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Thursday, November 24, 2011

How Diamonds are Formed

This shows the Earth's mantle where diamonds are formed.
Courtesy of the Debeers Group.

A vast amount of knowledge has been developed since the 1980s about how diamonds were formed.  It has been through the inclusions found in diamonds that much of the information about how they were formed and their origin was discovered,  Inclusions are all natural materials that are found in diamonds including some that are gaseous, liquid and solid inclusions.  Although theoretically it is possible to form diamonds by squeezing a lump of coal in nature that hasn’t happened although in the lab this had been done under conditions of extreme heat and pressure in giant hydraulic presses,

Some of the diamonds that have been found in meteorites that are more then 4.25 billion years old far exceeding the age of life on Earth.  However natural diamonds on earth were formed at least 990 million years ago.  One of the discoveries made since the 1980s shows that diamond is formed whenever you burn acetylene gas that forms as a coating on metal that has been exposed to burning acetylene that is often deposited during the course of oxy-acetylene welding.  This type of diamond coating has since been made use of industrially for imparting a hard coating onto metals.

How a diamond bearing pipe is formed.
Courtesy of the Debeers Group 

It takes extreme pressure that ranges from 40 to 50 thousand times higher then the pressure found at the earth’s surface of 14,7 pounds or 1,000 millimeters that is termed one bar.  This is the pressure encountered at depths ranging from 125 to 200 kilometers below the surface   Most diamonds are formed from kerogene that was deposited in vast quantities on earth during its formation.  There are other diamonds that were formed from carbonaceous material like fossil coral or shells that have been subducted deep into the earth’s crust where they were also exposed to extreme heat and pressure.  A few diamonds were even formed at depths ranging from 300 to 400 kilometers deep or even further, but these diamonds are extremely rare.  Most diamonds were formed at temperatures that exceeded 9000 to 1,300o Celsius.

Formation and Emplacement of a Kimberlite Pipe

At about 125 kilometers beneath the surface of the earth the pressure of the magma cracks the surrounding rock until the cracks reach the earth’s surface.  Much of this cracking is due to contained carbon dioxide in the magma that causes the magma to bubble and expand.  This is caused either by heat from below or reduced pressure as the magma column reaches towards the surface.  It is this expansion that eventually causes the magma to erupt explosively at the surface.

An eruption like this has never been observed by mankind because it has been millions of years since one occurred on earth predating mankind.  It has been theorized that an eruption of this type rather then being hot like most volcanic eruptions are; it is emplaced under freezing temperatures because of the rapid expansion of the volcanic gases.  In the final few kilometers these gases rise at the speed of sound where they form a pipe following the path of least resistance through the surrounding rock finally reaching the surface.

It is because of this rapid ascent of magma that allows the diamonds to reach the surface of the earth rather then being converted into graphite a far more stable substance at the earth’s surface.  The pressure him pardoned by dissolved gases in the magma eject the diamonds they don't have time to be degraded into graphite.  The diamonds are actually xenoliths, particles caught up in the magma train as foreign objects.  Once the volcanic cone cools it starts too erode releasing the diamonds into the soil. The diamonds that are not eroded state trapped in a kimberlite or lamproite where they are mined.

Different Types of Diamond Deposits

How diamonds are mined.
Courtesy of the Debeers Group.

Diamonds are found in two different types of deposits of which primary deposits come first.  These are deposits found in kimberlite, lamproite or komatiitef pipes and are mined lake any other mineral found in hard rock.  Secondary diamonds are found in river gravel deposits where they are concentrated after traveling for some distance from their primary source.  The gravel found in the Orange River in South Africa is an important source of diamonds.

Often overlooked by many is a third source of diamonds found in a rock type called conglomerate that are really fossilized river gravel.  One important formation like this is found near Marathon, Ontario that not only carries diamonds, but rubies and sapphires as well..