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