Minerals have a defined P-T stability field. Changing pressure and/or temperature initiates a phase transition towards equilibrium conditions. However, nanometre-sized crystals not always follow that rule. Nano-diamond formed by low-pressure vapour condensation is common in primitive chondrites⁽¹⁾. Nanocrystalline γ-iron (fcc), stable at high temperature, does not transform into α-iron (bcc) at ambient conditions ⁽²⁾. TEM and Raman spectroscopy studies demonstrated that nanometer-sized minerals retain their crystal structure acquired at high P-T conditions. Nanocrystalline inclusions of “Phase egg” (20 – 30 nm) in diamond crystallized with tetragonal structure did not transform into a monoclinic structure stable at ambient conditions. “Phase Egg” together with nanocrystalline stishovite (25 nm) evidence that this particular diamond originated at least from the upper Transition Zone ⁽³⁾.

Nanocrystalline inclusions of monoclinic (Mg,Fe,Cr)TiO₃ perovskite in olivine from Udachnaya kimberlite pipe (Siberia) occur together with nanocrystalline ilmenite. Ilmenite is the stable phase of FeTiO₃ polymorphs at ambient pressure. At higher PT-conditions ilmenite transforms into a lithium niobate structure. At pressure exceeding16 GPa (RT) another phase transformation into a perovskite-like phase with an orthorhombic cell takes place. This phase is not quenchable. The monoclinic perovskite nanoinclusions in olivine suggest that the host peridotite originated at a pressure of 8 – 10 GPa, which is higher than the last silicate equilibration (4.5 – 6.5 GPa) ⁽⁴⁾.

Structure preservation in nanocrystals is explained by: a) There is a stable container encasing the nanophase all the time during uplift without cracking. Positive ΔV associated with the phase transformation stabilizes the original high-PT phase. b) Nucleation problems with the stable phase.

References

¹⁾Daulton, T.L. (2005) D.M. Gruen et al.,(eds), Synthesis, Properties and Application of ultrananocrystalline diamond, 49-62, Springer.

²⁾Gleiter H. (1989) Nanocrystalline Materials. Progress in Materials Science 33, pp303.

³⁾Wirth, R., Vollmer, C., Brenker, F., Matsyuk, S., Kaminsky, F. (2007), EPSL, 259, 384-399.

⁴⁾Wirth, R., Matsyuk, S. (2005), EPSL, 233, 325-336.