The search for new nanostructural topologies composed of elemental carbon is driven by technological opportunities as well as the need to understand the structure and evolution of carbon materials formed by planetary shock impact events and in laboratory syntheses. We describe two new families of diamond-graphene (diaphite) phases constructed from layered and bonded sp3 and sp2 nanostructural units and provide a framework for classifying the members of this new class of materials. The nanocomposite structures are identified within both natural impact diamonds and laboratory-shocked samples and possess diffraction features that have previously been assigned to lonsdaleite and postgraphite phases. The diaphite nanocomposites represent a new class of high-performance carbon materials that are predicted to combine the superhard qualities of diamond with high fracture toughness and ductility enabled by the graphitic units and the atomically defined interfaces between the sp3- and sp2-bonded nanodomains.

The concept of the phonon-mode Grüneisen tensor is reviewed as method to determine the elastic strains across crystals from the changes in the wavenumbers of Raman-active phonon modes relative to an unstrained crystal. The symmetry constraints on the phonon-mode Grüneisen tensor are discussed and the consequences for which combinations of strains can be determined by this method are stated. A computer program for Windows, stRAinMAN, has been written to calculate strains from changes in Raman (or other phonon) mode wavenumbers, and vice-versa. It can be downloaded for free from www.rossangel.net.

Complex nanostructures in diamondMeteoritic diamonds formed during bolide impacts on Earth and diamond-related materials synthesized by compressing graphite contain a wide variety of complex nanostructures. This Comment highlights and classifies this structural complexity by a systematic hierarchical approach, and discusses the perspectives on nanostructure and properties engineering of diamond-related materials.

Metamorphic rocks are the records of plate tectonic processes whose reconstruction relies on correct estimates of the pressures and temperatures (P-T) experienced by these rocks through time. Unlike chemical geothermobarometry, elastic geobarometry does not rely on chemical equilibrium between minerals, so it has the potential to provide information on overstepping of reaction boundaries and to identify other examples of non-equilibrium behavior in rocks. Here we introduce a method that exploits the anisotropy in elastic properties of minerals to determine the unique P and T of entrapment from a single inclusion in a mineral host. We apply it to preserved quartz inclusions in garnet from eclogite xenoliths hosted in Yakutian kimberlites (Russia). Our results demonstrate that quartz trapped in garnet can be preserved when the rock reaches the stability field of coesite (the high-pressure and high-temperature polymorph of quartz) at 3 GPa and 850 °C. This supports a metamorphic origin for these xenoliths and sheds light on the mechanisms of craton accretion from a subducted crustal protolith. Furthermore, we show that interpreting P and T conditions reached by a rock from the simple phase identification of key inclusion minerals can be misleading.

Diamond is a material of immense technological importance and an ancient signifier for wealth and societal status. In geology, diamond forms as part of the deep carbon cycle and typically displays a highly ordered cubic crystal structure. Impact diamonds, however, often exhibit structural disorder in the form of complex combinations of cubic and hexagonal stacking motifs. The structural characterization of such diamonds remains a challenge. Here, impact diamonds from the Popigai crater were characterized with a range of techniques. Using the MCDIFFaX approach for analysing X-ray diffraction data, hexagonality indices up to 40% were found. The effects of increasing amounts of hexagonal stacking on the Raman spectra of diamond were investigated computationally and found to be in excellent agreement with trends in the experimental spectra. Electron microscopy revealed nanoscale twinning within the cubic diamond structure. Our analyses lead us to propose a systematic protocol for assigning specific hexagonality attributes to the mineral designated as lonsdaleite among natural and synthetic samples.

Elastic geobarometry for host-inclusion systems can provide new constraints to assess the pressure and temperature conditions attained during metamorphism. Current experimental approaches and theory are developed only for crystals immersed in a hydrostatic stress field, whereas inclusions experience deviatoric stress. We have developed a method to determine the strains in quartz inclusions from Raman spectroscopy using the concept of the phonon-mode Grüneisen tensor. We used ab initio Hartree-Fock/Density Functional Theory to calculate the wavenumbers of the Raman-active modes as a function of different strain conditions. Least-squares fits of the phonon-wavenumber shifts against strains have been used to obtain the components of the mode Grüneisen tensor of quartz (g 1 m and g 3 m) that can be used to calculate the strains in inclusions directly from the measured Raman shifts. The concept is demonstrated with the example of a natural quartz inclusion in eclogitic garnet from Mir kimberlite and has been validated against direct X-ray diffraction measurement of the strains in the same inclusion.

Studies of dense carbon materials formed by bolide impacts or produced by laboratory compression provide key information on the high-pressure behavior of carbon and for identifying and designing unique structures for technological applications. However, a major obstacle to studying and designing these materials is an incomplete understanding of their fundamental structures. Here, we report the remarkable structural diversity of cubic/hexagonally ( c / h ) stacked diamond and their association with diamond-graphite nanocomposites containing sp 3 -/sp 2 -bonding patterns, i.e., diaphites, from hard carbon materials formed by shock impact of graphite in the Canyon Diablo iron meteorite. We show evidence for a range of intergrowth types and nanostructures containing unusually short (0.31 nm) graphene spacings and demonstrate that previously neglected or misinterpreted Raman bands can be associated with diaphite structures. Our study provides a structural understanding of the material known as lonsdaleite, previously described as hexagonal diamond, and extends this understanding to other natural and synthetic ultrahard carbon phases. The unique three-dimensional carbon architectures encountered in shock-formed samples can place constraints on the pressure–temperature conditions experienced during an impact and provide exceptional opportunities to engineer the properties of carbon nanocomposite materials and phase assemblages.

New viscosity experiments at superliquidus temperatures and during cooling at a rate of 10 K/hr have been performed at different shear rates on a synthetic pyroxenite melt. Results revealed that this melt is extremely fluid at temperature between 1646 and 1530 K and measured viscosities are between 2.2 and 7.8 Pa·s. Such very low viscosities allow the lava to flow in turbulent regime as confirmed by the high Reynolds numbers, which are always >2,000. As a consequence, very long distance could be covered by the lava flow. If we consider this studied composition as proxy for Mars lava flows coupled with very high effusion rates, our results might explain the presence of extraordinary large volcanic channels, as recently hypothesized for the Kasei Valles on Mars, even considering that the gravity is approximately one third that of Earth. Few literature data tracking viscosity during cooling are available, and they reported shear thinning effect on different compositions. Our experiments performed at 0.1 and 1 s−1 have shown complex variation in the apparent viscosity, confirming that nonequilibrium rheology represents a still unexplored field of investigation useful to better understand the real geological scenarios occurring in magmatic and volcanic systems.