Geos 306, Lecture 14
Mineralogy of the Lower Mantle
The lower mantle covers the region of earth from 2891-670 km depth, bounded below by the core and the D"-layer
(spoken as D-double-prime) and above by the transition zone. The pressure and temperature ranges cover 23.83 GPa, 1950 K to 135.75 GPa, 3750 K.
The lower mantle contains more of the volume of the Earth than any other region.
There appears to be 3 minerals that are most important in the lower
mantle because of their abundance,
Aside from a few inclusions found in diamonds, the mineralogy of the lower
mantle is, for the most part, inferred from the results of experimental data. It is certainly
understood that at the pressures of the lower mantle, all Si is 6-coordinated
with O, forming SiO6 octahedral groups. Candidate phases for the lower mantle are
found by fitting the seismic data to crystallographic data.
The volume abundance's are in the order of 20 % magnesio-wüstite,
while the rest is represented by chemical variations of bridgmanite, adopting the perovskite structure, with a small amount of free silica in the form
of stishovite. You should be able to derive this proportion assuming the mantle contains the charge-balanced oxide component of the earth.
Stishovite will not form if Mg or Fe is available, instead bridgmanite is the stable phase.
This implies that MgSiO3 bridgmanite is the most abundant mineral in the earth since it occupies 80% of its largest part.
It has just been given an
official name because it was finally found in nature.
A cubic closest-packed array of O atoms, with 1/4 of the closest packed
sites filled with 12-coordinated Mg or Fe. The Si is in octahedral coordination. Perovskite
offers an excellent example of a displacive cubic
to orthorhombic phase transition as a function of pressure. The structure type is quite important
in industry because it is the proto-type structure for the
Here is an image of the perovskite structure that includes 2 monolayers.
The white spheres are O, the dark spheres are Fe/Mg and the smaller blue
spheres are Si. Notice that Fe/Mg substitutes for an anion in this closest packed structure.
- Future directions for superconductor applications include:
- There may be more perovskite-type mineral species in the lower mantle than we currently envision. For instance,
recent work with Dr Dera at Argonne National Labs demonstrated the transformation of the pyroxene CaFeSi2O6 hedenbergite
to (Ca,Fe)SiO3 perovskite at high pressure.
Magnesio-wüstite: rock salt structure. MgO is stable to the
highest pressure that we have attained in laboratory conditions. In contrast,
FeO undergoes an interesting displacive transition to a rhombohedral (hexagonal) phase
around P = 20 GPa and then at higher pressures, 105 GPa, to a NiAs type
structure. (Note that NaCl is a ccp AB compound, while NaAs is a hcp AB compound)
FeO, rocksalt to rhombohedral transition
Image of FeO in the NiAs structure. Fe is displayed as the larger
The NiAs structure is the hcp equivalent to the ccp rock salt structure.
However, in the hcp structure the Fe-Fe distances are shorter than in the ccp phase. This reconstructive
phase transition is a result of the diffuse electron density of the Fe
atoms overlapping with each other at high pressures. With increasing depth
the volume of the FeO structure decreases, the atoms pack closer together,
and Fe-Fe bonds eventually form, and so the rocksalt structure transforms
to the NiAs structure, where the Fe-Fe distances are about equal to the
Fe-O distances. As pressure is increased, Fe-Fe bonding dominates to the
extent that O is excluded from the structure altogether. The result is a
pure Fe phase. The region of the earth in which this process occurs is probably the D" layer.
The electron density of Mg is not nearly as diffuse as that of the larger
Fe, and that is probably why no transitions in MgO have yet been observed.
Stishovite: Distorted hcp array of oxygens with Si in the octahedral voids.
From Pauling's rules, s(SiO) = 4/6 = 2/3. Therefore CN(O) = 3.
Synthesized by Stishov in the late 1950's. First found at Meteor Crater,
Arizona in 1960's.
Undergoes a displacive phase transition around 50 GPa that is driven by
more efficient packing of the O atoms.
The development of mineralogy was greatly impacted by
(1) synthesizing stishovite in the lab and
(2) then finding it in nature, in a high-pressure environment.
This changed the mindset of scientists into realizing that if something could be made,
and if the conditions are duplicated in nature, then nature will make it too.