Paper No. 7
Presentation Time: 10:05 AM


WIRTH, R., 3.3 Chemistry and Physics of Earth Materials, GFZ Potsdam, Telegrafenberg, Potsdam, 14473, Germany,

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.


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.

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