Lonsdaleite — The Hexagonal Carbon Polymorph from Meteorite Impact
Lonsdaleite — The Hexagonal Carbon Polymorph from Meteorite Impact
A rare diamond-related mineral known mainly from impact sites and shocked meteorites
Lonsdaleite is a hexagonal polymorph of carbon, structurally related to cubic diamond but crystallising in the hexagonal rather than the cubic system. The mineral was first identified in 1967 in fragments of the Canyon Diablo meteorite, the impactor that formed Meteor Crater in Arizona, and has since been documented at terrestrial impact structures and in a small number of laboratory synthesis experiments. It is named after Dame Kathleen Lonsdale, the British crystallographer who pioneered X-ray diffraction studies of carbon and other materials. Lonsdaleite is genuinely rare in nature; it has never been recovered as macroscopic single crystals, and it has no role as a gemstone.
Crystal structure
Diamond and lonsdaleite are both built from sp3-bonded carbon atoms in tetrahedral coordination, with each atom bonded to four neighbours. The two minerals differ only in the stacking sequence of the carbon layers: diamond has cubic ABCABC stacking while lonsdaleite has hexagonal ABABAB stacking. The result is two minerals with similar density (around 3.5 g/cm3) and similar hardness (close to the maximum possible for a covalent network of carbon), but with distinct symmetry, distinct X-ray diffraction patterns, and slightly different elastic properties.
Theoretical models of pure single-crystal lonsdaleite predict elastic moduli somewhat higher than those of diamond, and a number of widely cited papers have suggested that lonsdaleite could be measurably harder than diamond. However, no naturally occurring or laboratory-synthesised lonsdaleite has yet been produced in pure enough form or in large enough single-crystal volume to test this prediction directly. Subsequent re-examination of natural lonsdaleite samples has shown that much of what was originally identified as lonsdaleite is actually nano-twinned diamond — cubic diamond with very fine stacking faults — rather than a separate hexagonal phase. The line between the two is genuinely blurred at the nanometre scale.
Where it occurs
Lonsdaleite occurs in two principal natural settings: shocked carbonaceous meteorites and terrestrial impact craters. In both cases, the formation mechanism is the rapid compression and heating of pre-existing graphite or other carbon material by a shock wave from a hypervelocity impact, which transforms the carbon to a high-pressure phase that retains some of the hexagonal stacking inherited from the precursor graphite. The Canyon Diablo meteorite, the Popigai impact structure in Siberia, the Ries Crater in Germany, and several other impact sites have yielded lonsdaleite-bearing material.
Crystals are universally microscopic — typically nanometres to a few micrometres across — and intergrown with cubic diamond, graphite, and other carbon phases. The mineral has never been documented as macroscopic single crystals in nature. Laboratory syntheses have produced lonsdaleite by shock-compression of graphite or by thermal decomposition of certain carbon precursors, but again only at small scales.
Why the mineral matters
Lonsdaleite is significant in three contexts despite never having entered the gem trade. The first is impact science: the presence of lonsdaleite is a marker for shock pressures sufficient to convert graphite to a diamond-related phase, which is useful for confirming hypervelocity impact origin at terrestrial structures and for distinguishing impact-derived from volcanic carbon phases. The second is materials science: the predicted superhardness of pure lonsdaleite has motivated synthesis attempts aimed at producing a material harder than diamond, with potential applications in cutting tools and abrasives. Whether such a material is achievable at scale remains an open question. The third is mineralogical: lonsdaleite is one of the textbook examples of polymorphism, and its existence demonstrates that even the most familiar minerals can have alternative crystal forms under sufficiently extreme conditions.