Ringwoodite
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Ringwoodite is a high-pressure phase of Mg2SiO4 (magnesium silicate) formed at high temperatures and pressures of the
Earth's mantle Earth's mantle is a layer of silicate mineral, silicate rock between the Earth's crust, crust and the Earth's outer core, outer core. It has a mass of and makes up 67% of the mass of Earth. It has a thickness of making up about 46% of Earth's ...
between depth. It may also contain
iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...
and
hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
. It is polymorphous with the
olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...
phase
forsterite Forsterite (Mg2SiO4; commonly abbreviated as Fo; also known as white olivine) is the magnesium-rich Endmember, end-member of the olivine solid solution series. It is Isomorphism (crystallography), isomorphous with the iron-rich end-member, fayalit ...
(a
magnesium Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...
iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...
silicate A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is also used ...
). Ringwoodite is notable for being able to contain
hydroxide Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It ...
ions within its structure. In this case two hydroxide ions usually take the place of a magnesium ion and two oxide ions. Combined with evidence of its occurrence deep in the Earth's mantle, this suggests that there is from one to three times the
world ocean The ocean is the body of salt water that covers approximately 70.8% of Earth. The ocean is conventionally divided into large bodies of water, which are also referred to as ''oceans'' (the Pacific, Atlantic, Indian, Antarctic/Southern, and ...
's equivalent of water in the mantle transition zone from 410 to 660 km deep. This mineral was first identified in the Tenham meteorite in 1969, and is inferred to be present in large quantities in the Earth's mantle.
Olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...
,
wadsleyite Wadsleyite is an orthorhombic mineral with the formula β-(Mg,Fe)2SiO4. It was first found in nature in the Peace River meteorite from Alberta, Canada. It is formed by a phase transformation from olivine (α-(Mg,Fe)2SiO4) under increasing press ...
, and ringwoodite are polymorphs found in the
upper mantle The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust (geology), crust (at about under the oceans and about under the continents) and ends at the top of the lower mantle (Earth), lower man ...
of the earth. At depths greater than about , other minerals, including some with the perovskite structure, are stable. The properties of these minerals determine many of the properties of the mantle. Ringwoodite was named after the
Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...
n earth scientist Ted Ringwood (1930–1993), who studied polymorphic phase transitions in the common mantle minerals olivine and
pyroxene The pyroxenes (commonly abbreviated Px) are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents ions of calcium (Ca), sodium (Na), iron ( ...
at pressures equivalent to depths as great as about 600 km.


Characteristics

Ringwoodite is polymorphous with forsterite, Mg2SiO4, and has a spinel structure. Spinel group minerals crystallize in the isometric system with an octahedral habit. Olivine is most abundant in the upper mantle, above about ; the olivine polymorphs wadsleyite and ringwoodite are thought to dominate the transition zone of the mantle, a zone present from about 410 to 660 km depth. Ringwoodite is thought to be the most abundant mineral phase in the lower part of Earth's transition zone. The physical and chemical property of this mineral partly determine properties of the mantle at those depths. The pressure range for stability of ringwoodite lies in the approximate range from 18 to 23  GPa. Natural ringwoodite has been found in many shocked chondritic meteorites, in which the ringwoodite occurs as fine-grained polycrystalline aggregates.Chen. M, El Goresy A., and Gillet P. (2004). "Ringwoodite lamellae in olivine: Clues to olivine–ringwoodite phase transition mechanisms in shocked meteorites and subducting slabs". ''PNAS''. Natural ringwoodite generally contains much more magnesium than iron and can form a gapless solid solution series from the pure magnesium
endmember An endmember (also end-member or end member) in mineralogy is a mineral that is at the extreme end of a mineral series in terms of purity of its chemical composition. Minerals often can be described as solid solutions with varying compositions of ...
to the pure iron endmember. The latter, the iron-rich endmember of the γ-olivine solid solution series, γ-Fe2SiO4, was named ahrensite in honor of US mineral physicist Thomas J. Ahrens (1936–2010).


Geological occurrences

In meteorites, ringwoodite occurs in the veinlets of quenched shock-melt cutting the matrix and replacing olivine probably produced during shock metamorphism. In Earth's interior, olivine occurs in the upper mantle at depths less than about 410 km, and ringwoodite is inferred to be present within the transition zone from about 520 to 660 km depth.
Seismic Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes (or generally, quakes) and the generation and propagation of elastic ...
activity discontinuities at about 410 km, 520 km, and at 660 km depth have been attributed to phase changes involving olivine and its polymorphs. The 520-km depth discontinuity is generally believed to be caused by the transition of the olivine polymorph
wadsleyite Wadsleyite is an orthorhombic mineral with the formula β-(Mg,Fe)2SiO4. It was first found in nature in the Peace River meteorite from Alberta, Canada. It is formed by a phase transformation from olivine (α-(Mg,Fe)2SiO4) under increasing press ...
(beta-phase) to ringwoodite (gamma-phase), while the 660-km depth discontinuity by the phase transformation of ringwoodite (gamma-phase) to a silicate perovskite plus
magnesiowüstite Ferropericlase or magnesiowüstite is a magnesium/iron oxide with the chemical formula that is interpreted to be one of the main constituents of the Earth's lower mantle (Earth), lower mantle together with the silicate perovskite (), a magnesium/i ...
. Ringwoodite in the lower half of the transition zone is inferred to play a pivotal role in mantle dynamics, and the plastic properties of ringwoodite are thought to be critical in determining flow of material in this part of the mantle. The ability of ringwoodite to incorporate
hydroxide Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It ...
is important because of its effect on
rheology Rheology (; ) is the study of the flow of matter, primarily in a fluid (liquid or gas) state but also as "soft solids" or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applie ...
. Ringwoodite has been synthesized at conditions appropriate to the transition zone, containing up to 2.6 weight percent water. Because the transition zone between the Earth's upper and lower mantle helps govern the scale of mass and heat transport throughout the Earth, the presence of water within this region, whether global or localized, may have a significant effect on mantle rheology and therefore mantle circulation. In subduction zones, the ringwoodite stability field hosts high levels of seismicity. An "ultradeep"
diamond Diamond is a Allotropes of carbon, solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Diamond is tasteless, odourless, strong, brittle solid, colourless in pure form, a poor conductor of e ...
(one that has risen from a great depth) found in Juína in western Brazil contained an inclusion of ringwoodite — at the time the only known sample of natural terrestrial origin — thus providing evidence of significant amounts of water as
hydroxide Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It ...
in the Earth's mantle. The gemstone, about 5mm long, was brought up by a diatreme eruption. The ringwoodite inclusion is too small to see with the naked eye. A second such diamond was later found. The mantle reservoir could contain about three times more water, in the form of hydroxide contained within the wadsleyite and ringwoodite crystal structure, than the Earth's oceans combined.


Synthetic

For experiments, hydrous ringwoodite has been synthesized by mixing powders of
forsterite Forsterite (Mg2SiO4; commonly abbreviated as Fo; also known as white olivine) is the magnesium-rich Endmember, end-member of the olivine solid solution series. It is Isomorphism (crystallography), isomorphous with the iron-rich end-member, fayalit ...
(),
brucite Brucite is the mineral form of magnesium hydroxide, with the chemical formula Magnesium, Mg(hydroxyl, OH)2. It is a common alteration product of periclase in marble; a low-temperature hydrothermal Vein (geology), vein mineral in metamorphosed li ...
(), and
silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...
() so as to give the desired final elemental composition. Putting this under 20
gigapascal The pascal (symbol: Pa) is the unit of pressure in the International System of Units (SI). It is also used to quantify internal pressure, stress, Young's modulus, and ultimate tensile strength. The unit, named after Blaise Pascal, is an SI ...
s of pressure at for three or four hours turns this into ringwoodite, which can then be cooled and depressurized.


Crystal structure

Ringwoodite has the spinel structure, in the
isometric crystal system In crystallography, the cubic (or isometric) crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals. There are three main varieties o ...
with
space group In mathematics, physics and chemistry, a space group is the symmetry group of a repeating pattern in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of the pattern that ...
''Fd'm'' (or ''F''3''m''). On an atomic scale, magnesium and silicon are in octahedral and tetrahedral coordination with oxygen, respectively. The Si-O and Mg-O bonds have mixed ionic and covalent character. The cubic unit cell parameter is 8.063 Å for pure Mg2SiO4 and 8.234 Å for pure Fe2SiO4.


Chemical composition

Ringwoodite compositions range from pure Mg2SiO4 to Fe2SiO4 in synthesis experiments. Ringwoodite can incorporate up to 2.6 percent by weight H2O.


Physical properties

The physical properties of ringwoodite are affected by pressure and temperature. At the pressure and temperature condition of the Mantle Transition Zone, the calculated density value of ringwoodite is 3.90 g/cm3 for pure Mg2SiO4; 4.13 g/cm3 for (Mg0.91,Fe0.09)2SiO4 of pyrolitic mantle; and 4.85 g/cm3 for Fe2SiO4. It is an isotropic mineral with an index of refraction n = 1.768. The colour of ringwoodite varies between the meteorites, between different ringwoodite bearing aggregates, and even in one single aggregate. The ringwoodite aggregates can show every shade of blue, purple, grey and green, or have no colour at all. A closer look at coloured aggregates shows that the colour is not homogeneous, but seems to originate from something with a size similar to the ringwoodite crystallites. In synthetic samples, pure Mg ringwoodite is colourless, whereas samples containing more than one mole percent Fe2SiO4 are deep blue in colour. The colour is thought to be due to Fe2+–Fe3+ charge transfer.


References


External links

{{Commonscat Magnesium minerals Iron minerals Nesosilicates Polymorphism (materials science) Spinel group Cubic minerals Minerals in space group 227 High pressure science