Wadsleyite is an orthorhombic

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

with the formula β-(Mg,Fe)₂SiO₄. It was first found in nature in the Peace River meteorite from

Alberta, Canada Alberta ( ) is one of the thirteen provinces and territories of Canada. It is part of Western Canada and is one of the three prairie provinces. Alberta is bordered by British Columbia to the west, Saskatchewan to the east, the Northwest Territ ...

. It is formed by a phase transformation from

olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...

(α-(Mg,Fe)₂SiO₄) under increasing pressure and eventually transforms into spinel-structured

ringwoodite Ringwoodite is a high-pressure phase of Mg2SiO4 (magnesium silicate) formed at high temperatures and pressures of the Earth's mantle between depth. It may also contain iron and hydrogen. It is polymorphous with the olivine phase forsterite (a ...

(γ-(Mg,Fe)₂SiO₄) as pressure increases further. The structure can take up a limited amount of other bivalent

cations An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by con ...

instead of magnesium, but contrary to the α and γ structures, a β structure with the sum formula Fe₂SiO₄ is not thermodynamically stable. Its cell parameters are approximately a = 5.7 Å, b = 11.71 Å and c = 8.24 Å. Wadsleyite is found to be stable in the upper part of the Transition Zone of the Earth's mantle between in depth. Because of oxygen atoms not bound to silicon in the Si₂O₇ groups of wadsleyite, it leaves some oxygen atoms insufficiently bonded. Thus, these oxygens are hydrated easily, allowing for high concentrations of hydrogen atoms in the mineral. Hydrous wadsleyite is considered a potential site for water storage in the Earth's mantle due to the low electrostatic potential of the under bonded oxygen atoms. Although wadsleyite does not contain H in its chemical formula, it may contain more than 3 percent by weight H₂O, and may coexist with a hydrous melt at transition zone pressure-temperature conditions. The solubility of water and the density of wadsleyite depend on the temperature and pressure in the Earth. Even though their maximum water storage capabilities might be reduced to about 0.5-1 wt% along the normal geotherm, the Transition Zone which holds up to 60 vol% wadsleyite could still be a major water reservoir in the Earth's interior. Furthermore, the transformation resulting in wadsleyite is thought to occur also in the shock event when a meteorite impacts the Earth or another planet at very high velocity. Wadsleyite was first identified by Ringwood and Major in 1966 and was confirmed to be a stable phase by Akimoto and Sato in 1968. The phase was originally known as β-Mg₂SiO₄ or "beta-phase". Wadsleyite was named for mineralogist Arthur David Wadsley (1918–1969).

Composition

In values of weight percent oxide, the pure magnesian variety of wadsleyite would be 42.7% SiO₂ and 57.3% MgO by mass. An analysis of trace elements within wadsleyite shows a large number of elements: rubidium (Rb), strontium (Sr), barium (Ba),

titanium Titanium is a chemical element with the Symbol (chemistry), symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resista ...

(Ti),

zirconium Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'' ...

(Zr), niobium (Nb),

hafnium Hafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in many zirconium minerals. Its existence was predicted by Dmitri M ...

(Hf),

tantalum Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as ''tantalium'', it is named after Tantalus, a villain in Greek mythology. Tantalum is a very hard, ductile, lustrous, blue-gray transition metal that ...

(Ta),

thorium Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately soft and malleable and has a high ...

(Th), and

uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...

(U). This suggests that the concentrations of these elements could be larger than what has been supposed in the transition zone of Earth's upper mantle. Moreover, these results help in understanding chemical differentiation and magmatism inside the Earth. Although nominally anhydrous, wadsleyite can incorporate more than 3 percent by weight H₂O, which means that it is capable of incorporating more water than Earth's oceans and may be a significant reservoir for H (or water) in the Earth's interior.

Geologic occurrence

Wadsleyite was found in the Peace River meteorite, an L6

hypersthene Hypersthene is a common rock-forming inosilicate mineral belonging to the group of orthorhombic pyroxenes. Its chemical formula is . It is found in igneous and some metamorphic rocks as well as in stony and iron meteorites. Many references have f ...

-olivine

chondrite A chondrite is a stony (non- metallic) meteorite that has not been modified, by either melting or differentiation of the parent body. They are formed when various types of dust and small grains in the early Solar System accreted to form p ...

from Peace River,

. The wadsleyite in this meteorite is believed to have formed at high pressure during the shock event related to the impact on Earth from the olivine in sulfide-rich veins of the meteorite. It occurs as microcrystalline rock fragments, often not surpassing in diameter.

Structure

Wadsleyite is a spinelloid, and the structure is based on a distorted cubic-closest packing of oxygen atoms as are the spinels. The a-axis and the b-axis is the half diagonal of the spinel unit. The

magnesium Magnesium is a chemical element with the 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 of the periodic ta ...

and the

silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ta ...

are completely ordered in the structure. There are three distinct octahedral sites, M1, M2, and M3, and a single tetrahedral site. Wadsleyite is a sorosilicate in which Si₂O₇ groups are present. There are four distinct oxygen atoms in the structure. O2 is a bridging oxygen shared between two tetrahedra, and O1 is a non-silicate oxygen (not bonded to Si). The potentially hydrated O1 atom lies at the center of four edge-sharing Mg²⁺

octahedra In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet a ...

. If this oxygen is hydrated (protonated), a Mg vacancy can occur at M3. If water incorporation exceeds about 1.5% the M3 vacancies can be ordered in violation of space group ''Imma'', reducing the symmetry to monoclinic ''I''2/''m'' with beta angle up to 90.4º. Wadsleyite II is a separate spinelloid phase with both a single (SiO₄) and double (Si₂O₇) tetrahedral units. It is a magnesium-iron silicate with variable composition that might occur between the stability regions of wadsleyite and

γ-Mg₂SiO₄, but computational models suggest that at least the pure magnesian form is not stable. One-fifth of the silicon atom is in isolated tetrahedral and four-fifths is in Si₂O₇ groups so that the structure can be thought of as a mixture of one-fifth spinel and four-fifths wadsleyite.

Crystallography and physical properties

Wadsleyite crystallizes in the orthorhombic crystal system and has a unit cell volume of 550.00 Å³. Its space group is ''Imma'' and its cell parameters are a = 5.6921 Å, b = 11.46 Å and c = 8.253 Å; an independent study found the cell parameters to be a = 5.698 Å, b = 11.438 Å and c = 8.257 Å. Pure magnesian wadsleyite is colorless, but iron-bearing varieties are dark green. The wadsleyite minerals generally have a

microcrystalline A microcrystalline material is a crystallized substance or rock that contains small crystals visible only through microscopic examination. There is little agreement on the range of crystal sizes that should be regarded as microcrystalline, but th ...

texture and are fractured. Because of small crystal size, detailed

optical Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultravio ...

data could not be obtained; however, wadsleyite is anisotropic with low first-order birefringence colors. It is biaxial with a mean

refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...

of n = 1.76 and has a calculated

specific gravity Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest ...

of 3.84. In X-ray

powder diffraction Powder diffraction is a scientific technique using X-ray, neutron, or electron diffraction on powder or microcrystalline samples for structural characterization of materials. An instrument dedicated to performing such powder measurements is call ...

, its strongest points in pattern are: 2.886(50)(040), 2.691(40)(013), 2.452(100,141), 2.038(80)(240), 1.442(80)(244).

Sound velocities

Sawamoto et al. (1984) firstly measured the P-wave velocity (Vp) and

S-wave __NOTOC__ In seismology and other areas involving elastic waves, S waves, secondary waves, or shear waves (sometimes called elastic S waves) are a type of elastic wave and are one of the two main types of elastic body waves, so named because th ...

velocity (Vs) of Mg-endmember of wadsleyite at ambient condition by the

Brillouin spectroscopy Brillouin spectroscopy is an empirical spectroscopy technique which allows the determination of elastic moduli of materials. The technique uses inelastic scattering of light when it encounters acoustic phonons in a crystal, a process known as Bril ...

. Their data suggested that olivine-wadsleyite phase transition would cause a Vp jump of ~13% and a Vs jump of ~14%. Therefore, the olivine-wadsleyite phase transition has been suggested as the main reason for the 410 km seismic discontinuity at the boundary between the

Upper Mantle The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust (at about under the oceans and about under the continents) and ends at the top of the lower mantle at . Temperatures range from appr ...

and the

Mantle Transition Zone The transition zone is part of the Earth's mantle, and is located between the lower mantle and the upper mantle, between a depth of 410 and 660 km (250 to 400 mi). The Earth's mantle, including the transition zone, consists primarily o ...

in Earth.

Namesake

Arthur David Wadsley (1918–1969) received the privilege of getting a mineral named after him due to his contributions to geology such as the crystallography of minerals and other

inorganic compounds In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemi ...

. The proposal to have wadsleyite named after Wadsley was approved by the Commission on New Minerals and Mineral Names of the International Mineralogical Association. The type specimen is now preserved in the collection of the Department of Geology at the

University of Alberta The University of Alberta, also known as U of A or UAlberta, is a Public university, public research university located in Edmonton, Alberta, Canada. It was founded in 1908 by Alexander Cameron Rutherford,"A Gentleman of Strathcona – Alexande ...

References

{{Meteorites Sorosilicates Meteorite minerals Orthorhombic minerals Minerals in space group 74