Rahab Diamonds

Diamonds never cease to excite our imagination and stimulate scientists to study them. Physicists working with powerful lasers have tried to answer a simple secondary chemistry question that has implications for the composition of distant planets.

Diamond, as we know, is a pure form of carbon, and is the hardest substance known to man. But from a chemical-physical point of view, the more stable form of carbon at room temperature and pressure is actually graphite - the black material of the pencil. Therefore, so far no answer has been given to the simple question: What is the melting point of a diamond?

If we heat a diamond it will indeed turn into liquid carbon, but on the way it will turn into graphite first, and therefore the substance that undergoes melting is graphite. But we all know that diamonds are formed under conditions of high pressure and temperature, under such conditions, it is the diamond, and not the graphite, that is the most stable form. Why then do diamonds not turn into graphite when set in jewelry? The answer, to the joy of the diamond industry, is that the process formulated:

_diamondC_(S) → _graphiteC_(S)

is a process slow Very - some argue that indeed, all diamonds slowly turn into graphite, so that in the (very) long term investing in diamonds is not worthwhile.

What is a liquid diamond anyway?

Journalists who reported on the find wrote about liquid diamonds. Is this even possible? Looks like the journalists missed some high school chemistry classes. Every chemistry student knows that a diamond is actually a description of crystalline carbon. In the diamond crystal, each carbon atom is bound to four neighbors by four single covalent bonds, and each carbon resides in a corner of a tetrahedron. This compact structure of the atomic crystal gives it its difficulty. Graphite, on the other hand, is made up of huge surfaces of carbon arranged in a structure of hexagons, like a beehive. The melting of the diamond will therefore break the covalent bonds and the resulting liquid can no longer be considered a diamond. The authors of the original article do avoid the use of the term "liquid diamond". They describe the liquid as a metallic liquid, and at even higher temperatures and pressures, as a liquid with a "complex structure".

Therefore, to solve the issue of the melting point of diamonds, a research team from the American Lawrence Livermore National Laboratory, led by John Eggert, tried to heat diamonds at extremely high temperatures and pressures. They heated a small diamond, about a tenth of a carat (0.02g), using a powerful laser at a pressure of 40 million atmospheres (a pressure of one atmosphere is the pressure prevailing on Earth at sea level) - under these conditions, the diamond turned into a drop of liquid. So the scientists began to gradually lower the pressure and temperature. When the pressure dropped to 11 million atmospheres and the temperature reached about 50,000 degrees Celsius, the pressure continued to drop but the temperature drop stopped - a hallmark of a phase transition - the liquid began to freeze and solid diamonds began to form.

But here we are in for a surprise. Most solids are denser than their liquids, therefore, a lump of iron will settle in liquid iron, and a lump of solid oil will settle in a bottle of liquid oil. One exception we all know: water. Ice floats on the surface of water because of the rare property of water: the solid is less dense than the liquid. Well, water has a partner - a diamond. The tiny diamonds that began to form floated on the surface of the "liquid diamond" drop, evidence of their lower density.

The scientists hypothesize that in the core of giant planets, such as Uranus (or in its new Hebrew name "Oron") or Neptune ("Rahab"), conditions may exist where diamonds are stable enough. These planets belong to the "gas giant" family (along with Jupiter and Saturn) because most of their mass is gaseous. The atmosphere of Uron and Rahab contains hydrocarbon compounds (containing carbon and hydrogen), and according to the hypothesis, in the heart of these planets, conditions prevail where the hydrocarbons collapse and break down into liquid carbon and hydrogen. The pressure and temperature conditions also match those of the experiment, so it is not impossible that diamonds float there on the surface of a liquid like icebergs on the surface of the ocean.

In order to confirm this hypothesis, it is necessary to send a spacecraft to study the planets (although it is difficult to assume that such a spacecraft could reach the core of the planets and survive there) or to build an imaging facility on Earth - two expensive options that do not seem realistic in the near future.

Sources

• Abstract of the original article in the journal Nature Physics
• Report on the finding in the journal Discovery
• More report on the Centaury Dreams blog