Erice, 2009 Compression Systematics in Minerals Dr Robert T Downs Department of Geosciences University of Arizona Tucson, AZ
O, Fe, Mg, Si
The Earth represents a P,T,x diagram of Fe,Mg,Si,O on a grand scale.
Therefore, most of the material in the Earth, and perhaps in the Universe, exists at elevated pressure and temperature conditions.
Other reasons to study minerals at high-pressure Synthesis of new phases P&T
Peanut butter to diamonds with Kevin Righter, Lunar and Planetary Lab, U of Arizona
Other reasons to study minerals at high-pressure Synthesis of new phases Study of new properties Superconductor Hg-1223 Pressure (GPa) T C (K)
Other reasons to study minerals at high-pressure Synthesis of new phases Study of new properties Probe of interatomic forces 3 /z C (Å 3 /esu) K p -1 (Mbar -1 )
Apply Pressure to a Solid The volume of the solid must decrease. Thus the atoms must get closer together. My interest lies in understanding the various ways that the crystal structure can accommodate volume decrease.
Spinel structure e.g. Al 2 MgO 4
Spinel structure e.g. Al 2 MgO 4
Spinel structure e.g. Fe 2 FeO 4
Spinel structure e.g. Mg 2 SiO 4
quartz
cristobalite
Silica polymorphs
Compression Mechanisms 1.Bond compression 2.Angle bending 3.Combination of both
Pyroxene
Pyroxene Chemistry M2 M1 Si 2 O 6 jadeite NaAlSi 2 O 6 diopside CaMgSi 2 O 6 enstatite Mg 2 Si 2 O 6
Diopside
KTP (potassium titanyl phosphate)
KTP
Allan and Nelmes, 1996 KTP
Quantum calculation of Na-O bond formation
Diopside
Spodumene Arlt and Angel, 2000
jadeite
Kanoite, MnMgSi 2 O 6 Arlt & co-workers,
Images courtesy of Thilo Arlt
Bridgman’s Law: For isostructural materials, cell volumes at ambient conditions are inversely correlated with the bulk modulus. i.e. Larger the cell, the smaller the bulk modulus
Diopside
Sympathetic bonds Antipathetic bonds McCarthy et al, (2008)
Garnet
Compression Mechanisms 1.Bond compression 2.Angle bending 3.Combination of both 4.Bonding changes 5.Reconstructive phase changes
Reconstructive Phase Changes e.g. graphite to diamond e.g. albite to jadeite + quartz
MgO at pressure
NiAs structure, hcp analogue to rocksalt