Wadsleyite

Wadsleyite is a high-pressure phase of Mg₂SiO₄ and is polymorphous with the olivine phase forsterite. An orthorhombic mineral with the formula β-Mg₂SiO₄, it was first found in nature in the Peace River meteorite from Alberta, Canada. It is formed by a phase transformation from forsterite (α-Mg₂SiO₄) under increasing pressure and eventually transforms into spinel-structured ringwoodite (γ-Mg₂SiO₄) as pressure increases further. The structure can take up a limited amount of other bivalent cations 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 = 11.7 Å and c = 8.24 Å.

Wadsleyite is found to be stable in the upper part of the transition zone of the Earth’s upper mantle between 410–520 kilometres (250–320 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, 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 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 solubility of water and the density of wadsleyite depend on the temperature and pressure in the Earth. 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 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 and suggest 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 et al. 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 the Peace River meteorite, an L6 hypersthene-olivine chondrite from Peace River, Alberta, Canada. 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 0.5 mm (0.020 in) in diameter (van de Moortèle et al. 2007; Price et al. 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 et al. 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. 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 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 ringwoodite γ-Mg₂SiO₄ (Kleppe, 2006), but computational models suggest that at least the pure magnesian form is not stable (Tokár et al. 2013). 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).

Crystallography and physical properties

Molar volume vs. pressure at room temperature

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 (Price et al. 1983); an independent study found 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 et al. 1983) It is biaxial with a mean refractive index of n = 1.76 and 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 et al. 1983).

Biographic sketch

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 (Price et al. 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. 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