Mineralogical Magazine, April 2000, Vol. 64(2), pp. 157–184
Pressure-induced transformations
in deep mantle and core minerals
R. J. HEMLEY*, H. K. MAO
AND
S. A. GRAMSCH
Geophysical Laboratory and Center for High-Pressure Research, Carnegie Institution of Washington,
5251 Broad Branch Road N.W., Washington D.C. 20015, USA
ABSTR ACT
Recent experimental and theoretical studies provide new insight into the variety of high-pressure
transformations in minerals that comprise the Earth’s deep mantle and core. Representative examples of
reconstructive, displacive, electronic and magnetic transformations studied by new diamond-anvil cell
techniques are examined. Despite reports for various transitions in (Mg,Fe)SiO3-perovskite, the stability
field of the orthorhombic phase expands relative to magnesiowüstite + SiO2 with increasing pressure
and temperature. The partitioning of Fe and Mg between Mg-rich silicate perovskite and
magnesiowüstite depends strongly on pressure, temperature, bulk Fe/Mg ratio, and ferric iron content.
The soft-mode transition in SiO2 from the rutile- to CaCl2-type structure, originally documented by
X-ray powder diffraction, Raman scattering, and first-principles theory has been explored in detail by
single crystal diffraction, and transitions to higher-pressure forms have been examined. The effect of H
on the transformations of various nominally anhydrous phases and transitions in dense hydrous Mgsilicates are also examined. New studies of the phase diagram of FeO include the transition to
rhombohedral and higher-pressure NiAs polymorphs, and provide prototypical examples of coupled
structural, electronic, and magnetic transitions. High-spin/low-spin transitions in FeO have been
examined by high-resolution X-ray emission spectroscopy to 150 GPa, and the results are compared
with similar studies of Fe2O3 and FeS. Finally, laser-heating studies to above 150 GPa and 2500 K
show that (hcp) e-Fe has a large P-T stability field. Radial XRD measurements carried out at room
temperature to 220 GPa have constrained the elasticity, rheology and sound velocities of e-Fe at core
pressures.
K EY WORDS : high-pressure, diamond cell, perovskite, stishovite, X-ray emission spectroscopy, elastic contents,
iron.
Introduction
RECENT observations of the deep mantle and core
are leading to new views of the structure,
composition and dynamics of our planet. The
lower mantle is characterized by generally smooth
variations in seismic velocities and density, but
signiŽ cant lateral variations in seismic velocities
have been uncovered, with possible weaker
discontinuities reported (Bina, 1998; LeStunff et
al., 1995). Seismic tomographic studies provide
* E-mail: [email protected]
# 2000 The Mineralogical Society
evidence for slab penetration deep within the
mantle in some regions, with slab buildup at the
base of the upper mantle in other locales (e.g. van
der Hilst et al., 1997). Hidden chemical
boundaries deep within the lower mantle (e.g.
below 1000 km depth) have been proposed to
reconci l e geophysi cal and geochem ical
constraints (Kellogg et al., 1999; van der Hilst
and Karason, 1999). SigniŽ cant heterogeneity is
found at the base of the mantle (D’’ and the coremantle boundary), with evidence for distinctly
low velocities (Jeanloz and Williams, 1998;
Williams et al., 1998). Recently, a number of
new Ž ndings about the core have also emerged
R. J. HEMLEY ETAL.
(Stixrude and Brown, 1998), including the
existence of elastic anisotropy and the possibility
of super-rotation of the inner core (Song and
Helmberger, 1993; Song and Richards, 1996; Su
et al., 1996).
Transformations in Earth materials induced by
pressure, as well as the combined effects of
pressure and temperature, are the key to interpreting these observations. Understanding the full
array of dynamic processes therefore requires
detailed studies of the structure, bonding and
electronic properties of component minerals and
their effect on physical and chemical properties
under extreme conditions. Experimental investigations of these phenomena are becoming possible
with newly developed diamond-cell techniques
that now permit detailed studies of transformations
in minerals in this very high pressure range (Mao
and Hemley, 1998). These techniques include in
situ methods such as synchrotron X-ray diffraction
and spectroscopy, synchrotron infrared spectroscopy and optical techniques (e.g. Raman and
Brillouin scattering). These new probes indicate
that the properties of many materials are strongly
altered under deep Earth conditions, giving rise to
transformations that may not be apparent or
ascertained from studies of these same materials
near ambient conditions.
We present an overview of several recent
developments in the study of transformations in
deep Earth materials induced by the combined
effects of pressure and temperature. Several model
materials of the deep mantle and core illustrate the
variety of pressure-induced transformations occurring in deep Earth materials. These include
crystallographic transformations, (both reconstructive and displacive), electronic changes (including
band structure, bonding and insulator-metal transitions), magnetic transitions and Ž nally the ways in
which these are coupled with chemical changes.
The examples, taken primarily from recent studies
from our laboratory, include dense silicates,
hydrous phases, simple oxides and iron. We
focus on subsolidus transitions, although the
melting of iron at core pressures is considered.
The role of iron provides a unifying theme, as it
gives rise to novel transformations that produce
physical and chemical behaviour of critical
importance for understanding the deep Earth. In
this discussion, we emphasize the crystal-chemical
principles that govern pressure-induced transformations, as well as the correspondence between
observed properties and those predicted by various
levels of theory.
Silicate perovskites
158
It is now established that the dominant minerals of
both the upper mantle ­ olivine, pyroxenes, garnet
­ and subducted crust break down with increasing
depth (below 660 km) to form phase assemblages
dominated by silicate perovskites (Bina, 1998;
Hemley and Cohen, 1992; Hirose et al., 1999;
Kesson et al., 1994). These materials have
structures consisting of corner-linked SiO6 octahedra, with the remaining cations in the larger,
approximately dodecahedral, sites. Essentially two
types of silicate perovskite are observed: the major
phase is the Mg-rich mineral with the distorted
orthorhombic structure (space group Pbnm) and
the minor phase is essentially pure CaSiO3, which
adopts the high-symmetry cubic structure (Pm3m)
(Hemley and Cohen, 1992). A large number of
experimental studies have shown that many
properties of the lower mantle determined from
geophysical observations can be explained in terms
of these mineral assemblages.
Recent experimental studies suggest the possibility of structural transitions in perovskite within
the lower mantle, a result that could be detected
by seismology. Thermodynamic data indicate
small free energy differences between the silicate
perovskite and mixed oxides (e.g.
MgSiO3 = SiO2 + MgO). Evidence that MgSiO3
perovskite breaks down to simple oxides (MgO
and SiO2) at higher pressure has been reported.
New peaks have been observed in XRD patterns
measured from laser-heated (Mg,Fe)SiO3 perovskite samples, both in situ (Meade et al., 1995) and
quenched to 300 K at high pressure (Saxena et al.,
1996). Saxena et al. (1998) also report evidence
for a transition to a pseudo-cubic form (at
1485 K), followed by decomposition to MgO +
SiO2 at 1600 K. To understand the observations
for (Mg,Fe)SiO3, we note that incorporation of Fe
destabilizes the perovskite in the sense that
perovskite breaks down to oxides when the
maximum solubility of Fe is reached. We also
note that the stability of perovskite relative to
mixed oxides is highly sensitive to the entropies
and relative densities of the three phases, so that
pressure- or temperature-induced disproportionation may occur at constant Fe content. Because of
the temperature uncertainties and gradients
associated with diamond-cell work, the reported
breakdown reactions (Saxena et al., 1996) could
be interpreted as partial melting or phase
separation, perhaps coupled with P-T gradients.
In fact, Fe partitioning measurements between
TRANSFORMATIONS IN MANTLE AND CORE MINERALS
perovskite and magnesiowüstite to 85 GPa show
increasing solubility of Fe in MgSiO3, indicating
the enhanced stability of the perovskite.
Moreover, Serghiou et al. (1998) report Raman
data indicating that perovskite can be synthesized
from mixed oxides up to 100 GPa. Fiquet et al.
(1998) found that the perovskite is stable to
57 GPa and ~2500 K. Recent work has shown
that the orthorhombic structure is stable to at least
95 GPa at these temperatures (Fiquet et al., in
prep.).
The question of stability in these materials is
thus related to Mg-Fe partitioning and the
maximum solubility of Fe in Mg-rich silicate
perovskite. To address this question, we examined
pressure, temperature and combined P-T effects
on the partitioning of Fe2+ to lower mantle
conditions, starting with olivines, pyroxenes and
hematite (variable Fe3+ in some cases) (Mao et
al., 1997b). Diamond-cell samples were heated
from both sides simultaneously with the split
beams of a multi-mode, near-infrared YAG laser
(Mao et al., 1998a). Such a technique allows
constant and uniform heating of the sample across
a 30 ­ 50 mm area with a temperature uncertainty
of ~50 K (at 2000 K). Because the diamond cell
is a closed system, no ferric iron is produced from
ferrous iron during high P-T processing. The
samples were P-T quenched after being subjected
to pressures of 30 ­ 55 GPa. The Fe content
determined from lattice parameters reveals that
the partitioning depends on pressure, temperature
and bulk Fe/Mg ratio (Fig. 1). These results
indicate that given a constant bulk composition,
the relative amounts of perovskite and magnesiowüstite vary with depth. Equation of state data for
perovskite and magnesiowüstite can be combined
with the results of the depth-dependent partitioning experiments to interpret seismic data. This
must be taken into account in developing
seismological and geodynamic models (Bina,
1998). In particular, comparison of the density
and bulk sound velocity along adiabats as a
function of depth indicate a decreasing perovskite
fraction with depth (Bina and Hemley, in prep.).
The variable solubility is also expected to be
coupled with the partitioning of minor and trace
elements (Tschanuer et al., 1999).
A possible phase transformation in ferromagnesian perovskite from an orthorhombic structure to a
tetragonal or cubic structure has been suggested
based on observations of extensive twinning in
samples quenched from high P and T (Wang et al.,
1990). First-principles total-energy calculations for
159
MgSiO3 perovskite as a function of strain (from
cubic to tetragonal to orthorhombic)have predicted
that the low-symmetry orthorhombic Pbnm structure would be stable throughout the P-T range of
the lower mantle (Stixrude and Cohen, 1993;
Warren and Ackland, 1996; Wentzcovitch et al.,
1995; Warren et al., 1998). Consistent with these
predictions, Funamori and Yagi (1993) and
Funamori et al. (1996) demonstrated by in situ
XRD that MgSiO3 persists in the orthorhombic
structure at P-T conditions of the topmost lower
mantle (36 GPa and ~2000 K). The recent
diamond-cell diffraction measurements by Fiquet
et al. (1998, and in prep.) demonstrate that the
structure persists to at least 95 GPa and 2500 K.
Experiments employing CO 2 lasers to heat
perovskites containing 10­ 15 mol.% Fe suggest
possible transitions at higher pressures (Meade et
al., 1995). The observations can be attributed to the
pre-existing texture in the orthorhombic sample
observed before and after heating. Theoretical
calculations (Stixrude et al., 1996) predict that
CaSiO3 is in fact not cubic under ambient
conditions, as widely believed, but has a weak
orthorhombic distortion associated with rotated
octahedra (like MgSiO3 ). X-ray diffraction
measurements show the persistence of the cubic
form over most of the pressure range of the lower
mantle, although an accurate determination may
require higher resolution than has been provided by
existing measurements.
Electronic transitions associated with the
presence of Fe2+ and Fe3+ in the perovskite
structure have been observed in silicate perovskites. Fei et al. (1994) found evidence in
quenched samples for a thermally activated
electron delocalization requiring site occupancies
for both the Fe2+ and the Fe3+ in the same
s t r u c t u r e . M ö s s b a u e r s p e c t r a o f
(Mg0.95Fe0.95)SiO3 synthesized at low fO2 indicates Fe3+ in the octahedral site, whereas higher
fO2 conditions result in Fe3+ on both octahedral
and dodecahedral sites (McCammon, 1998). A
subsequent high-resolution powder XRD study
and Rietveld structure reŽ nement showed no
evidence for Fe 3+ in the octahedral site
(Jephcoat et al., 1999). Zhang (1997) and Zhang
et al. (1999) measured the Mössbauer effect in
(Mg,Fe)SiO3 perovskite and clinopyroxene at
high pressure by nuclear resonant forward
scattering with synchrotron radiation. The thermally activated electron delocalization found
earlier in quenched samples was depressed at
high pressure; i.e. in the stability Ž eld of the
R. J. HEMLEY ETAL.
FIG. 1. Partitioning of Fe in ferromagnesian silicate perovskite (pv) and magnesowüstite (mw) showing the combined
dependence on pressure, temperature and Mg/Si ratio. (a) Maximum Fe content in (Mg,Fe)SiO 3 perovskite.
(b) Maximum Fe content in (Mg,Fe)O. (c opposite ) Partition coefŽ cient K(pv-mw) as a function of P, T and starting
composition (forsterite/ fayalite composition in synthetic olivines) (Mao et al., 1997b). The results are compared to
earlier measurements by Fei et al. (1996). The partitioning also depends on the total ferric iron content.
perovskite, where the resolution of the spectrum
also improved. Further studies are required to
constrain and better understand the mechanism of
160
electrical conductivity in (Mg,Fe)SiO3 perovskites (see Hemley et al., 1998b; Katsura et al.,
1998; Peyronneau and Poirier, 1998). The
TRANSFORMATIONS IN MANTLE AND CORE MINERALS
FIG. 1(c).
possibility of pressure-inducedhigh-spin/low-spin
transitions in the iron component of (Mg,Fe)SiO3
remains an important question that can now be
probed by new X-ray spectroscopic techniques (as
described below for iron oxides and sulŽ des).
Silica
Silica is considered a secondary mineral in current
models of the deep mantle (Bina, 1998) and it is
believed to be produced in chemical reactions
(e.g. at the core-mantle boundary) or present in
regions indicated by lateral heterogeneity.
Transformations in silica, as well as their
seismological signatures, may be useful in
deciphering chemical variability within the
lower mantle. As the high-pressure polymorph
containing octahedrally-coordinated silicon,
stishovite is a model compound for understanding
the crystal chemistry in deep mantle silicates,
including their transformations. The pressureinduced transition from stishovite to the CaCl2type structure involves an orthorhombic distortion
of the tetragonal rutile structure, Ž rst identiŽ ed by
powder XRD (Tsuchida and Yagi, 1989).
Previous structural studies have employed X-ray
powder diffraction techniques without a pressure
medium (Andrault et al., 1998; Kingma et al.,
161
1996; Tsuchida and Yagi, 1989). The transition is
driven by a shear (c11 ­ c12) instability, which is in
turn coupled with a soft Raman mode, yielding a
pseudo-proper ferroelastic transition. In fact,
detailed measurements of the soft mode established the transition pressure near 50 GPa at room
temperature (Kingma et al., 1995). The result was
in excellent accord with prior Ž rst-principles
theoretical predictions (Cohen, 1992), as well as
subsequent theoretical calculations (Karki et al.,
1997; Teter et al., 1998).
Single-crystal diffraction allows the separation
of Bragg peaks in reciprocal space; this is
particularly important for the correct and
unambiguous identiŽ cation of peak splittings in
reciprocal space (angles between re ections) such
as those associated with the rutile-CaCl2 transition. The Raman study of Kingma et al. (1995)
also showed that hydrostatic media are required
for identifying the pressure regime of the
transition, as non-hydrostatic pressure has a
large effect on the properties of stishovite (see
Hemley et al., 1994). Previously, measurements
on single crystals in hydrostatic media have
genera ll y bee n l i m i t ed t o < 5 ­ 10 GPa.
Synchrotron energy-dispersive XRD methods
have been used to examine the transition (Mao
and Hemley, 1996). A 5610 mm single crystal of
R. J. HEMLEY ETAL.
FIG. 2. Pressure-induced transition in stishovite. (a) Displacement pattern for the transition. (b and c) Lattice
parameters as a function of pressure. (d opposite) Square of the symmetry-breaking strain through the transition. The
single-crystal XRD results are shown by the solid squares (compression) and open squares (decompression).
Triangles: low-pressure single-crystal XRD (Hemley et al., 2000); circles, polycrystalline diffraction with no
medium (Andrault et al., 1998). The solid line shows the results of the Landau model Ž t (Carpenter et al., 2000).
stishovite was mounted in a single-crystal
diamond cell and suspended in a hydrogen
medium, which has been shown to remain very
hydrostatic to megabar pressures. X-ray diffaction
measurements were carried out during compression to 65 GPa and decompression (at room
temperature). The splittings of lines diagnostic
162
of the transition are observed at 58 GPa and
indicate an orthorhombic distortion of the
tetragonal phase. The orthorhombic form persists
on decompression back to 40 GPa, giving a
hysteresis loop of ~20 GPa.
These data allow one to calculate the
spontaneous strain. The spontaneous strains e1-e3
TRANSFORMATIONS IN MANTLE AND CORE MINERALS
FIG. 2(d).
for a tetragonal ? orthorhombic transition are
given by e 1 = (a ­ a0 )/a0, e2 = (b ­ a0 )/a0 and
e3 = (c­ c0)/c0, where a, b and c are lattice
parameters of the orthorhombic phase, while a0
and c0 are the lattice parameters of the tetragonal
phase extrapolated into the stability Ž eld of the
orthorhombic phase (Carpenter et al., 2000). The
symmetry-breaking strain is
…e1 ­ e2 † ˆ
a­ b
a0
…1†
which is proportional to the order parameter Q.
The order parameter scales as (e1 ­ e2)2; as
expected, it is linear in pressure (Fig. 2).
Extrapolating the higher pressure data to low
pressure gives a transition pressure of 51 GPa
(Carpenter et al., 2000), in excellent agreement
with Raman data (and near the midpoint of the
hysteresis loop). All of the spectroscopic, XRD
and theoretical results have been combined to
develop an order-parameter model for the
transition based on Landau theory (Carpenter et
al., 2000). The c11 ­ c12 instability at the transition
gives rise to an anomalous decrease in the shear
wave velocity, which provides a seismic signature
that could be diagnostic of free silica in the deep
163
mantle. For example, the transition itself may play
a role in contributing to anomalous seismic
structure in the D’’ region. Thus, the presence of
free silica can be ascertained by the anomalous
softening associated with the transition.
There is current interest in the possibility of
transitions to still denser forms of silica
(Dubrovinsky et al., 1997; Teter et al., 1998,
Sharp et al., 1999). Evidence for possible
polymorphism of silica at higher pressures has
been obtained from crystal chemical considerations, Ž rst-principles theory and recent highpressure experiments. A large family of dense
structures can be constructed starting with a closepacked (or nearly close-packed) array of oxygen
atoms; distinct structures are obtained depending
on the ordering of the Si atoms in the octahedral
sites. This produces chains of SiO6 octahedra with
different degrees of kinking; the structure with no
kinks is that of stishovite and has the CaCl2-type
structure. Within this scheme, the number of kinks
increases the density, the a-PbO2-type structure
being the most dense in this series (Fig. 3). Some
experiments indicate that this structure forms at
high pressure (>80 GPa), although the study of
Andrault et al. (1998) cited above reported that the
CaCl2-type phase is stable to 120 GPa. The
R. J. HEMLEY ETAL.
existence of this transition at pressures below
135 GPa at high temperatures would indicate that
the higher-pressure phase could exist at the base of
the mantle. This is especially signiŽ cant in view of
the evidence for silica-forming reactions between
mantle silicates and the core (Goarant et al., 1992;
Knittle and Jeanloz, 1991).
Tentative evidence has been reported for
a-PbO 2 -type silica in the SNC Shergotty
meteorite (Sharp et al., 1998, 1999). Early
shock-wave data from recovered samples
(German et al., 1973) were used as the basis for
the identiŽ cation of the new phase (i.e. similar
lattice parameters obtained from limited diffraction data). There are major differences in the axial
ratios, a result that persists when comparing the
experimental data with the theoretical predictions
at all calculated pressures. In addition, there have
been reports of baddelyite-type SiO2 (with Si in
seven-fold coordination) (Goresy et al., 1998).
This is not supported by theoretical calculations
(Fig. 3), which accurately predict the measured
high-pressure sequence of transitions. There is
evidence for extensive metastability in silica at
high pressure in both the lower-pressure, tetrahedrally coordinated Si phases and higherpressure, octahedrally-coordinated phases, a fact
that complicates the unambiguous identiŽ cation
of equilibrium phases and the determination of
thermodynamic P-T stability Ž elds (Hemley et al.,
1994). These results thus conŽ rm the propensity
of SiO2 to exhibit extensive metastability in both
the low-pressure and the high-pressure phases.
Dense hydrous phases
A central issue in deep Earth mineralogy is the
fate of ‘volatile’-bearing phases, including nomin-
FIG. 3. Higher pressure transitions predicted for SiO2 (Teter et al., 1998), including both the dense polytypes related
to stishovite and a-PbO 2, the proposed baddeylite, I2/a structures (Tse et al., 1992) and the Pa3 structure. The latter
is predicted to be the stable phase above 200 GPa but has not been observed experimentally; see also Cohen (1994).
164
TRANSFORMATIONS IN MANTLE AND CORE MINERALS
ally anhydrous and dense hydrous silicates.
Efforts have been made to establish the effect of
water on the phase relations among high pressure
polymorphs (a, b and g phases) of (Mg,Fe)2SiO4
because of the signiŽ cant implications for the
transition zone in the Earth’s upper mantle
(Gasparik, 1993). These polymorphs exhibit
distinct structure in their infrared spectra due to
their unique symmetries and different numbers of
infrared bands (Rossman, 1996). Infrared re ectance spectra have been used to identify the
phases in multiphase, high pressure-temperature
sample charges (Fig. 4). Systematic study of
phase relations as a function of pressure and
temperature has demonstrated that the stability
Ž eld of the b-phase expands into both the a- and
g-Ž elds when H2O and Fe are present (i.e. San
Carlos olivine, which contains 11% Fe) (Kagi et
al., 1997). The b- and g-phases were identiŽ ed
unambiguously because of their different symmetries and distributions of IR bands (orthorhombic
vs cubic). By contrast, there is only a 1­ 2%
FIG. 4. Synchrotron infrared spectra of (Mg,Fe)2SiO4 polymorphs and dense hydrous silicates (phases E and D)
measured under ambient conditions. (a) lower frequency (silicate) region; (b) higher frequency (OH stretching)
region. Infrared re ectance spectra were collected from sample areas as small as 6 mm (with the synchrotron beam)
to identify the phases and to study structural variations within a capsule due to the presence of hydroxyl (and
possibly H2O).
165
R. J. HEMLEY ETAL.
difference in Fe content between b- and
g-(Mg,Fe)SiO4 on the basis of phase equilibrium
experiments, making phase identiŽ cation by
conventional electron probe techniques difŽ cult.
Dense hydrous Mg silicates include the socalled alphabet phases (A, B, D, etc.). The phase
relations of the highest-pressure hydrous silicates
have been reviewed by Frost (1999). There has
been much confusion about these phases; for
example, phases F and G appear to be identical to
phase D (Ohtani et al., 1998; Yang et al., 1997).
Phase D has been reported to coexist with silicate
perovskite and stishovite (Li and Jeanloz, 1991).
The structural behaviour of hydrogen in such
phases has been addressed in particular with the
combination of single-crystal XRD and vibrational spectroscopy. Phase D
[(Mg1.11Si 1.89H2.22O6)] has also been shown to
be stable to lower mantle conditions (Frost and
Fei, 1998), and has all of the silicon in octahedral
coordination (Lu et al., 1996). The re ectance
spectrum (Fig. 4) shows that only SiO6 structural
units (near 700 cm ­ 1 ) are present, with no
evidence for SiO4 tetrahedra (expected between
1000 and 1500 cm ­ 1), typical of crustal and upper
mantle silicate minerals. This has been subsequently conŽ rmed by single-crystal XRD (Yang
et al., 1997). Moreover, the vibrational spectra
reveal variable hydrogen bonds, with a low
hydroxyl frequency at ~2850 cm ­ 1, correÊ
sponding to an OHO bond length of 2.67 A
(Nakamoto et al., 1955; Novak, 1974), found later
in the X-ray structure reŽ nement (Yang et al.,
1997).
High P-T experiments to >50 GPa and 2100 K
show that dense hydrous Mg silicate phases
decompose sequentially as the pressure increases,
Ž nally releasing H2O at the highest P-T conditions
(Shieh et al., 1998). Thermodynamic calculations,
based on the equation of state of phase D,
indicated that phase D is only marginally denser
than the high-temperature dehydrated assemblage
at 30 GPa (Frost and Fei, 1999). Superhydrous
phase B (sB) is stable to the bottom of the
transition zone and top of the lower mantle,
whereas phase D decomposes along slab
geotherms at ~1250 km depth; this may deŽ ne
the lower depth limit for dense hydrous Mg
silicates (Fig. 5). Combined laser-heating/in situ
XRD experiments show that phase E, sB and D
are indeed stable at high P-T conditions and are
quenchable to ambient conditions (Shieh et al.,
2000). However, there is evidence for new, nonquenchable phases. Indeed, work by Bina and
166
Navrotsky (pers. comm.) suggests that ice VII, an
unquenchable phase, may be stable in the coolest
regions of the slab. The structure and equation of
state both depend on Fe content.
The possibility of phase transformations in
dense hydrous Mg silicates prior to breakdown
(dehydration) is also of interest. Variations in
hydrogen bonding may be a key factor in
controlling polymorphism in hydrous phases at
high pressure (Faust and Williams, 1996). As
indicated above, dense hydrous silicates present
different degrees of hydrogen bonding under
ambient conditions. The conventional, ambientpressure concept of the O ­ HO linkage (involving
a hydrogen bond/covalent bond) is, however, no
longer valid in such materials under high
compression. High-pressure measurements can
be used to evaluate the approach toward such a
situation (i.e. a symmetric hydrogen-bonded state,
which is also associated with a weakened OH
covalent bond). The prototypical case is the
symmetric hydrogen-bonded phase of H 2O
found at 60 GPa (Aoki et al., 1996; Goncharov
et al., 1996). Compression of the O ­ HO linkage
in this system gives symmetric hydrogen bonding
Ê .
with distances of 2.38­ 2.40 A
Phase D exhibits a range of O ­ O distances,
with the shortest linkage identiŽ ed as that
involved in the hydrogen bonding
Ê , as mentioned above. The
r(O ­ HO) = 2.67 A
degree of compression of this linkage can be
estimated from the measurement of the equation
of state (Frost and Fei, 1999). Using the measured
pressure dependence of the lattice parameters to
30 GPa and no changes in the framework
Ê and
structure, the O ­ HO linkage is 2.54 A
Ê at 50 GPa. Weakened
decreases to 2.51 A
covalent OH bonds in phase D (or other DHMS)
under pressure may give rise to large anharmonic
effects prior to melting, including possible
diffusive behaviour or superionic conductivity
(e.g. as predicted for subsolidus H2O; Cavazonni
et al., 1999). Such behaviour would give rise to
entropic stabilization of the solid as well as
seismic attenuation (in the absence of the
production of a free H2O-rich  uid phase).
Further insight into the high-pressure behaviour
of dense hydrous silicates can be gained by
comparison with studies of simple hydroxides.
Pressure-induced disordering of crystals may be
intimately associated with the behaviour of the
hydrogen through sublattice amorphization or
‘melting’ (e.g. Duffy et al., 1995b; Nguyen et
al., 1997; Parise et al., 1998). The pressure
TRANSFORMATIONS IN MANTLE AND CORE MINERALS
FIG. 5. (a) Structure of phase D (Yang et al., 1997). (b) Phase relations showing the breakdown of serpentine at deep
mantle conditions (Shieh et al., 1998).
dependence of the O ­ H stretching modes shows a
tendency toward increased hydrogen bonding, but
decreased hydrogen bonding is also observed
(Faust and Williams, 1996; Hemley et al., 1998a).
These results, together with the evidence for
disordering, point towards the importance of
hydrogen-hydrogen repulsions.
Simple oxides and sulfides
Long considered an important model compound
for high-pressure phases of the deep Earth, FeO
exhibits a rich polymorphism, with B1 (NaCl),
rhombohedral and B8 (NiAs) structures at mantle
pressures and temperatures (Fig. 6). The struc167
tural chemistry of FeO contrasts markedly with
the high-pressure behaviour of MgO, which
remains in the NaCl (B1) structure over this
range (to at least 227 GPa (Duffy et al., 1995a)).
FeO belongs to the group of highly correlated
transition-metal compounds and, according to
theoretical phase diagrams determined for this
class of materials, FeO is expected to be
intermediate in character, between the pure
charge-transfer and the pure Mott-Hubbard
insulators (Bocquet et al., 1992; Zaanen et al.,
1985). Accordingly, the band gap appears to be
intermediate between a d-d and p-d gap (Saitoh et
al., 1999). The phase diagram, determined by in
situ XRD combined with both resistive and
R. J. HEMLEY ETAL.
FIG. 6. Structures of FeO polymorphs. Large spheres
represent oxygen atoms.
double-sided laser heating and shock-wave
measurements, is shown in Fig. 7.
The B1 phase is a paramagnetic insulator that
undergoes a transition to an anti-ferromagnetic
state at low temperature. FeO is non-stoichiometric (x~1) and the Néel temperature depends
slightly on the cation deŽ ciency x (McCammon,
168
1992) (TN =184 K for x ~ 0.08 used in studies
reported below). The room-temperature transition
to a rhombohedral phase occurs at 17 GPa
(Fig. 8). (Mao et al., 1996; Shu et al., 1998).
This transition is very sensitive to stress. Singlecrystal experiments carried out in hydrostatic
media reveal unusually large strains as the single
crystal breaks up into four microcrystals along the
four body diagonals (Shu et al., 1998). Radial
diffraction measurements show that the transition
is driven by a soft C44 elastic constant (Singh et
al., 1998b). The transition appears to be largely
independent of the defect concentration. In
contrast, the single-crystal elastic moduli of
MgO increases monotonically with pressure,
with no sign of an instability (Zha et al., 1997;
Sinogeiken and Bass, 1999).
The high P-T phase of FeO was Ž rst observed
in shock-wave experiments (Jeanloz and Ahrens,
1980) and was shown to have the NiAs-type
structure (Fei and Mao, 1994) with metallic
conductivity (Knittle and Jeanloz, 1986). The
structure is hexagonal and is related to the B1 and
rhombohedral phases by a martensitic-type sliding
of the oxygen planes (Fig. 6). Recent theoretical
calculations indicate that the inverse NiAs type
structure is energetically close to the normal NiAs
type (Mazin et al., 1998; Fang et al., 1999),
giving rise to the possibility of extensive
polytypism in this system (Mazin et al., 1998).
The reported relative intensities of the diffraction
peaks suggest the possibility of the formation of
the inverse form (Mazin et al., 1998; Fang et al.,
1999), but a quantitative assessment is not
possible because of the existence of preferred
orientation in the samples (Fei and Mao, 1994).
The origin of the polymorphism in FeO (and
related transition metal monoxides) can be found
in the changes in electronic structure and bonding
induced by the combined effects of pressure and
temperature. Theoretical study of the origin of the
B1-rhombohedral transition using density functional calculations carried out within the local
density approximation (LDA) indicates an
increase in charge in the regions between the
iron when the crystal distorts, that is, metal-metal
bonding develops (Hemley and Cohen, 1996).
Gramsch (in prep.) has shown that the sign of the
rhombohedral strain (elongation vs compression
along the cubic [111] direction) can be explained
by the effect that strain has on the orbital
ordering. Elongation along [111] results in a
‘crystal-Ž eld’ stabilization of a single spin-down
t2g level whereas compression results in the
TRANSFORMATIONS IN MANTLE AND CORE MINERALS
FIG. 7. FeO phase diagram obtained by in situ high P-T X-ray diffraction. The open triangles and diamonds (at lower
temperatures) are from resistive heating experiments (Fei and Mao, 1994); the closed symbols (higher temperature)
correspond to laser-heating measurements (Shen et al., in prep.). The open circle is the shock-wave measurement
from Jeanloz and Ahrens (1980).
FIG. 8. Interplanar spacings from single-crystal XRD of FeO in a He medium at room temperature. The results show
the displacive transition at 17 GPa (Shu et al., 1998).
169
R. J. HEMLEY ETAL.
stablization of a doubly degenerate pair, exactly
the opposite electronic effect. FeO (d 6) has one
electron in the spin-down manifold, which means
that crystal Ž eld stabilization provided by [111]
elongation is favoured over compression, as
observed (Fig. 9). This contrasts with the
behaviour of CoO (d 7), however, which has two
spin-down electrons in the t2g favouring a
compressive distortion as observed (see below).
With a further increase in pressure, a large
decrease in magnetic moment is predicted
(Cohen et al., 1997, 1998). This is likely to be a
universal phenomenon in transition-metal bearing
minerals at deep mantle (for example core-mantle
boundary) pressures.
These theoretical predictions of electronic
structure changes can be examined by new
X-ray spectroscopic techniques (Fig. 10). The
spin state of iron in FeO can be determined by
the analysis of the Fe-Kb emission line. The high
sensitivity of this technique to local atomic
moments has been established in transition
metal systems (Peng et al., 1994; Taguchi et al.,
1997; Tsutsumi et al., 1976). The emission
spectrum of high-spin iron is characterized by a
main peak with an energy of 7058 eV and a
satellite peak located at lower energy due to the
3p core -3d hole exchange interaction in the
emission Ž nal state. In the 2+ oxidation state of
iron (d6 conŽ guration), the low-spin state is
characterized by a total magnetic moment equal
to zero. This collapse of the 3d magnetic moment
has a distinct signature, since the exchange
interaction (and therefore the lower energy
satellite) vanishes and the resulting spectrum
consists of a single narrow line. Because the
diamond and Re gasket both absorb in the X-ray
region below ~10 keV, we used X-ray transparent
Be gaskets and performed measurements through
the gasket. The energies of interest (Fe-Kb edge)
require that both incident and emitted beams pass
through the gasket. Megabar pressures can be
attained with these new Be-gasketing techniques
(Hemley et al., 1997).
Figure 11a shows the Fe-Kb X-ray emission
spectra of FeO to 143 GPa (Badro et al., 1999).
X-ray diffraction was combined with the emission
measurements at each pressure. Diffraction
measured at the highest pressure indicates that
the material is rhombohedral. In contrast, Fe2O3
undergoes a high-spin to low-spin transition
(Fig. 11b). The emission spectrum in the region
of the valence band reveals information on the
width of the band. This part of the spectrum
provides a direct measure of the symmetryprojected anion p bandwidth. The results also
show that the symmetry-projected local electronic
density of states of the valence band does not
broaden, consistent with a preserved high-spin
state. The results for FeO contrast with the
interpretation of recent high-pressure Mössbauer
measurements, which revealed a quadrupole-split
component between 60 and 90 GPa that was
assigned to a diamagnetic low-spin state of Fe2+
(Pasternak et al., 1997). This effect increases with
pressure to 120 GPa, leading the authors to
conclude that that the iron in FeO would be
entirely in a low-spin state at 140 GPa. We can
FIG. 9. Crystal Ž eld model predicting the direction of the spontaneous strain in FeO arising from the distortion of the
FeO6 octahedron along two possible [111 ] distortion pathways.
170
TRANSFORMATIONS IN MANTLE AND CORE MINERALS
FIG. 10. (a) The radial XRD technique showing the relative absorbance of diamond and Be. (b) Schematic energy
diagram for the Kb emission process for 3d transition metals (J. Badro, pers. comm.).
171
R. J. HEMLEY ETAL.
FIG. 11. Fe X-ray emission spectra of (a) FeO and (b) Fe2O3 at selected pressures (Badro et al., 1999). The satellite at
7045.5 eV is characteristic of a high-spin state. The ambient pressure spectrum was collected from a large sample
outside the diamond cell. FeO remains in the high-spin state to 143 GPa. In contrast, Fe2O3 undergoes a spin-state
transition coupled with a structural transition. In the latter material, the iron is in the +3 state (3d5 conŽ guration) and
the total 3d magnetic moment is not equal to zero in the low-spin state. The inset shows a detail of the high-energy
region. The peak is characteristic of emission from the valence band and the Ž nal state has a valence band core-hole.
The shape of the peak is indicative of the valence-band electronic density of states projected by point-group
symmetry Oh. The experiments were conducted on the undulator insertion device beamline at Sector 13 of the APS.
The measurements were made using a 870 mm diameter, horizontally-oriented, Rowland circle-type spectrometer
for the sample-analyser-detector system.
172
TRANSFORMATIONS IN MANTLE AND CORE MINERALS
FIG. 12. High-spin/low-spin transition in FeS by high-resolution emission spectroscopy (Rueff et al., 1999). Emission
spectra of FeS at 0 GPa (high spin) and 11 GPa (low spin) as compared to FeS2 which is in the low-spin state at
ambient pressure (0.1 MPa; all measurements at room temperature).
reconcile their observations with the X-ray
emission results by proposing that there is a
maximum TN above 300 K and 40­ 60 GPa with
re-entrant behaviour back to the paramagnetic
phase above 80 GPa (Badro et al., 1999).
Mössbauer spectra for Fe2O3 (Pasternak et al.,
1999), are in accord with the X-ray emission
measurements, although the proposed high-pressure crystal structures need further study. We also
point out that several high P-T transformations in
Fe2O3 have been observed with the laser-heating/in
situ XRD technique (Ma et al., 1999); structural
reŽ nements are in progress.
These results may be compared with the highpressure behaviour of other transition metal oxides.
MnO has been shown to undergo a Ž rst-order
transition at ~90 GPa on shock compression
(Syono et al., 1998). Recent static compression
studies reveal a distortion of the B1 phase at
~40 GPa, followed by transitions at 90 and
120 GPa (Yagi et al., 1998). The 40 GPa transition
is thus similar to the rhombohedral distortion
observed in FeO, although the extent and sign of
the distortion remains to be determined. The
diffraction pattern of the higher-pressure phase,
which also appears metallic, can be Ž t with the
NiAs (B8) structure, whereas the intermediate
173
phase is unclear, although a mixed phase (or
polytype) is likely (e.g. analogous to the polytype
structure proposed for FeO; Mazin et al. (1998)).
No transition has been detected in NiO to 100 GPa
(Shieh et al., 2000).
Of particular importance to the mantle is the
magnesiowüstite solid solution. Study of
(Mg,Fe)O shows that the 17 GPa transition
sh i f t s t o 2 5 G P a i n t he c o m p o si t i o n
(Mg0.1Fe0.9)O. Although magnesiowüstite forms
a complete solid solution at low and moderately
high pressure, it is not known if this behaviour
persists throughout the P-T range of the lower
mantle; the observation of pressure-induced
changes in the local electronic conŽ guration of
Fe suggests that Mg-Fe solid solution behaviour is
greatly altered at deep mantle conditions. There
would be a signiŽ cant effect on the partitioning of
minor elements that would preclude extrapolations from ambient pressure measurements.
The results for the iron oxides may be
compared to the behaviour of FeS. The phase
relations in FeS have been the subject of
numerous studies over the years. A transition
was Ž rst observed by XRD at ~7 GPa.
Subsequently, King et al. (1978) found by highpressure Mössbauer measurements that the transi-
R. J. HEMLEY ETAL.
tion is associated with a loss of quadrupole
splitting in the high-pressure phase. Recent
powder XRD structure reŽ nements provide
information on the structural changes associated
with the transition (Fei et al., 1998; Kusaba et al.,
1997; Nelmes et al., 1998). The high-pressure
structure [FeS(III)] is monoclinic, with space
group P21/a. X-ray spectra of FeS through the
transition are shown in Fig. 12 (Rueff et al.,
1999). A strong emission line and a lower energy
satellite peak are observed. In comparison, pyrite
is in the low-spin state at zero pressure. A plot of
intensities extracted from Ž tting the data reveals a
major change at 7 GPa, consistent with the X-ray
data. The structure reŽ nement reveals that the
shorter Fe ­ Fe distances in FeS(III) are associated
with the change in spin state (Fig. 13). It has been
suggested that FeS(III) is metallic; however, midinfrared/visible spectra indicate no signiŽ cant
increases in re ectivity associated with these
transitions on compression at room temperature
and no sign of metallic behaviour to at least
50 GPa (see Hemley et al., 1998b). The insulating
state of FeS(III), at least at lower pressures, has
been conŽ rmed by recent electrical conductivity
measurements (Takele and Hearne, 1999).
Transitions in iron
The above studies of FeO and FeS also provide
insight into the mineralogy of the core. The most
inaccessible region of the planet, the core, has
been the focus of a considerable attention during
the past two years because of the evidence for
elastic anisotropy and rotation of the inner core
(Song and Helmberger, 1993; Song and Richards,
1996). Knowledge of the elasticity and texture of
iron is crucial for understanding recent seismological observations, such as the high Q, the low
shear velocity and the anisotropy of the compressional velocity. The transformations of pure iron
are central to understanding the nature of the core,
both experimentally and theoretically. The phase
relations of iron have been controversial in recent
years, in part because of the lack of in situ studies
at very high P-T conditions. Over the past two
years, we have undertaken a series of studies of
the phase diagram of Fe using the double-sided
laser heating technique coupled with in situ XRD.
In situ XRD studies to 84 GPa and 3500 K
established that the the subsolidus phase of Fe is
hcp (Shen et al., 1998). More recent measurements of the diffraction data of Fe at 155 GPa and
2500 K are shown in Fig. 14, along with
174
experimental (both static and shock) and theoretical determinations of the melting line. e-Fe
exhibits a wide stability Ž eld that increases with
pressure; recent shock-wave data are consistent
with this proposal (Nguyen and Holmes, 1998).
Notably, there have been reports of the existence
of other phases [e.g. dhcp (Saxena et al., 1995)
and an orthorhombic form (Andrault et al., 1997)]
within the stability Ž eld that we have identiŽ ed
for e-Fe. In these studies, additional diffraction
lines were used to identify the proposed new
structures. We Ž nd that we can reproduce such
diffraction lines when the transformation from the
e to g phases is incomplete (e.g. incomplete
heating of the sample) resulting in either a
metastable polytype structure, a mixed phase
sample (i.e. e- and g-Fe), or a sample containing
a single phase (e.g. e-Fe), at distinct pressures
induced by localized heating (i.e. phase heterogeneity associated with large pressure gradients of
>10 GPa over a few mm). This can occur with
single-sided laser heating, double-sided laser
heating with improper alignment, or internal
wire heating. When samples are thoroughly
heated and transformations driven to completion,
a simple pattern emerges (e.g. >60 GPa only e-Fe
is observed in measurements carried out so far to
155 GPa and 2500 K). The persistence of the hcp
phase to high P-T conditions is supported by Ž rstprinciples calculations (Laio et al., 2000; Price et
al., 1999; Vocaldo et al., 1999). In the absence of
additional data, we take this as the leading
candidate for the structure of the inner core
alloy. The melting temperature at core pressures
is, however, still poorly constrained.
These results provide a basis for using studies
of e-Fe as a starting point for understanding the
properties of the inner core. The density and bulk
modulus of the hexagonal-close-packed (hcp) iron
have been measured previously to core pressures
by static and dynamic methods (Brown and
McQueen, 1986; Mao et al., 1990). The
development of the transparent Be gasket
technique described above (Hemley et al., 1997)
has allowed the origin of the elastic anisotropy of
the core materials to be addressed (Mao et al.,
1998b). Compression of solids under non-hydrostatic conditions produces differential stresses.
Although usually considered a problem in highpressure studies, the differential stress can be
turned to our advantage by measuring the strain as
a function of diffraction angle relative to the
principal stress in non-hydrostatic experiments
(Fig. 15a). Recent theoretical developments have
TRANSFORMATIONS IN MANTLE AND CORE MINERALS
a
b
FIG. 13. (a) Powder XRD pattern and Rietveld reŽ nement of FeS(III) (Fei et al., 1998). (b) Structure of FeS(II) and
FeS(III).
provided methods to extract the second elastic
moduli from such measurements on polycrystalline samples (Singh et al., 1998a,b).
175
Recently, we studied the shear modulus (G),
single-crystal elasticity tensor (Cij), aggregate VP
and VS and orientation dependence of VP and VS
R. J. HEMLEY ETAL.
FIG. 14. (a) Schematic diagram of the double-sided laser heating/ in situ XRD technique used to study the phase
diagram of iron. (b) P-T phase diagram of Fe determined from Shen et al. (1998) and Ma et al. (1999) using this
technique. (c opposite ) Extended P-T phase diagram. Static pressure (diamond cell) studies: thick solid lines, Shen et
al. (1998) and Ma et al. (1999); short dashed line, Boehler (1990) and Boehler et al. (1993); and thick dash-dot line,
Williams et al. (1987). Shock wave estimates (schematic) of the melting line: thick dotted line, Yoo et al. (1993);
thick dash-dot-dot line, Williams et al. (1987); solid diamond, Brown and McQueen (1986). Theory: thin dotted line,
Alfè et al. (1999); thin dash-dot-dot line, Laio et al. (2000).
176
TRANSFORMATIONS IN MANTLE AND CORE MINERALS
FIG. 14(c).
of iron with the radial XRD techniques mentioned
above; ultrasonic measurements were also
preformed at 14 GPa as a further test of the
method (Mao et al., 1998b). The resulting V P and
VS are shown as a function of pressure in Fig. 15b.
Notably, there is excellent agreement with
ultrasonic measurements carried out at 14 GPa.
The agreement with theory tends to improve at
higher pressures. The disagreement at lower
pressures appears to arise in part from the
neglect of magnetic effects (Stenle-Neumann et
al., 1999; Stixrude et al., 1997). Subsequent spinpolarized calculations indicate that the e-Fe
retains some magnetic properties in the lower
pressure range (e.g. <50 GPa); calculations at
this level of theory improve the agreement with
experiment (Price et al., 1999). Figure 15 also
shows the results of shock-wave data obtained at
high temperatures. We can use these results to
estimate the effect of temperature on velocities at
possible core temperatures. If we assume a core
temperature of 6000 K, with ­ dVP/dT = 2.9 6
10 ­ 4 km s ­ 1 K ­ 1 (Brown and McQueen, 1986),
we Ž nd velocities at core conditions that are
somewhat higher than those determined by
seismological studies of the core. These results
thus provide new constraints (i.e. in addition to
the density constraints) on the presence of light
elements in the inner core.
177
The resulting C ij (second-order stiffness)
tensors are linear in pressure over this pressure
range (to 220 GPa) (Mao et al., 1997a). The
results show quite good agreement with recent
Ž rst-principles calculations (Table 1) (Laio et al.,
2000). On the other hand, the values of key
moduli, including C44, differ from other theoretical predictions (Stenle-Neumann et al., 1999).
This is apparent from the directional dependence
of the sound velocities determined from this
analysis of the experimental data as compared to
theory. The inner core VS is softer than the
aggregate VS of iron, suggesting the presence of
low-VS components or premelting-induced softening in the core. Observation of a strong lattice
strain anisotropy in iron samples indicates a large
(24%) VP anisotropy under the isostress assumption and therefore a perfect alignment of crystals,
or a single-crystal of iron, may not necessarily be
needed to explain the seismic observations.
Alternatively, the strain anisotropy may indicate
the effects of texture, which were not included in
the model but are readily observed for Fe under
pressure. There may also be a stress variation due
to preferred slip systems; i.e. effectively an
orientation dependence of the maximum unaxial
component t (a constant t, which is deŽ ned as the
difference in stress components, s3 ­ s1, was
assumed in this study). As pointed out by Mao
R. J. HEMLEY ETAL.
FIG. 15. (a) Schematic diagram of the radial XRD technique (Mao et al., 1998b). (b) Aggregate velocities of Fe as a
function of pressure. The open squares and circles were obtained by radial XRD techniques (two different
approaches). The grey square shows the results of high pressure ultrasonic measurements. The theory is from
Stixrude and Cohen (1995). The shock-wave results are from Brown and McQueen (1986). The crosses are the
seismological results for the inner core and the open diamonds show the calculated velocities at core conditions
obtained from the static compression data (Mao et al., 1998b).
et al. (1998b), the measurements would then
implicitly also provide information on the
pressure dependence of the shear strength and
rheology of the material to core pressures. Work
is in progress to include the effects of the
orientation dependence of t. In addition, preliminary studies in which this technique has been
combined with double-sided laser heating and
multi-element solid-state detectors, indicate the
feasibility of high P-T measurements.
Conclusions
178
Recent technical developments now permit a wide
range of studies of the variety of pressure-induced
transformations in minerals under conditions
TRANSFORMATIONS IN MANTLE AND CORE MINERALS
TABLE 1. Elasticity of e-Fe at 298 K and high pressures (Mao et al., 1998) compared to theoretical predictions
(300 K).
P
GPa
Density
g cm ­ 3
C11
GPa
C12
GPa
C33
GPa
C13
GPa
C44
GPa
Exp
Exp
Theory b
Exp
Theory b
16.5
39
39
211
211
9.00
9.67 a
10.09
12.61 c
12.80
500
747
1533
1697
275
301
846
809
491
802
1544
1799
284
297
865
757
235
215
583
421
Theory d
210
12.48
1554
742
1749
820
414
K
GPa
G
GPa
297
351
455
1071
1093#
1093+
1062#
1069+
108
134
224
396
446 #
449 +
411 #
412 +
VP
km s ­ 1
VS
km s ­ 1
6.95
7.40
7.64
11.26
11.45
3.47
3.73
3.72
5.61
5.90
11.41
5.76
a
Jephcoat et al. (1986)
Stixrude and Cohen (1995)
Mao et al. (1990)
d
Laio et al. (2000)
#
Reuss bounds
+
Voigt bounds
b
c
appropriate to the deep Earth. Most notable are
the in situ measurements, where there have been
major improvements in sensitivity, accuracy and
precision, allowing a detailed comparison of
experiment, Ž rst-principles theory and geophysical observation. Rather than present a comprehensive review of transformations in deep Earth
materials, we have focused on key examples that
shed light upon different kinds of relevant
pressure-inducedtransformations, including structural, electronic and magnetic changes and the
ways in which these changes may be coupled
together. For example, changes in electronic
structure and bonding obviously produce
changes in chemical reactivity and phase
behaviour, such as the multivariable dependence
of Fe/Mg partitioning in silicate perovskite and
the possible changes in solid solution behaviour in
magnesiowüstite. An important new development
made possible by these techniques is the ability to
study complex multiphase assemblages, including
whole rock samples, at deep mantle P-T
conditions. Such studies promise to provide
direct information on the stability and physical
properties of materials at deep mantle conditions
and allow an interpretation of phenomena such as
the ultralow velocity zones, the origin of plume
formation, lateral heterogeneity and interactions
at the core-mantle boundary.
Acknowledgements
We thank the following collaborators who
contributed in signiŽ cant ways to the work
described above: J. Badro, M.A. Carpenter, R.E.
Cohen, T.S. Duffy, Y. Fei, L.W. Finger, P. Gillet,
A.F. Goncharov, C.C. Kao, K.J. Kingma, R. Lu,
Y. Ma, S. Merkel, C.T. Prewitt, M.L. Rivers, J.P.
Rueff, G. Shen, A.K. Singh, M. Somayazulu, L.
Stixrude, D.M. Teter, S. Shieh, V.V. Struzhkin, Y.
Timofeev, H. Yang and C.S. Zha. We also thank
C.T. Prewitt, H. Yang, Y. Fei and S. Shieh for
help with the Ž gures. This work was supported by
the NSF, NASA and DOE. Experiments
performed at Sector 13 (GSECARS) of the APS
were supported in part by the NSF, DOE and the
W.M. Keck Foundation.
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[Manuscript received 31 August 1999:
revised 12 November 1999 ]
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