Impact Diamonds

January 26, 2015

Diamonds Are Forever is a 1971 James Bond film staring Sean Connery.[1] The title comes from the popular notion that diamonds are indestructible. Diamonds, like other carbon allotropes, will actually burn. As I outlined in a previous article (Making Diamonds, December 16, 2013), the Gibbs free energy shows that oxidation is favored even at room temperature; viz.,[2]

C^(Diamond) + O₂^(gas) -> CO₂^(gas)
ΔG_f(Diamond) = 0.693 kcal/mole
ΔG_f(Oxygen) = 0 kcal/mole
ΔG_f(Carbon Dioxide) = -94.258 kcal/mole
ΔG(Reaction) = -94.951 kcal/mole

The negative sign indicates a favorable reaction. However, an activation energy must be overcome before the reaction initiates. For this reason, diamond will only ignite at a temperature of 850-1,000 °C in air, or 720-800 °C in pure oxygen.

Theory is one thing, but scientists are skeptical people, and they require proof in the form of an experiment. In 1772, Antoine Lavoisier used a lens to focus the Sun's rays onto a diamond in an oxygen atmosphere to heat it sufficiently to produce carbon dioxide.

Diamond is the stable allotrope of carbon only under high pressure, as its Phase Diagram shows (see figure). Fortunately for jewelers, once diamond is formed it will exist metastably at room temperature. Natural diamonds form from carbonaceous minerals at extreme depths (about a hundred miles) within Earth's mantle where such high pressures exist. This process was completed only after a considerable fraction of the age of the Earth had passed. The diamonds thus formed were transported to the surface by volcanic eruptions in which they are embedded in igneous rock, such as kimberlite.

The carbon phase diagram. The triple point (TP) is around 4600 K and 10.8 MPa. (Wikimedia Commons image, from data in ref. 3, modified using Inkscape.)[3]

Geologic pressure can be obtained in another, spectacular way by meteor impact. Meteor impact happens at about 17 km/s, but impacts large enough to leave a significant crater (a kilometer in diameter) happen only every five thousand years. The energy involved in such impacts is about 40 megatons, with extinction level 50 gigaton events happening a few times every million years.

There goes the neighborhood! An artist's impression of the Cretaceous-Tertiary (K-T) meteor impact event, now known as the Cretaceous-Paleogene (K-Pg) impact event. This meteor impact, 65 million years ago, is thought to have been responsible for dinosaur extinction. (Image: Don Davis/NASA, via Wikimedia Commons.)

Many meteors contain graphitic carbon, and the class of meteors called carbonaceous chondrites even have carbon in their names. Impact of these meteors creates small diamonds from the simultaneous heat and pressure, and these diamonds were identified as a type with a crystal structure similar to the original graphite. This mineral phase, discovered in 1967 in the remains of the Canyon Diablo meteorite, was named lonsdaleite.

This "lonsdaleite" grain from the Canyon Diablo meteorite looks more like coal than diamond. The scale bar is one millimeter. (Photograph: Arizona State University/Laurence Garvie, cropped and annotated.)[5]

An international team of scientists from the Hungarian Academy of Sciences (Budapest, Hungary), Arizona State University (Tempe, Arizona), and the University of Bayreuth (Bayreuth, Germany) decided to look more closely at the crystal structure of lonsdaleite. The motivation for this was the inability to synthesize lonsdaleite as a separate, pure material. In a recent paper in Nature Communications, they provide evidence that lonsdaleite is actually a structurally disordered form of ordinary diamond.[4-5]

Not only has the presence of lonsdaleite been used as an indicator of meteor impact, it's been conjectured that lonsdaleite might have mechanical properties, such as high strength, superior to ordinary diamond. The lure of such high strength has inspired efforts for its synthesis, and the failure of such synthesis has puzzled scientists for decades.[5]

The crystal structure of diamond (left), and the presumed structure of lonsdaleite (right). The carbon atoms are tetrahedrally-bonded in each, but the overall structure shows a subtle difference between alternate layers of carbon atoms. (Arizona State University image by Péter Németh.)[5]

The research team attempted synthesis of lonsdaleite, and it also examined Canyon Diablo specimens of the mineral using advanced electron microscope techniques. They discovered diamond with extensive {113} twins and {111} stacking faults, and they identified these novel defects also in their synthetic samples.[4-5] These defects cause the same X-ray and electron reflections reported for lonsdaleite.[4] Their conclusion is that lonsdaleite is just the regular cubic form of diamond in which such defects have been generated by shock or pressure.[5]

Says Péter Németh of the Hungarian Academy of Sciences, a former visiting scientist at Arizona State University, and coauthor of the study, "So-called lonsdaleite is actually the long-familiar cubic form of diamond, but it's full of defects."[5] Still, "lonsdaleite" might show some interesting mechanical properties, including exceptional hardness.[5]

References:

1. Diamonds Are Forever (1971, Guy Hamilton, Director) on the Internet Movie Database.
2. Free energy data from L. B. Pankratz, "Thermodynamic Properties of Elements and Oxides," U. S. Bureau of Mines Bulletin 672, U. S. Government Printing Office (1982).
3. J.M. Zazula, "On Graphite Transformations at High Temperature and Pressure Induced by Absorption of the LHC Beam," LHC Project Note 78, CERN, January 18, 1997 (PDF File).
4. Péter Németh, Laurence A. J. Garvie, Toshihiro Aoki, Natalia Dubrovinskaia, Leonid Dubrovinsky, and Peter R. Buseck, "Lonsdaleite is faulted and twinned cubic diamond and does not exist as a discrete material," Nature Communications, vol. 5, Article No. 5447 (November 20, 2014), doi:10.1038/ncomms6447.
5. Asteroid impacts on Earth make structurally bizarre diamonds, say ASU scientists, Arizona State University Press Release, November 20, 2014.