erice, 2009 compression systematics in minerals dr robert t downs department of geosciences...
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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)
TC (
K)
Other reasons to study minerals at high-pressure
• Synthesis of new phases
• Study of new properties
• Probe of interatomic forces
<d>3/zC (Å3/esu)
Kp-1
(M
bar-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. Al2MgO4
Spinel structure
e.g. Al2MgO4
Spinel structure
e.g. Fe2FeO4
Spinel structure
e.g. Mg2SiO4
quartz
quartz
quartz
cristobalite
cristobalite
Silica polymorphs
Compression Mechanisms
1. Bond compression
2. Angle bending
3. Combination of both
Pyroxene
Pyroxene Chemistry
•M2 M1 Si2O6
• jadeite NaAlSi2O6
• diopside CaMgSi2O6
• enstatite Mg2Si2O6
Diopside
KTP(potassium titanyl phosphate)
KTP
Allan and Nelmes, 1996
KTP
KTP
Quantum calculation of Na-O bond formation
Diopside
Spodumene
Arlt and Angel, 2000
jadeitejadeite
Kanoite, MnMgSi2O6
Arlt & co-workers, 1997-2000
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