UNUSUAL STABILITY OF NANO-CRYSTALS IN GEOMATERIALS
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.
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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.