Wadsleyite

Wadsleyite is a high-pressure polymorph of olivine, an orthorhombic mineral found in the Peace River meteorite in Alberta, Canada. In the phase transformations from Mg₂SiO₄-Fe₂SiO₄ (forsterite-fayalite), olivine is transformed to wadsleyite β-Mg₂SiO₄ and then to a spinel-structured γ-Mg₂SiO₄ with increasing pressure. This series of transformations is thought to occur during an extraterrestrial shock event on the earth. With a formula of (Mg,Fe²⁺)₂(SiO₄), its cell parameters are as follows: a = 5.7Å, b = 11.7Å and c = 8.24Å. It is polymorphous with ringwoodite and is found to be stable in the transition zone of the Earth’s upper mantle. These regions are between 400 and 525km in depth. Because of oxygens not bound to silicon in the Si₂O₇ groups of wadsleyite, it leaves oxygens unoccupied, and as a result, these oxygens are hydrated easily. As a result, there are 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 its low electrostatic potential. It exists with a hydrous melt at transition zone pressure-temperature conditions. The water solubility and density of wadsleyite is ultimately affected by the temperature and pressure inside of the Earth.

Wadsleyite was first pointed out 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 “Phase B” and is a polymorph of olivine, along with minerals ringwoodite. Finally, wadsleyite was named after Dr. Arthur David Wadsley in his honor of his contributions to the field of geology.

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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 γ-Mg₂SiO₄ which is a spinel structure. (Agrell, Price, Putnis and Smith, 1983) Wadsleyite is synthesized stably at 1400˚C and under 17GPa of pressure between depths of 410 and 660km. Figure 1 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. (Fujii, Mibe, Nakai and Orihashi, 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. (Fujii, Mibe, Nakai and Orihashi, 2006)

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 during an extraterrestrial shock event on earth, beginning a solution-series of olivine. It occurs as microcrystalline rock fragments, often not surpassing 0.5mm in diameter, that pseudomorph pre-existing olivine parts within the mineral. (Agrell, Price, Putnis and Smith, 1983) To the left, Figure 2 shows a microcrystalline image of wadsleyite. 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. (Agrell, Price, Putnis and Smith, 1983)

Structure

Wasdsleyite’s structure is based on a somewhat distorted cubic-closest packing of oxygen atoms. The α-axis and the β-axis is the half diagonal of the spinel unit. The magnesium and the silicon are completely ordered in the structure. The hydrated O2 atom lies at the center of four edge-sharing Mg³⁺ octahedra. The compression of the unoccupied oxygen polyhedra plays a significant role in density increase of “pressure-induced” segments of transformations. (Smyth, 1994) This mineral contains three distinct tetrahedral sites, six different octahedral sites and eight distinct oxygen sites. (Kleppe, 2006) .The Mg³⁺ atoms occupy the octahedral sites; Si⁴⁺ atoms occupy the tetrahedral sites. Within the oxygen sites, some oxygens are unoccupied in the polyhedra, creating a shallow electrostatic potential for the mineral. (Horiuchi and Sawamoto, 1981)This meets the criteria for potential hydroxyl sites, in which the unoccupied oxygens become hydrated.

Wadsleyite is a pyrosilicate and a polymerized tetrahedral in which mostly Si₂O₇ groups are present. (Ashbrook, Berry, Farnanf, Le Polle, Pickard and Wimperise, 2006) A structure of β-Mg₂SiO₄ is shown in Figure 3. Wadsleyite II has both a single (SiO₄) and coupled tetrahedral (Si₂O₇). 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 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)

Physical properties

As an orthorhombic crystal system, wadsleyite’s unit cell volume is V 550.00Ǻ³. Its space group is Imma and its cell parameters are as follows: a = 5.6921Ǻ, b = 11.46Ǻ and c = 8.253Ǻ. (Agrell, Price, Putnis and Smith, 1983) A more recent structure of wadsleyite confirms the cell parameters to be a = 5.698Ǻ, b = 11.438Ǻ and c = 8.257Ǻ. (Horiuchi and Sawamoto, 1981) The condition of its diaphaneity is transparent and its color is light gray to light blue. As shown in the illustrations, Figure 5a shows wadsleyite in its purest state whereas Figure 5b shows a variety of samples of wadsleyite from pure to impure, clear to midnight blue.

The wadsleyite minerals generally have a felsitic 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. (Agrell, Price, Putnis and Smith, 1983) Its mean refractive index is n = 1.76, D = 3.84 and it is biaxial. 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). (Agrell, Price, Putnis and Smith, 1983)

Biographic sketch

Late Dr. A. D. Wadsley 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. (Agrell, Price, Putnis and Smith, 1983) His impressive investigations of minerals were all known to the mass in his field and to many others in the studies of geology. The proposal to have Wadsleyite named after Wadsley was approved by the Commission on New Minerals and Mineral Names of the International Mineralogical Association. (Agrell, Price, Putnis and Smith, 1983) It is now preserved as an official mineral specimen in the Department of Geology’s collection at the University of Alberta. Dr. Wadsley accepted this honor for his exceptional knowledge in the geologic field of study.

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