Unusual Stability of Nano-Crystals in Geomaterials

Richard Wirth Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences

What are nanocrystalline materials? Nanocrystalline materials consist of a large volume fraction of defects (grain boundaries) and crystals < 10 nm.

Gleiter, Progress in Materials Science, 33 (4), 223-316, (1989)

What is interesting with nanocrystalline materials Properties of nanocrystalline materials are different from those of the crystalline state.

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Enhanced diffusivity due to high grain boundary percentage.

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Elastic constants in metallic nanocrystalline materials are reduced by 30%.

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The hardness of nanocrystalline TiO 2 rutile.

is about two or more times the hardness of crystalline

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Low-temperature ductility of nanocrystalline ceramics (TiO 2 ).

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Inhibition of phase transformations.

Gleiter, Progress in Materials Science, 33 (4), 223-316, (1989)

Why should geologists consider nanocrystalline materials? Natural occurrence in rocks.

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Nanometre-sized diamonds are common in primitive chondrites (Kennet et al., 2009). Nanometre-sized diamonds in melt inclusions in Hawaiian lavas (Wirth & Rocholl, 2003).

Nanometre-sized inclusions in minerals.

Nanometre-sized apatite and TiO 2 phases (Brookite) in sediments.

Nanometre-sized biomineralization products (Banfield et al., 2000).

Nanocrystalline TiO 2 phases in sedimentary rocks Nanodiamonds in the Younger Dryas Boundary Sediment Layer

These diamonds provide strong evidence for Earth's collision with a rare swarm of carbonaceous chondrites or comets. Kennett et al., Science 2 January 2009: Vol. 323 no. 5910 p. 94

Diamond nanoinclusions in mantle xenoliths, Hawaii

Wirth & Rocholl, EPSL, 211, 2003

Nanocrystalline TiO 2 phases in sedimentary rocks Nanocrystalline TiO 2 in sedimentary rocks is often Brookite

Phase transformation in theα- γ Fe system

911

°

C fcc bcc

Phase transformation in theα- γ Fe system

911

°

C fcc “The martensitic phase transformation from γ-Fe to α-Fe observed in coarse grained polycrystals does not occur in the isolated nano-sized crystals as well as in nanocrystalline material obtained by consolidation of the nanometer-sized crystals”. (Gleiter, 1989) bcc

Gleiter, H. (1989) Nanocrystalline materials. Progress in Materials Science, 33, 233-315.

Nanoinclusions of Phase Egg in diamond from Juina, Brazil Phase Egg (AlSiO 3 (OH)) + Stishovite + Porosity (Liquid).

Phase Egg (AlSiO 3 (OH) nanocrystal

Wirth,R., Vollmer,C., Brenker,F., Matsyuk,S., Kaminsky, F (2007), EPSL, 259, pp. 384-39.

Nanoinclusions of Phase Egg in diamond from Juina, Brazil

a*

c*

5 nm

Synthetic Phase Egg has monoclinic crystal structure determined at ambient conditions (Schmidt et al., 1998).

Nanoinclusions of Phase Egg in diamond from Juina predominantly show tetragonal crystal structure. Only occasionally, monoclinic nanocrystals were observed (Wirth et al., 2007).

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The tetragonal structure is the high-pressure structure.

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Phase Egg crystallized with tetragonal structure and did only partially transform into the monoclinic structure stable at RT.

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Nanocrystalline grain size inhibits phase transformation!

Wirth,R., Vollmer,C., Brenker,F., Matsyuk,S., Kaminsky, F (2007), EPSL, 259, pp. 384-39.

Schmidt,M.W., Finger,L.W., Ross, R.J., Dinnebier, R.E. (1998), Amer. Min., 83, pp. 881-888.

Nanoinclusions of Phase Egg in diamond, Juina, Brazil

Stishovite nanocrystals in pores in Phase Egg matrix.

a* Phase Egg + fluid (porosity) + stishovite

c*

Wirth,R., Vollmer,C., Brenker,F., Matsyuk,S., Kaminsky, F (2007), EPSL, 259, pp. 384-39.

Retrograde formation of Phase Egg Sano et al., Journal of Physics and Chemistry of Solids 65 (2004) 1547-1554

Nanocrystalline (Mg,Fe,Cr)TiO 3 perovskite (monoclinic) in olivine from Udachnaya kimberlite pipe, Siberia

Mineral association:

Olivine + (Mg,Fe,Cr)TiO 3 Ilmenite + magnetite +

(Mg,Fe,Cr)TiO 3 olivine

Wirth, R., Matsyuk, S. (2005), EPSL, 233, pp. 325-226

Nanocrystalline (Mg,Fe,Cr)TiO 3 perovskite (monoclinic )

Wirth, R., Matsyuk, S. (2005), EPSL, 233, pp. 325-226

Nanocrystalline (Mg, Fe, Cr)TiO 3 perovskite (monoclinic) Structural data of ilmenite (Mg,Fe) TiO 3 polymorphs ---------------------------------------------------------------------- FeTiO 3 Ilmenite a Li-NbO 3 structure b Perovskite c Perovskite d trigonal trigonal orthorhombic monoclinic - 3.46 GPa 16 GPa 16 – 22 GPa ------------------------------------------------------------------------------- a 0 (nm) 0.5088 0.51233 0.5022 1.09(5) b 0 (nm) 0.5169 0.5169 c 0 (nm) 1.4088 1.37602 0.7239 0.74(2) β 95 a)Wechsler & Prewitt (1984) b)Leinenweber (1995) c)Leinenweber (1991) d)Wirth & Matsyuk (2005) ° ------------------------------------------------------------------------------- Orthorhombic perovskite not quenchable in experiments. Could be recovered only in Li-NbO 3 structure.

Nanocrystalline (Mg,Fe,Cr)TiO 3 perovskite (monoclinic) 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)

Wirth, R., Matsyuk, S. (2005), EPSL, 233, pp. 325-226

Why can nanocrystals be stable outside their stability field ?

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ΔG α of the interface in α-Fe nanocrystals is larger then ΔG γ nanocrystals for a given excess volume ΔV

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in γ-Fe Meng et al., Acta Materialia 50 (2002) 4563-4570.

Why can nanocrystals be stable?

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ΔG α of the interface in α-Fe nanocrystals is larger then ΔG (2002) 4563-4570.

γ in γ-Fe nanocrystals for a given excess volume ΔV (Meng et al., Acta Materialia 50

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The γ-Fe nanocrystals do not contain suitable nucleation sites for the α-Fe phase (Gleiter, Progress in Materials Science, 33 (4), 223-316, (1989).

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Growth of the α-Fe phase is inhibited (Gleiter, Progress in Materials Science, 33 (4), 223-316, (1989).

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Positive ΔV during phase transformation might stabilize the high-pressure phase if a sufficient tough container is enclosing the nanocrystal.

Future work on nanocrystalline materials in geosciences 

Nanocrystalline perovskite inclusions preserved in diamond ?

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Nanocrystalline Ringwoodite preserved in diamond ?

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Why crystallizes nanocrysalline TiO 2 in sediments in Brookite structure?

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Formation of nanocrystalline materials in geosciences via sol-gel processes?