Geos 306, Fall 2007, Lecture 13, The Lower Mantle

Geos 306, Fall 2007, Lecture 13
Mineralogy of the Lower Mantle

There appears to be 3 minerals that are most important in the lower mantle because of their abundance,

Magnesium-silicate perovskite: (Mg,Fe)SiO₃

Magnesio-wüstite: (Mg,Fe)O

Stishovite, SiO₂

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, and the rest adopts the perovskite structure, with a small amount of free silica in the form of stishovite. Stishovite will not form if Mg or Fe is available, instead the perovskite is the stable phase. This implies that MgSiO₃ perovskite is the most abundant mineral in the earth, since the lower mantle is the largest part of the earth.

Perovskite: Correctly stated, iron-magnesium-silicate perovskite. A cubic closest-packed array of O atoms, with 1/4 of the closest packed sites filled with Mg or Fe. The Si is in octahedral coordination. Perovskite offers an excellent example of a displacive cubic to orthorhombic phase transition. The structure type is quite important in industry because it is the proto-type structure for the high-temperature superconductors. 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.

Future directions for superconductor applications include: rocket launchers, Interstellar space ships, etc.

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 phase around P = 20 GPa and then at higher pressures, 105 GPa, to a NiAs type structure.

FeO, rocksalt to rhombohedral transition

Image of FeO in the NiAs structure. Fe is displayed as the larger spheres.

The NiAs structure is the hcp equivalent to the ccp rock salt structure. However, in the hcp structure the Fe-Fe distances are shorter. 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 and the atoms get 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 for this process 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 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 would have made it too.