DIAMOND’S FORMATION

Introduction about diamonds.

How are diamonds born naturally and artificially ?

I. Natural diamond.

1. Initial conditions.

2. Crystallization.

3. Surface evolution.

II. Synthetic diamond.

1. Principle.

2. Imitation.

III. Sociological and economical impact.

Conclusion.

DIAMOND’ S FORMATION

Projet d’anglais STU

Année 2001-2002

OFFANT Yohann DEUG STU

2ème année

Summary :

Carbon crystallizes in the cubic system, it generally contains some nitrgen and bore

traces. The chimical composition of the diamond is simply made of carbon pure. Diamond is

the hardest material on earth (according to the Mohs scale : 10/10) and it’s impossible to

make stripes on it. This octaedric cristal is very searched and has such a successful reputation.

That it stands for the symbol of beauty and of riches. Man dig everywhere all around the

world (south africa, australian, bresilia, india…) in very basic magmatic rocks such as

kimberlit rocks and in the alluvium. This is why diamonds, have a very big renowm on the

one hand in jeweller’s shop, on the other, in industry.

In this report, we will try to understand how and where this fabulous mineral is

created.

DIAMOND’ S FORMATION

Introduction.

Diamond is the most searched of all the precious stones because it is the hardest

substance. In value, it surpasses Ruby, Sapphir and Emerald. The biggest diamond found in

the world weighs 3106 carats (or 620gr) ; this stone has therefore a priceless value…

Nevertheless, it has an industrial interest. We must know that only 20 % of diamonds are cut

and the rest is used for an industrial purpose (hacksaw, abrasive…). However, it simply

concerns crystallized carbon. This is why man have tried to synthetise them. Gradually, they

got closer to the real constitution of diamond.

It will be interesting to know how diamond naturally and artificially born, and the

consequent economical stake.

I- Natural diamond.

1) Initial conditions.

Diamond crystallize in the cubic system and they are majoritarly found under the

octaedric form. In order to crystallize carbon, we must have temperatures around 1500°c and

pressures near 50-60 kbars ! This extreme conditions can be discovered in the superficial

leayers of the mantel, that is to say between 150-200 km of deepth. As the eclogitic diamond

are a let more common. We will focus our study on this type of diamond, that has always

been found out near the subduction areas.

Before all, what is the particularity of this zone, that give birth to diamond ?

2) The crystallization.

The oceanic plaque, which contain basalte and sediment (riches in organic material),

bury under the lithosphere entel 660 km. Temperature and pressure increase and the basalt is

transformed in eclogitic rock. For this transformation, the basalt needs to reach a temperatur

between 1100 and 1600°c, or between 150 and 200 km of deepth. But we found this eclogit

included in diamonds. Thus, the diamond’s Formation is linked with the responsible

processus of the basalt transformation. Diamonds are caracterised by free development of his

cristallographic side. Diamond’s growth is there-for related to a fluid. With the depth, the slab

warms up and is deshydrated. As soon as basalt transformes, the material (from the oceanic

material) meets fluid riches in water and CO2 (from the organic material). If it encounters a

silicate area, it is very likely that the diamond crystallizes. This fluid rises in the mantel and

stabilizes at 1600°c and 60 kbars. If these conditions are respected diamonds organize their

carbon atoms. Because of the great pressure, they will be organized in a cubic octaedric form,

that will have to be very compact. The growth of this diamond depends on its cooling down.

Firsly, the diamond cools down while it is getting away from the magma and crystallizes in

the eclogitic rock. Secondly, with the accumulation on the sides of the mantel, the eclogic’s

leayer collapses and are brought to the stop of the subduction zone. As the eclogitic rock( is

metamorphosed to resist such temperatures in that depth), it will preserve the initial state of

the diamond.

But how come on the surface ?

3) Surface evolution.

At 660 km, is also created the kimberlitic magma. They have an essential role in the

rise of the diamond to the surface. Indeed, the kimberlitic magma is lighter than its gluing

environnement, begins a mouvement to the surface (thanks to a strong convection flow). The

eclogitic rock stocked in this area, are brought with this mouvement to the surface. Here, none

diamonds have ever crystallized in the kimberlit, as it is just a vehicle. Often in mouvement

under the cratons, the kimberlitic mass hurts the weakest areas. It creates a gap and penetrates

the craton. At 3 km below the surface, the kimberlitic mass explodes, provocating a violent

decrease of temperature. This is why diamonds are not transformed in graphite.

Lastly, the magma comes at the surface and made some outflows. After an alteration

process, we can found the diamond near the river which drain the volcanic slope.

We have seen how do diamond born under the ground, but how human have tried to

build them ?

II- Synthetic diamond

1) Principle.

The controlled carbon crystallization supposes that the diamond needs a

simultaneous realisation, during a periode of time that is long enough that fits into 3

parameters, in order to be in the stable zone of the diamond :

- Pressures : It needs 50-80 kbars. This high pressures are obtained with a

machine called “belts”.

- Temperatures : It requires 1500-2000°c. These high temperatures are

obtained with electric current or by a chimical reaction.

- The accelerator of reaction : It is necessary for promoted this

crystallization. We use chemical element promoted like : iron, nickel,

cobalt…

As soon as the diamond crystallizes, it must be cooled down near the ambient

temperature before decrease the pressure (to prevent the graphit from transformation). The

crystal that can be obtained are very many, but small (0.2mm). To obtain big crystals, we

must control precisely all these factors and particularly the decrease of the pressures ; in other

words the half of the XX century.

2) Imitation.

Since 1831, there had been a lot of technical imitations like the verneuil process, the

YAG, “methode par tirage”. With this process, we can obtain the same form, the same

luminosity, and hardness of 8. They are more and more similar to the real constitution of the

diamond. And nowadays they are used in jeweller’s shop and to cut some rejoint of the most

important diamond as the Hope Diamond (50 carats).

III- Sociological and economical impact.

The search of cheap material can be fulfilled by their cut, their colors, and their

purity, which led to the fabrication of the artifial stone. The real diamond are more and more

difficult to identify as the new processes are sophisticated. This is why it is necessary to keep

the synthetic diamond’s trade-name. Even it artificial diamond are on the market, this

synthetic diamond is still part of the experimental field, because of its high price in order to

obtain a small 1 carat specimen. On the contrary, for the saphyr or the Emerald, the synthetic

stone market is very interresting. But consequently like in south america or in Africa.

Will it be the same for diamond soon ?

CONCLUSION.

Today, there are as many hypotheses on the diamond forming than there are on the

methodes for its artificial creation. They all turn around a high pressure, a high temperature

and slow and gradual cool down. Even if the natural diamond’s market is relatively protected,

buyers are very distrustful and septical facing an interesting offer. It is high time for scientists

to have made a real identify card of the diamond in order to legitimize their authenticity.