Rare hexagonal diamond formed by crash of dwarf planet and asteroid, scientists believe A rare type of diamond that's even stronger than the usual form of the crystal was created when a large asteroid smashed into an ancient dwarf planet 4.5 billion years ago, researchers assert. This type of diamond is called Lonsdaleite and has a hexagonal crystal structure, unlike the cubic structure found in most diamonds. …
Yes they can go up to 11 plus, I got some big several pound carbonado diamonds, I and two other guys threw one 30+ times at a backhoe attachment and could barely scratch it. I have some posted on Rumble my channel is Gibsmedats. The pure diamonds and the lonsdaleites are formed by vapor deposition after the meteorite impact and I have the specimen to prove it.
Use a search engine to look up "conchoidal".
(edit - the article also states that diamond has a cubic structure, which might imply that it does have planes along which it can fracture, however, it is actually tetragonal, and does not. This new crystalline phase is apparently hexagonal, but the article doesn't state how this differs from graphite, which is famously pretty soft, so this doesn't really explain whether it has cleavage planes or not.)
However the tetragonal structure of diamond does have a periodicity which is simple cubic. Mistaking the periodicity of a translation unit for the lattice structure of the atoms themselves is the kind of mistake that Wikipedians and Vultures are both to likely to fall into.
By contrast a translation unit of lonsdaleite is hexagonal, though in reference to the cross-section of individual layers rather than actual hexagonal prisms. One might hazard that, because it is not isotropic but layered, one might expect its hardness to vary depending on what angle you hit it. But before finding that out, we need to be able to prepare specimens both big and pure enough to test.
The hardness of diamond is down to the combination of bond length, strength, and bonding angle. The tetragonal bonding angle is particularly stable because it is both the most space-efficient packing for uniform spheres, and because the sp3 hybrid orbital is the most energetically favourable for carbon, and is exactly tetragonal.
A hexagonal lattice pattern would suggest a combination of sp2 and p orbitals. However, that's the bonding pattern in graphite, with the hybrid sp2 orbitals forming the hexagonal lattice, and the left over p-orbitals pointing straight up/down from this plane. These then form a macro delocalised pi-bond across the entire plane, which explains both graphite's "slipperyness" and the good conductivity in that plane (and, incidentally, why pure sheets of graphite are good insulators in the perpendicular axis, although this is seldom seen in real-world situations, because graphite samples tend to be disordered).
This has to be something other than sheets of graphite, but still reportedly hexagonal. Is this akin to graphite but with inter-layer p-p bonding, so essentially sheets of graphite compressed so that they bond together? sp3 bonds are longer than sp2, but both are shorter than p-p bonds, so, as you suggest, this would be expected to be softer than diamond in one direction, and harder in the others.
edit - googling the structure of lonsdaleite suggests that it, like diamond, has fully hybridised sp3 orbitals, but rather than being a repeated tetrahedral unit, is composed of tetrahedra which are alternately upside-down from each other. I'm not sure why this would make it any stronger than diamond, but then I am no crystallographer...
When you look at the hexagons in lonsdaleite they are not planar as in graphite, but skew. The bonding orbitals are SP3 all right, with the fourth electrons bonding alternately to the layer above/below. This preserves the optimal tetrahedral angle, while also allowing bond lengths, and hence energies, to normalise.
It is slightly less dense than conventional diamond, so something interesting is going on with those calculations that predicted it would be harder (and look like they may actually be right).
This type of diamond is called Lonsdaleite and has a hexagonal crystal structure, unlike the cubic structure found in most diamonds. Scientists believe the hexagonal structure could make Lonsdaleite up to 58 per cent harder than other diamonds.
Because hexagons are the bestagons! (Youtube video)
It is high time to steer a large asteroid into earth in an attempt to get these harder than diamonds... Lonsdaleite... Slam the asteroid into the RainForest, "After all it's just a bunch of Trees... and who wouldn't rather have Lonsdaleites?"
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