat from below or reduced pressure from above.

This expansion causes the magma to erupt explosively up through the cracks, like an uncorked champagne bottle.

The resulting explosion is vastly more powerful than most volcanoes, with molten rock rising to the surface at the speed of sound, forming a ‘pipe’ through the path of least resistance to the surface.

The magma rises so quickly that diamonds do not have time to convert to graphite, which is the more stable form of carbon at the Earth’s surface. Once the diamonds cool down, they don’t have enough energy to re-form their crystal structure into graphite.

The volcanic cone eventually cools and the magma hardens into kimberlite.  It then starts to be eroded and weathered away by the elements. Eventually, it becomes almost undetectable on the surface.

Now THIS was what got my attention!

Types of Diamond Deposits

Primary deposits
Diamondiferous pipes, which are ‘pipes’ of mineral-rich volcanic rock containing diamonds, are known as primary deposits.

Diamondiferous pipes are the solidified cores of kimberlite or lamproite volcanoes.


Secondary deposits
Deposits that contain diamonds which have travelled some distance from their original source are referred to as secondary deposits.

The diamonds that are now found in Namibia, for example, have travelled over 1,000 miles from their original source in southern Africa, transported by the Orange River.

Kimberlite, lamproite and komatiite are the only types of rock known to have transported diamonds from the Earth’s interior up to the surface in significant quantities.

So there you go, a lesson in Geology! 🙂

Souce: DeBeers

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2 Comments »

  1. nice article very informative thanks

    Comment by rockhunter1 — September 7, 2008 @ 6:35 pm

  2. Your welcome! 🙂

    Comment by diamondsurplus — September 10, 2008 @ 6:21 pm


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