Last lecture we discussed the mineralogy of the lower mantle.
There appear to be three phases of importance found there, stishovite,
magnesio-wüstite, and perovskite, though we have no direct
evidence for this. Instead we assume the P and T conditions and,
with the help of laboratory experiments, we deduce that these
are the major phases.
The mineralogy of the upper mantle is better constrained.
We can observe many of the phases directly, in rocks.
The major phases are olivine (~60%) and pyroxene (~30%),
with a minor aluminous component (~10%)
(plagioclase, spinel or garnet, depending upon depth).
In general, the deeper in the earth, the greater the density of the minerals. At the relatively shallow depths of the upper mantle, we have distorted closest-packed arrays of O atoms, with Si in the tetrahedral sites and Mg/Fe in the octahedral sites. As pressure increases, the atoms must pack even more tightly, so the collection of atoms undergo phase transitions to structure with intrinsically denser packing. One of the ways to do this is to put Si into octahedral coordination. The transition zone is made up of minerals with a mixture of 4 and 6 coordinated Si. Below the transition zone, in the upper mantle, all Si is 6 coordinated. Above the transition zone, in the upper mantle, all Si is 4 coordinated. In order for Si to be able to fit in a six-coordinated site, the O atoms must shrink with increasing pressure at a faster rate than the Si atoms.
Olivine: (Mg,Fe)2SiO4, with an assumed Fe component equal to 10-12%. This is the observed concentration from many localities, such as San Carlos, Arizona. The structure is based upon a hcp array of O atoms, with Fe/Mg in octahedral sites and Si in tetrahedral.
It is the most common phase in the upper mantle. The structure is capable of taking up some Ca into an octahedral site. The amount of Ca that it can take is dependent upon pressure, so it may be possible to use the Ca content of olivine as a geobarometer. However, the amount of Ca is also very sensitive to temperature, and we do not know temperature very well. Therefore, the Ca content is not really very useful.
Mg2SiO4 is called forsterite, Fo,
Fe2SiO4 is called fayalite, Fa, and
Ca2SiO4 is called larnite, La.
Larnite is fictitious. We can synthesize it in the lab, but it is
not found in nature. Therefore, we summarize the composition of
mantle olivines by writing, for example, Fo.9Fa.1.
Olivines are orthorhombic, and thus are intrinsically anisotropic.
However, olivine displays anisotropic behavior to a greater extent than
other minerals. It is twice as compressible along the b-cell edge as along
the a or c directions.
At pressures around the 410-km discontinuity, Fo-rich olivine transforms to a ccp structure called wadsleyite. Most researchers believe that this transformation is the source of the discontinuity. Iron rich olivines do not undergo this transformation. At higher pressures, the Fa-rich olivine and wadsleyite transform to a spinel structure, (Mg,Fe)2SiO4, called ringwoodite. These phases are often known as the a -b -g phases.
Pyroxene: (Mg,Fe)2Si2O6.
The end members are ferrosilite, FeSiO3, and enstatite, MgSiO3.
(Note that these are the same formula as for perovskite).
Pyroxene can be considered to be a distorted ccp array of O atoms with Si in
tetrahedral coordination, and Fe/Mg in octahedral. The SiO4 groups
form chains that are linked together through octahedral chains.
One of the octahedral sites is extremely distorted and the structure is often
illustrated as if this site were a sphere, as shown above.
There are a wide variety of elements that substitute into the pyroxene structure.
For instance, jadeite, NaAlSi2O6, is another example. There are
also several different symmetries. Ferrosilite and enstatite are orthorhombic at room
conditions, however, at high pressure, (below 220 km) they transform to monoclinic.
Pyroxenes are categorized as orthopyroxenes and clinopyroxenes.
The mixing of Fe with Mg, and of clinopyroxenes with orthopyroxenes
is an extremely sensitive indicator of pressure and temperature conditions.
Thus, we can determine the pressure and temperature conditions of the formation
of a mantle rock by examining the mixing states of pyroxene.
Other important pyroxenes are:
Enstatite Mg2Si2O6
Ferrosilite Fe2Si2O6
Jadeite NaAlSi2O6
Diopside CaMgSi2O6
Hedenbergite, CaFeSi2O6
Spodumene LiAlSi2O6
Garnet, A3B2(SiO4)3.
This is a framework structure constructed of corner-linked octahedra (the B-site) and tetrahedra,
with a larger cation in an 8-coordinated site (the A-site, or dodecahedral site).
Important garnets fall along two well-known trends are given nicknames,
pyralspite (pyrope, almandine, spessartine)
and ugrandite (uvarovite, grossular, andradite).
Pyrope Mg3Al2(SiO4)3
Almandine Fe3Al2(SiO4)3
Spessartine Mn3Al2(SiO4)3
Uvarovite Ca3Cr2(SiO4)3
Grossular Ca3Al2(SiO4)3
Andradite Ca3Fe2(SiO4)3
The symmetry is cubic at low pressures, but at higher pressures, in the transition zone, Si can occupy up to 1/2 of the octahedral sites and so ordering of the octahedral cations induces tetragonal symmetry. For instance, majorite, Mg3MgSi(SiO4)3 = MgSiO3, (note the same chemistry as enstatite and perovskite).
Garnet is also an important high tech material. It forms the crystals that are the pixels of a TV or computer screen. By putting minor amounts of a rare-earth element into the dodecahedral site, the crystals will glow different colors when excited by electrons. For instance, one composition produces red, another produces green and another can produce blue. So, the pixels of a TV screen are made of 3 separate crystals, each producing one of the colors. Exciting the crystals gives the colors seen on the screen.
