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| Wadsleyite | |
|---|---|
| Name | Wadsleyite |
| Formula | (Mg,Fe)2SiO4 |
| Category | Silicate mineral (spinelloid) |
| System | Orthorhombic |
| Symmetry | Imma |
| Color | Olive to dark green |
| Hardness | ~6 (Mohs) |
| Density | ~3.65–3.8 g/cm3 |
Wadsleyite is an orthorhombic high‑pressure mineral phase of the olivine group that occurs under conditions characteristic of the Earth's upper mantle and transition zone. It is significant for understanding subduction zone processes, the 410 km discontinuity, and large‑scale mantle convection. Named after a notable researcher, the phase connects laboratory shock experiment results and seismic observations from regions beneath Japan, Chile, and the Marianas Trench.
Wadsleyite plays a central role in models of the upper mantle and mantle transition zone by representing one of the major high‑pressure polymorphs of forsteritic olivine. Its presence or absence affects interpretations of the 410 km discontinuity, seismic tomography images beneath the Pacific Ocean, and geophysical inferences about Lehmann discontinuity analogues in other planetary bodies such as Mars and Moon. Studies by institutions like the Carnegie Institution for Science, Stanford University, ETH Zurich, and Massachusetts Institute of Technology have combined mineral physics, geochemistry, and seismology to constrain its properties.
Wadsleyite was first synthesized and characterized following experimental work on high‑pressure transformations of forsterite carried out in laboratories including the Geophysical Laboratory (Carnegie) and the Lawrence Livermore National Laboratory. The name honors geophysicist Brian Wadsley (note: proper nouns only) for contributions to high‑pressure mineralogy, following conventions used by the International Mineralogical Association and traditions exemplified by naming practices for perovskite, ringwoodite, and bridgmanite. Early reports appeared alongside landmark studies from researchers affiliated with Caltech, University of Cambridge, University of Tokyo, and University of Oxford.
Wadsleyite has an orthorhombic lattice (space group Imma) with a compact spinelloid framework that accommodates Si in both tetrahedral and network configurations, analogous to structural themes found in spinel and garnet phases. Its general formula is (Mg,Fe)2SiO4, allowing substitution of divalent cations such as Fe2+ and minor trivalent species introduced through interactions with aluminum and hydrous components during mantle processes; comparable substitutional behavior has been studied in clinopyroxene and orthopyroxene. Single‑crystal and powder diffraction studies at facilities like the European Synchrotron Radiation Facility, National Synchrotron Light Source, and Advanced Photon Source have refined lattice parameters and site occupancies, following analytical standards set by the International Union of Crystallography.
Wadsleyite exhibits densities (~3.65–3.8 g/cm3) and elastic moduli that are higher than olivine but lower than ringwoodite, influencing seismic velocity contrasts across depth ranges studied by International Seismological Centre datasets and US Geological Survey models. Its hardness (~6 Mohs) and optical properties were characterized alongside spectroscopic signatures measured with Fourier transform infrared spectroscopy, Raman spectroscopy, and Mössbauer spectroscopy. Chemically, wadsleyite can incorporate up to several weight percent H2O through hydroxyl substitution, a capacity that parallels hydrated phases investigated in serpentine studies and has implications for volatile storage examined by researchers at Scripps Institution of Oceanography and Woods Hole Oceanographic Institution.
Wadsleyite forms from olivine at pressures corresponding to depths of roughly 410 km and is stable over a pressure‑temperature window controlled by bulk composition, presence of volatiles, and thermal gradients associated with subducting slab intersections and mantle plume upwellings. Phase relations have been mapped using multi‑anvil presses and diamond anvil cell techniques at centers like Bayerisches Geoinstitut, Tohoku University, and Institut de Physique du Globe de Paris, informing thermodynamic models used by USGS and NOAA for interpreting mantle heterogeneity. Experiments show that hydration, iron content, and aluminum partitioning shift the olivine→wadsleyite→ringwoodite equilibria, echoing compositional effects documented for peridotite and eclogite.
The olivine‑to‑wadsleyite transition contributes to the prominent seismic discontinuity near 410 km depth observed in global receiver function studies, seismic reflection profiles across cratons like the Canadian Shield and active margins like the Andes, and body wave travel time inversions beneath regions such as Iceland and the Hawaii hotspot. Variations in transition depth reflect thermal anomalies linked to cold subduction and hot mantle plume scenarios investigated in joint studies by groups at Brown University, University of California, Berkeley, and Princeton University. Wadsleyite's water storage capacity modulates mantle viscosity and electrical conductivity in the transition zone, factors constrained by magnetotelluric surveys of continents like Australia and island arcs such as Aleutian Islands.
Research on wadsleyite spans high‑pressure experimentation, computational mineral physics, and field studies. Multi‑anvil and diamond anvil experiments at Oak Ridge National Laboratory, GFZ Potsdam, and CNRS have quantified phase boundaries, elastic properties, and dehydration melting. First‑principles calculations using methods developed at Argonne National Laboratory, Princeton University, and Harvard University have predicted defect chemistry, hydrogen incorporation, and anisotropic elasticity, complementing experimental constraints from synchrotron scattering at SLAC National Accelerator Laboratory and Diamond Light Source. Collaborative projects between agencies such as NASA, NSF, and European Research Council continue to integrate wadsleyite data into global geodynamic and planetary evolution models.
Category:Minerals