Wadsleyite

Wadsleyite is a high-pressure polymorph of olivine, and is an orthorhombic mineral found in the Peace River meteorite in Alberta, Canada. In phase transformations with increasing pressure from Mg₂SiO₄-Fe₂SiO₄ (forsterite – fayalite), olivine is transformed to wadsleyite (β-Mg₂SiO₄) and then to a spinel-structured ringwoodite (γ-Mg₂SiO₄). This series of transformations is thought to occur during an extraterrestrial shock event in the meteorite prior to its fall on Earth. With a formula of (Mg,Fe²⁺)₂(SiO₄), its cell parameters are as follows: a = 5.7 Å, b = 11.7 Å and c = 8.24 Å. It is polymorphous with ringwoodite and is found to be stable in the transition zone of the Earth’s upper mantle. These regions are from 400–525 kilometres (249–326 mi) in depth. Because of oxygens not bound to silicon in the Si₂O₇ groups of wadsleyite, it leaves some oxygen atoms underbonded, and as a result, these oxygens are hydrated easily. As a result, there can be high concentrations of hydrogen atoms in the mineral. Hydrous wadsleyite is a considered a potential site for water storage in the Earth’s mantle due to the low electrostatic potential of the underbonded oxygen atoms. Although wadsleyite does not contain H in its chemical formula, it may contain more that 3 percent by weight H₂O, and may coexist with a hydrous melt at transition zone pressure-temperature conditions. The water solubility and density of wadsleyite are ultimately affected by the temperature and pressure inside of the Earth.

Wadsleyite was first identified by Ringwood and Major in 1966 and was confirmed to be a stable phase by Akimoto and Sato in 1968.(Horiuchi and Sawamoto, 1981) The phase was originally known as β-Mg₂SiO₄ or “beta-phase ” and is a polymorph of olivine, along with minerals ringwoodite. Wadsleyite was named for mineralogist Arthur David Wadsley (1918-1969).

Composition

Wadsleyite is a polymorph of forsterite Mg₂SiO₄, an end-member of the solid-solution series of olivine(Horiuchi and Sawamoto, 1981). In the phase transformations of forsterite to fayalite Mg₂SiO₄-Fe₂SiO₄, this magnesium-rich olivine α-Mg₂SiO₄ changes to wadsleyite β-Mg₂SiO₄ under certain pressure and temperature conditions, and then with increasing pressure, it transforms to ringwoodite γ-Mg₂SiO₄ which is a spinel structure (Price, Putnis, Agrell and Smith, 1983). Wadsleyite is synthesized stably at 1,000–1,500 °C (1,800–2,700 °F) and 13 to 18 GPa of pressure between depths of 410–525 kilometres (255–326 mi). Figure 1 of the reference shows the high pressure phases of olivine polymorphs beginning with wadsleyite β-Mg₂SiO₄. Geologically speaking, it is a very fine-grained “reactive” forsterite that had been synthesized from hydrous starting materials.

In values of weight percent oxide, the pure magnesian variety of wadsleyite would be 44.5% SiO₂, 52.2% MgO, and 3.33% H₂O. The average microprobe analysis for wadsleyite yielded: MgO 38.21, SiO₂ 38.7, CaO 0.07, Cr₂O₃ 0.01, MnO 0.43, GeO 22.37, NiO 0.11 and ZnO 0.10.(Smyth, 1987) A recalculation of the number of cations on the basis of four oxygens will yield MgO 1.51, SiO₂ 1.03, CaO 0.0019, Cr₂O₃ 0.0002, MnO 0.0096, GeO 0.4032, NiO 0.0023 and ZnO 0.0019. An analysis of trace elements in wadsleyite suggests that there are a number of elements included in it. Results demonstrate traces of rubidium Rb, strontium Sr, barium Ba, titanium Ti, zirconium Zr, niobium Nb, hafnium Hf, tantalum Ta, thorium Th, and uranium U in wadsleyite relative to olivine.(Mibe, Nakai, Orihashi, and Fujii 2006) This information 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 (Mibe, Nakai, Orihashi, and Fujii 2006).

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 Peace River meteorite in Peace River, Alberta, Canada. This meteorite, an L6 hypersthene-olivine chrondite, is believed to have formed at high pressure during an extraterrestrial shock event. It occurs as microcrystalline rock fragments, often not surpassing 0.5 millimetres (0.020 in) in diameter, that pseudomorph pre-existing olivine parts within the mineral.(Price, Putnis, Agrell and Smith, 1983) The meteor or asteroid that impacted the earth generated the mineral phase transformations observed in shocked chrondites of the Peace River meteorite.(Beck, Gillet, Jahn, McMillan, Reynard, Van De Moortele and Wilson, 2007) It contains sulfide-rich veins of olivine and is believed, like other meteorite specimens, to have undergone a shock event, causing the grain components of olivine to transform into significant amounts of high-density wadsleyite.(Price, Putnis, Agrell and Smith, 1983)

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 and the silicon 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 (Ashbrook, Berry, Farnanf, Le Polle, Pickard and Wimperise, 2006). 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 (Smyth, 1987, 1994). If this oxygen is hydrated (protonated), a Mg vacancy can occur at M3. A structure of β-Mg₂SiO₄ is shown in reference figure 3. If water incorporation exceeds about 1.5% the M3 vacancies can order in violation of space group Imma, reducing the symmetry to monoclinic I2/m with beta angle up to 90.4º.

Wadsleyite II is a separate spinelloid phase that might occur between the fields of wadsleyite and ringwoodite. It has both a single (SiO₄) and double (Si₂O₇) tetrahedral units. It is a hydrous magnesium-iron silicate with variable composition that occurs between the stability regions of wadsleyite and ringwoodite γ-Mg₂SiO₄.(Kleppe, 2006) 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.(Horiuchi and Sawamoto, 1981). In the phase of wadsleyite II, there is considered to be possible host hydrogen in the transition zone of the Earth’s mantle. Since forsterite is thought to be about or little over 50% of the mantle, the transition region in the upper mantle could be an important water reservoir.(Kleppe, 2006) Wadsleyite is very water soluble and can accept up to 3 wt. % H₂O as hydroxyl at this site. Its water content is very significant in understanding the way Earth developed. Synthetic hydrous wadsleyite II is pictured in reference figure 4. Wadsleyite II in a variably hydrous magnesium-iron silicate phase. It is a potential host for hydrogen in the transition zone of the Earth's mantle. However, if the water composition of wadsleyite surpasses a 0.1–0.2 wt% amount, it could cause partial melting. As a result, an upwelling flow of water could affect the distribution of particular elements in the Earth.(Huang, Karato and Xu, 2005)

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 as follows: a = 5.6921 Å, b = 11.46 Å and c = 8.253 Å.(Price, Putnis, Agrell and Smith, 1983) A more recent structure of wadsleyite confirms the cell parameters to be a = 5.698 Å, b = 11.438 Å and c = 8.257 Å.(Horiuchi and Sawamoto, 1981). Pure magnesian wadsleyite is colorless, but iron bearing varieties are dark green.

The wadsleyite minerals generally have a microcrystalline texture and are fractured. Because of small crystal size, detailed optical data could not be obtained; however, wadsleyite is anisotropic with low first-order birefringence colors.(Price, Putnis, Agrell and Smith, 1983) It is biaxial with a mean refractive index of n = 1.76. I has a calculated specific gravity of 3.84. In X-ray powder diffraction, 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).(Price, Putnis, Agrell and Smith, 1983)

Biographic sketch

Arthur David Wadsley (1918-1969) received the privilege of getting a mineral named after him due his contributions to geology such as the crystallography of minerals and other inorganic compounds.(Price, Putnis, Agrell and Smith, 1983) The proposal to have wadsleyite named after Wadsley was approved by the Commission on New Minerals and Mineral Names of the International Mineralogical Association.(Price, Putnis, Agrell and Smith, 1983) The type specimen is now preserved in the collection of the Department of Geology at the University of Alberta.

See also

• Glossary of meteoritics

References