Geoscience Reference
In-Depth Information
Chapter 22
Squeezing: phase changes and
mantle mineralogy
It is my opinion that the Earth is very
noble and admirable...andifithad
contained an immense globe of
crystal, wherein nothing had ever
changed, I should have esteemed it a
wretched lump of no benefit to the
Universe.
ious phases or minerals -- the mineralogy. These,
in turn, depend on temperature, pressure and
composition. In general, one cannot assume that
the mineralogy is constant as one varies tem-
perature and pressure. Lateral and radial varia-
tions of physical properties in the Earth are pri-
marily due to changes in mineralogy. Changes of
composition with depth in the mantle are subtle
and there can be chemical discontinuities with
little jump in seismic velocity. The mineralogy of
the mantle changes at constant pressure, if the
temperature or composition changes. Tomogra-
phy maps the lateral changes in seismic velocity.
These changes are due to changes in mineralogy
and composition, and changes in crystal orienta-
tion, or fabric.
Galileo
Overview
Before one can infer the composition of the man-
tle from physical properties, one must deal with
the mineralogy of the mantle, and the role of par-
tial melting and solid--solid phase changes. These
issues straddle the disciplines of petrology and
mineral physics. Pressure-induced phase changes
and chemical variations are important in under-
standing the radial structure of the Earth. The
advent of tomography has made it important to
understand lateral changes in physical proper-
ties. Phase changes and compositional changes
are probably more important than temperature
changes in the interpretation of tomographic
images. Tomographic cross-sections are not maps
of temperature.
The densities and seismic velocities of rocks
are relatively weak functions of temperature,
pressure and composition unless these are
accompanied by a drastic change in mineral-
ogy. The physical properties of a rock depend
on the proportions and compositions of the var-
Spherical ions and crystal structure
It is useful to think of a crystal as a packing of
different-size spheres (ions), the small spheres --
usually cations -- occupying interstices in a frame-
work of larger ones -- usually oxygen. In ionic
crystals each ion can be treated as a ball with cer-
tain radius and charge. The arrangement of these
balls, the crystal structure, follows certain sim-
ple rules. The crystal must contain ions in ratios
such that the crystal is electrically neutral. Maxi-
mum stability is associated with regular arrange-
ments that place as many cations around anions
as possible, and vice versa, without putting ions
with similar charge closer together than their
radii allow while bringing cations and anions
as close together as possible. In other words, we
