Lonsdaleite has been used as an indicator of shock from cratering events, but does it exist?, P. Nemeth, L. A. Garvie and P. R. Buseck, American Geophysical Union, Fall Meeting 2013, abstract #P41F-1984
In 1967 a new diamond polymorph was described from the Canyon Diablo iron meteorite and called lonsdaleite (also referred to as hexagonal diamond. It was identified from reflections (e.g., at 0.218, 0.193, and 0.150 nm), additional to those in diamond, that were indexed in terms of a hexagonal cell. Lonsdaleite was attributed to shock-induced transformation of graphite within the iron meteorite upon impact and has subsequently been used as an indicator of shock and meteorite impact. Given the importance of lonsdaleite, we reinvestigated the structure of the shock-formed diamond and lonsdaleite from the Canyon Diablo meteorite with an aberration-corrected ultra-high-resolution scanning transmission electron microscope (STEM), with the view of providing further insights into the shock-forming mechanism. The STEM images allowed direct structural interpretation at 0.1-nm resolution and showed that the samples consist of single-crystal and twinned diamond, as well as graphite intimately associated at the nanoscale. A characteristic feature of the STEM images is stacking faults and twins (111, 200, 113) that interrupt the regularity of the crystal structure. Uncommon, subnanometer-sized regions occur with two- and four-layer hexagonal symmetry, though these regions merge into diamond with stacking faults. Although we did not find lonsdaleite, the defects can give rise to extra reflections like those attributed to lonsdaleite. For example, the (113) diamond twin results in a 0.216-nm spacing that matches that of the broad 0.218-nm lonsdaleite peak. Our observations from Canyon Diablo provide a new understanding of shocked diamond structures and question the existence of lonsdaleite and its inferred geologic implication, although the abundance of diamond twinning and stacking faults may be indicative of shock metamorphism.
See also :
Transformation of graphite to diamond via a topotactic mechanism, Laurence A.J. Garvie, Péter Németh and Peter R. Buseck, American Mineralogist, v. 99 no. 2-3 p. 531-538, February – March 2014
Several mechanisms and intermediate steps have been proposed to explain the transformation of graphite to diamond. However, the mechanism continues to be debated, in part because graphite that is incompletely transformed to diamond has not been reported; although such material could be used to better understand the diamond-forming process. Here we report the discovery of nano-sized grains of interstratified graphite and diamond from Gujba, an extraterrestrially shocked meteorite. We use high-resolution transmission electron microscopy (HRTEM) data from these grains to show that diamond formed via a reconstructive, topotactic rather than martensitic mechanism. Electron diffraction and HRTEM images show the following three-dimensional crystallographic relationships between the interstratified graphite and diamond: (001)g||(111)d, (100)g||(21̄1̄)d, and (12̄0)g||(01̄1)d. These relationships yield the transition matrix linking the graphite and diamond unit cells, which become coincident for graphite compressed to 7 GPa. The specific product, whether single-crystal or twinned diamond, is dictated by the initial graphite polytype and transformation route. The derivation of a three-dimensional transition matrix is consistent with a topotactic relationship between graphite and the newly formed diamond.
Transformation of graphite to lonsdaleite and diamond in the Goalpara ureilite directly observed by TEM, Yoshihiro Nakamuta and Shoichi Toh, American Mineralogist, v. 98 no. 4 p. 574-581, February – March 2014
This study reports on the structural relationship between graphite, lonsdaleite, and diamond extracted from the Goalpara ureilite and propose a model for the formation of lonsdaleite and diamond in these stony meteorites. The study is based on data from reflected-light microscopy and laser Raman spectroscopy of a polished thin section (PTS) of the Goalpara ureilite and X-ray powder diffraction (XRPD) analyses of the grains taken out of it. Selected-area electron diffraction (SAED) analyses and high-resolution TEM (HRTEM) observations were carried out in the three unique directions of pristine graphite with two thin slices prepared from a carbon grain directly taken out of a PTS. SAED patterns reveal the relative crystal-axes orientations between graphite (Gr), lonsdaleite (Lo), and diamond (Di) as (001)Gr//(100)Lo//(111)Di, Gr//Lo//[21̄1̄]Di, and (12̄0)Gr//(1̄20)Lo//(02̄2)Di. The shapes of diffraction spots in the SAED patterns reveal that the transformation of graphite to lonsdaleite and diamond is initiated by sliding of hexagonal carbon planes of graphite along the  of the graphite structure. These results suggest that lonsdaleite and diamond in ureilites formed directly from graphite through boat-type buckling and chair-type puckering of hexagonal carbon planes of graphite, respectively. The results of this study confirm the shock origin of diamond in ureilites.
So who didn’t see this coming? Kennett et al. are completely off the hook.