Post-perovskite
Encyclopedia
Post-perovskite is a high-pressure phase of magnesium
silicate
(MgSiO3). It is composed of the prime oxide constituents of the Earth's rocky mantle (MgO and SiO2), and its pressure and temperature for stability imply that it is likely to occur in portions of the lowermost few hundred km of Earth's mantle.
The post-perovskite phase has implications for the D′′-layer
that influences the convective mixing
in the mantle
responsible for plate tectonics
.
Post-perovskite has the same crystal structure as the synthetic solid compound CaIrO3, and is often referred to as the "CaIrO3-type phase of MgSiO3" in the literature. The crystal system
of post-perovskite is orthorhombic, its space group
is Cmcm, and its structure is a stacked SiO6-octahedral sheet along the b axis. The name "post-perovskite" derives from the name of the stable phase of MgSiO3 throughout most of Earth's mantle, which has the perovskite structure. The prefix "post-" refers to the fact that it occurs after perovskite structured MgSiO3 as pressure increases (and historically, the progression of high pressure mineral physics). At upper mantle pressures, nearest Earth's surface, MgSiO3 persists as the silicate mineral enstatite
, a pyroxene
rock forming mineral found in igneous
and metamorphic rock
s of the crust
.
(LHDAC) technique by a group at the Tokyo Institute of Technology
and, independently, by researchers from the Swiss Federal Institute of Technology
(ETH Zurich) and Japan Agency for Marine-Earth Science and Technology who used a combination of quantum-mechanical simulations and LHDAC experiments. The TIT group's paper appeared in the journal Science
. The ETH/JAM-EST collaborative paper and TIT group's second paper appeared two months later in the journal Nature
. This simultaneous discovery was preceded by S. Ono's experimental discovery of a similar phase, possessing exactly the same structure, in Fe2O3.
at 2500 K
, and exhibits a positive Clapeyron slope
such that the transformation pressure increases with temperature. Because these conditions correspond to a depth of about 2600 km and the D" seismic discontinuity
occurs at similar depths, the perovskite to post-perovskite phase change is considered to be the origin of such seismic discontinuities in this region. Post-perovskite also holds great promise for mapping experimentally determined information regarding the temperatures and pressures of its transformation into direct information regarding temperature variations in the D" layer once the seismic discontinuities attributed to this transformation have been sufficiently mapped out. Such information can be used, for example, to:
For these reasons the finding of the MgSiO3-post-perovskite phase transition is considered by many geophysicists to be the most important discovery in deep Earth science in several decades, and was only made possible by the concerted efforts of mineral physics scientists around the world as they sought to increase the range and quality of LHDAC experiments and as ab initio calculations attained predictive power.
may yield the morphology of a platy crystal habit
parallel to the (010) plane; the seismic anisotropy observed in the D" region might qualitatively (but not quantitatively) be explained by this characteristic. Theory predicted the (110) slip associated with particularly favorable stacking faults and confirmed by later experiments. Some theorists predicted other slip systems, which await experimental confirmation.
In 2005 and 2006 Ono and Oganov published two papers predicting that post-perovskite should have high electrical conductivity, perhaps 2 orders of magnitude higher than perovskite's conductivity. In 2008 Hirose's group published an experimental report confirming this prediction. A highly conductive post-perovskite layer provides an explanation for the observed decadal variations of the length of day.
) describing the increase in the pressure of the phase transition with increasing temperature is known to be relatively high in comparison to other solid-solid phase transitions in the Earth's mantle, however, the experimentally determined value varies from about 5 MPa/K to as high as 13 MPa/K. Ab initio calculations give a tighter range, between 7.5 MPa/K and 9.6 MPa/K, and are probably the most reliable estimates available today. The difference between experimental estimates arises primarily because different materials were used as pressure standards in LHDAC experiments. A well-characterized equation of state for the pressure standard, when combined with high energy synchrotron generated X-ray diffraction patterns of the pressure standard (which is mixed in with the experimental sample material), yields information on the pressure-temperature conditions of the experiment. However, as these extreme pressures and temperatures have not been sufficiently explored in experiments, the equations of state for many popular pressure standards are not yet well characterized and often yield different results. Another source of uncertainty in LHDAC experiments is the measurement of temperature from a sample's thermal radiation, which is required to obtain the pressure from the equation of state of the pressure standard. In laser-heated experiments at such high pressures (over 1 million atmospheres), the samples are necessarily small and numerous approximations (e.g., gray body) are required to obtain estimates of the temperature.
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
(MgSiO3). It is composed of the prime oxide constituents of the Earth's rocky mantle (MgO and SiO2), and its pressure and temperature for stability imply that it is likely to occur in portions of the lowermost few hundred km of Earth's mantle.
The post-perovskite phase has implications for the D′′-layer
Core–mantle boundary
The core–mantle boundary lies between the Earth's silicate mantle and its liquid iron-nickel outer core. This boundary is located at approximately 2900 km of depth beneath the Earth's surface. The boundary is observed via the discontinuity in seismic wave velocities at that depth...
that influences the convective mixing
Mantle convection
Mantle convection is the slow creeping motion of Earth's rocky mantle caused by convection currents carrying heat from the interior of the Earth to the surface. The Earth's surface lithosphere, which rides atop the asthenosphere , is divided into a number of plates that are continuously being...
in the mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
responsible for plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
.
Post-perovskite has the same crystal structure as the synthetic solid compound CaIrO3, and is often referred to as the "CaIrO3-type phase of MgSiO3" in the literature. The crystal system
Crystal system
In crystallography, the terms crystal system, crystal family, and lattice system each refer to one of several classes of space groups, lattices, point groups, or crystals...
of post-perovskite is orthorhombic, its space group
Space group
In mathematics and geometry, a space group is a symmetry group, usually for three dimensions, that divides space into discrete repeatable domains.In three dimensions, there are 219 unique types, or counted as 230 if chiral copies are considered distinct...
is Cmcm, and its structure is a stacked SiO6-octahedral sheet along the b axis. The name "post-perovskite" derives from the name of the stable phase of MgSiO3 throughout most of Earth's mantle, which has the perovskite structure. The prefix "post-" refers to the fact that it occurs after perovskite structured MgSiO3 as pressure increases (and historically, the progression of high pressure mineral physics). At upper mantle pressures, nearest Earth's surface, MgSiO3 persists as the silicate mineral enstatite
Enstatite
Enstatite is the magnesium endmember of the pyroxene silicate mineral series enstatite - ferrosilite . The magnesium rich members of the solid solution series are common rock-forming minerals found in igneous and metamorphic rocks...
, a pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
rock forming mineral found in igneous
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
and metamorphic rock
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
s of the crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
.
History
The CaIrO3-type phase of MgSiO3 phase was discovered in 2004 using the laser-heated diamond anvil cellDiamond anvil cell
A diamond anvil cell is a device used in scientific experiments. It allows compressing a small piece of material to extreme pressures, which can exceed 3,000,000 atmospheres ....
(LHDAC) technique by a group at the Tokyo Institute of Technology
Tokyo Institute of Technology
The Tokyo Institute of Technology is a public research university located in Greater Tokyo Area, Japan. Tokyo Tech is the largest institution for higher education in Japan dedicated to science and technology. Tokyo Tech enrolled 4,850 undergaraduates and 5006 graduate students for 2009-2010...
and, independently, by researchers from the Swiss Federal Institute of Technology
ETH Zurich
The Swiss Federal Institute of Technology Zurich or ETH Zürich is an engineering, science, technology, mathematics and management university in the City of Zurich, Switzerland....
(ETH Zurich) and Japan Agency for Marine-Earth Science and Technology who used a combination of quantum-mechanical simulations and LHDAC experiments. The TIT group's paper appeared in the journal Science
Science (journal)
Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....
. The ETH/JAM-EST collaborative paper and TIT group's second paper appeared two months later in the journal Nature
Nature (journal)
Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...
. This simultaneous discovery was preceded by S. Ono's experimental discovery of a similar phase, possessing exactly the same structure, in Fe2O3.
Importance in Earth's mantle
Post-perovskite phase is stable above 120 GPaPascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
at 2500 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
, and exhibits a positive Clapeyron slope
Clausius-Clapeyron relation
The Clausius–Clapeyron relation, named after Rudolf Clausius and Benoît Paul Émile Clapeyron, who defined it sometime after 1834, is a way of characterizing a discontinuous phase transition between two phases of matter. On a pressure–temperature diagram, the line separating the two phases is known...
such that the transformation pressure increases with temperature. Because these conditions correspond to a depth of about 2600 km and the D" seismic discontinuity
Core–mantle boundary
The core–mantle boundary lies between the Earth's silicate mantle and its liquid iron-nickel outer core. This boundary is located at approximately 2900 km of depth beneath the Earth's surface. The boundary is observed via the discontinuity in seismic wave velocities at that depth...
occurs at similar depths, the perovskite to post-perovskite phase change is considered to be the origin of such seismic discontinuities in this region. Post-perovskite also holds great promise for mapping experimentally determined information regarding the temperatures and pressures of its transformation into direct information regarding temperature variations in the D" layer once the seismic discontinuities attributed to this transformation have been sufficiently mapped out. Such information can be used, for example, to:
- 1) better constrain the amount of heat leaving Earth's core
- 2) determine whether or not subducted slabs of oceanic lithosphereLithosphereThe lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
reach the base of the mantle - 3) help delineate the degree of chemical heterogeneity in the lower mantle
- 4) find out whether or not the lowermost mantle is unstable to convective instabilities that result in upwelling hot thermal plumes of rock which rise up and possibly trace out volcanic hot spot tracks at Earth's surface.
For these reasons the finding of the MgSiO3-post-perovskite phase transition is considered by many geophysicists to be the most important discovery in deep Earth science in several decades, and was only made possible by the concerted efforts of mineral physics scientists around the world as they sought to increase the range and quality of LHDAC experiments and as ab initio calculations attained predictive power.
Physical properties
The sheet structure of post-perovskite makes the compressibility of the b axis higher than that of the a or c axis. This anisotropyAnisotropy
Anisotropy is the property of being directionally dependent, as opposed to isotropy, which implies identical properties in all directions. It can be defined as a difference, when measured along different axes, in a material's physical or mechanical properties An example of anisotropy is the light...
may yield the morphology of a platy crystal habit
Crystal habit
Crystal habit is an overall description of the visible external shape of a mineral. This description can apply to an individual crystal or an assembly of crystals or aggregates....
parallel to the (010) plane; the seismic anisotropy observed in the D" region might qualitatively (but not quantitatively) be explained by this characteristic. Theory predicted the (110) slip associated with particularly favorable stacking faults and confirmed by later experiments. Some theorists predicted other slip systems, which await experimental confirmation.
In 2005 and 2006 Ono and Oganov published two papers predicting that post-perovskite should have high electrical conductivity, perhaps 2 orders of magnitude higher than perovskite's conductivity. In 2008 Hirose's group published an experimental report confirming this prediction. A highly conductive post-perovskite layer provides an explanation for the observed decadal variations of the length of day.
Chemical properties
Another potentially important effect that needs to be better characterized for the post-perovskite phase transition is the influence of other chemical components that are known to be present to some degree in Earth's lowermost mantle. The phase transition pressure (characterized by a two-phase loop in this system), was initially thought to decrease as the FeO content increases, but some recent experiments suggest the opposite. However, it is possible that the effect of Fe2O3 is more relevant as most of iron in post-perovskite is likely to be trivalent (ferric). Such components as Al2O3 or the more oxidized Fe2O3 also affect the phase transition pressure, and might have strong mutual interactions with one another. The influence of variable chemistry present in the Earth's lowermost mantle upon the post-perovskite phase transition raises the issue of both thermal and chemical modulation of its possible appearance (along with any associated discontinuities) in the D" layer.Summary
Experimental and theoretical work on the perovskite/post-perovskite phase transition continues, while many important features of this phase transition remain ill-constrained. For example, the Clapeyron slope (characterized by the Clausius-Clapeyron relationClausius-Clapeyron relation
The Clausius–Clapeyron relation, named after Rudolf Clausius and Benoît Paul Émile Clapeyron, who defined it sometime after 1834, is a way of characterizing a discontinuous phase transition between two phases of matter. On a pressure–temperature diagram, the line separating the two phases is known...
) describing the increase in the pressure of the phase transition with increasing temperature is known to be relatively high in comparison to other solid-solid phase transitions in the Earth's mantle, however, the experimentally determined value varies from about 5 MPa/K to as high as 13 MPa/K. Ab initio calculations give a tighter range, between 7.5 MPa/K and 9.6 MPa/K, and are probably the most reliable estimates available today. The difference between experimental estimates arises primarily because different materials were used as pressure standards in LHDAC experiments. A well-characterized equation of state for the pressure standard, when combined with high energy synchrotron generated X-ray diffraction patterns of the pressure standard (which is mixed in with the experimental sample material), yields information on the pressure-temperature conditions of the experiment. However, as these extreme pressures and temperatures have not been sufficiently explored in experiments, the equations of state for many popular pressure standards are not yet well characterized and often yield different results. Another source of uncertainty in LHDAC experiments is the measurement of temperature from a sample's thermal radiation, which is required to obtain the pressure from the equation of state of the pressure standard. In laser-heated experiments at such high pressures (over 1 million atmospheres), the samples are necessarily small and numerous approximations (e.g., gray body) are required to obtain estimates of the temperature.