  The transformation pathways of carbon at high pressures are of broad interest
for synthesis of novel materials and for revealing the Earth's geological
history. We have applied large plastic shear on graphite in rotational anvils
to form hexagonal and nanocrystalline cubic diamond at extremely low pressures
of 0.4 and 0.7 GPa, which are 50 and 100 times lower than the transformation
pressures under hydrostatic compression and well below the phase equilibrium.
Large shearing accompanied with pressure elevation to 3 GPa also leads to
formation of a new orthorhombic diamond phase. Our results demonstrate new
mechanisms and new means for plastic shear-controlled material synthesis at
drastically reduced pressures, enabling new technologies for material
synthesis. The results indicate that the micro-diamonds found in the low
pressure-temperature crust could have formed during a large shear producing
event, such as tectonic rifting and continued plate collision, without the need
to postulate subduction to the mantle.
