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Diamonds: Chemistry & Structural Properties



Diamond Chemistry

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Structural Properties of Diamond - Diamond Formation

Diamonds are formed when carbon deposits are exposed to high pressure and high temperature for prolonged periods of time. Deep within the earth's crust there are regions that have a high enough temperature (900C to 1400C) and pressure (5 to 6 GPa) that it is thermodynamically possible for liquified carbon to form into diamonds.

Under the continental crust, diamonds form at depths of between 60 miles (100 kilometers) and 120 miles (200 km), in the diamond-stable conditions defined by the "graphite-diamond equilibrium boundary" [2].



At these depths, pressure is roughly 5 gigapascals and the temperature is around 2,200 degrees Fahrenheit (1,200 degrees Celsius). Diamond formation under oceanic crust takes place at greater depths due to lower surface temperatures. Therefore, diamond formation within the oceanic crust requires a higher pressure for formation. Long periods of exposure to these higher pressures and temperatures allow diamond crystals to grow larger than under land masses.

When diamonds are not located within a "kimberlite pipe," and excavated via a hard-rock or open pit mine, they are found in alluvial stream-beds known or "secondary deposits."


Diamond Chemistry - Rough Octahedral Diamonds

Diamond-bearing kimberlite is an ultrapotassic, ultramafic, igneous rock composed of garnet, olivine, phlogopite, and pyroxene, with a variety of trace minerals. Kimberlite occurs in the Earth's crust in vertical, upwardly-thrusting structures known as kimberlite pipes, which resemble a champagne flute.


Basic Physical Properties of Diamond

Diamond is the hardest naturally occurring material on earth, with a relative hardness of 10 on the Mohs scale. Diamond is one of several allotropes of carbon, with the principle allotrope being graphite. The word "allotrope" or "allotropy" specifically refers to the structural chemical bond between atoms. A diamond is a transparent, optically isotropic crystal with a high dispersion of 0.044, a refractive index of 2.42, and a specific gravity of 3.52.


Diamond Crystal Structure & Hardness

The unique chemical and molecular structure of crystalline diamond is what gives this gemstone its hardness, and differentiates it from simple graphite. The name "diamond," which is also known as "adamant," is derived from the Greek adamas, or "invincible," "untamable," and "unconquerable," referring to its incredible hardness.



Rough Diamond Shapes


A Type 2-A diamond has a hardness value of 167 GPa (6) when scratched with an ultrahard fullerite tip, and a hardness value of 231 GPa (5) when scratched with a diamond tip. The material "boron nitride," when found in a crystalline form that is structurally similar to diamond, is nearly as hard as diamond. Additionally, a currently hypothetical material, beta carbon nitride, may also be as hard or harder than diamond.

A diamond's incredible hardness was the subject of curiosity dating back to the Roman empire, where it was shown to combust in scientific experiments, although the reason for its combustion was not understood at the time.

Experimentation during the late 18th century demonstrated that diamonds were made of carbon, by igniting a diamond in an oxygen atmosphere, with the end byproduct of the combustion being carbonic-acid gas, or carbon dioxide.


Diamond Crystal Habit

Diamonds have a characteristic crystalline structure, and therefore, a predictable crystal growth pattern known as its "crystal habit." This means that diamond crystals usually "grow" in an orderly and symmetrical arrangement. The natural crystal form, and crystal habit of a diamond is octahedral (photo, above), although in nature, perfectly formed crystals are rare.



Octahedral Crystal Structure of Diamond


The external shape of the crystal, whether it is cubic, octahedral, or dodecahedral, does not always reflect the internal arrangement of its atoms. When a gemstone has an irregular external shape or asymmetrical arrangement of its crystal facets, it is termed as "subhedral," or "anhedral."



Diamond - Dodecahedral Crystal Structure


Trace impurities, crystal twinning, and varying growth conditions of heat, pressure and space can also affect the final shape of a formed crystal.


Diamond Graphitization

In extremely high temperature environments above 1700 C, graphite can develop internally and on the diamond's surface. Internally formed crystallographic graphite inclusions often create intense strain on the surrounding diamond, causing stress fractures or feathers.



Carbon Inclusion in Diamond

Carbon Inclusion - AGS Labs

   Hexagonal Platelet Inclusion

Hexagonal (Graphite) Platelet Inclusion - AGS Labs


Diamond Toughness

Within the fields of metallurgy and materials science, the term "toughness" describes the resistance of a given material to fracture when it is stressed or impacted.

Although diamond is the "hardest," and therefore, most scratch resistant mineral on earth, with a Mohs scale rating of 10, its "toughness" rating is moderate, due to its ability to fracture along cleavage planes. By comparrison, sapphire has a hardness rating of 9, meaning that a diamond is 4 times "harder" than sapphire, yet sapphire has a toughness rating of excellent. Hematite has a hardness of only 5.5 to 6.5, but its toughness rating is also excellent.



A material's toughness is measured in units of "joules" per cubic meter (J/m3) in the SI system, and "pound-force" per square-inch in US units of measurement. Unlike "hardness," which only denotes a diamond's high resistance to scratching, a diamond's "toughness" is only fair to good.

Particular cuts of diamond are more prone to breakage along cleavage planes, and therefore may be uninsurable by reputable insurance companies. The culet facet at the bottom of the pavilion, is a facet specifically designed to resist breakage. Additionally, very thin girdles on brilliant cut diamonds are also prone to breakage.


Thermal Properties of Diamonds

Diamond is a good conductor of heat, acting as a "thermal conductor." If you were to place a large enough diamond on your tongue it would draw heat away, making it seem cold. Many natural blue diamonds contain boron atoms which replace carbon atoms within the crystal matrix, increasing thermal conductance.

Purified synthetic diamond can have the highest thermal conductivity (2000-2500 W/m-K) of any solid material at room temperature - nearly five times greater than pure copper. Due to diamond's high thermal conductance, it is used in the manufacturing of semiconductors, to prevent silicon and other semiconducting materials from overheating.


Electromagnetic Properties of Diamond - Insulators or Semiconductors

Diamond is a relatively good electrical insulator, with the exception of natural blue diamonds, which are in fact semiconductors. Natural blue diamonds containing boron atoms, and synthetic diamonds that are doped with boron, are known as p-type semiconductors. If an n-type semiconductor can be synthesized, electronic circuits could be manufactured from diamonds in the future [8].


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Bibliography on Diamond Chemistry

1. Nature, The Diamond Chip . www.nature.com

2. Dept of Geology, UofG, Mineral inclusions from African & Brazilian Diamond . www.minsocam.org

3. University of Bristol, Allotropes of Carbon . www.chemsoc.org

4. , Inclusions in Diamonds from K14 and K10 Kimberlites. www.sciencedirect.com

5. V R Howes 1962 Proc. Phys. Soc., The Graphitization of Diamond . www.iop.org

6. Nature Article, Growing Diamond Crystals . www.nature.com

7. Steve Sque, University of Exeter, Physical Properties of Diamonds . www.newton.ex.ac.uk

8. Wired magazine, The New Diamond Age . www.wired.com

9. Nano Diamonds, The High Strength-To-Weight Ratio Diamonds . www.nanodiamond.info

10. Chemical & Engineering News, Man Made Diamonds . www.pubs.acs.org

11. Adamas Gemological Laboratory, Spectrophotometers to Measure Gem Color . www.gis.net

12. PBS Nature, Diamonds . www.pbs.org

13. American Museum of Natural History, The Nature of Diamonds . www.amnh.org

14. Smithsonian exhibit, Colored Diamonds . www.mnh.si.edu

15. GIA, Gemological Institute of America . www.gia.edu

16. AGSL, American Gem Society Laboratories . www.ags.org





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