Lonsdaleite
A form of carbon reported from meteorites and impact sites, described as hexagonal diamond.
Lonsdaleite is a carbon phase reported as having a hexagonal stacking of carbon layers, unlike the cubic stacking of normal diamond. It was described in 1967 from the Canyon Diablo iron, associated with Meteor Crater in Arizona, and named after Kathleen Lonsdale, the British crystallographer who showed by X-ray methods that the benzene ring is flat. It has since been reported from ureilites and from impact structures such as Popigai in Siberia, and it is produced in shock experiments on graphite.
Lonsdaleite is widely used as an indicator of shock, since it forms when graphite is rapidly compressed. However, its status as a distinct mineral is debated. Studies in the 2010s argued that much material called lonsdaleite is actually cubic diamond containing dense stacking faults and twins, which mimic a hexagonal structure in diffraction patterns. Other work, including studies of ureilites, has argued for genuine hexagonal domains.
It cannot be recognised in hand specimens. Identification relies on X-ray diffraction, electron diffraction, high-resolution transmission electron microscopy and Raman spectroscopy, ideally combined, because each technique alone can be ambiguous.
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