Deformation Behavior across the Zircon-Scheelite Phase Transition - Drs. Fang Hong & Bin Chen
Shanghai Sept. 23, 2016 — Dynamic flow and stress information may be reconstructed from materials’ preferred orientations. Recent experimental work from a team of HPSTAR scientists led by Drs. Fang Hong and Bin Chen, show that the high-pressure mineral Scheelite can inherit texture from its lower-pressure Zircon phase, suggesting new ways of interpreting flow in the upper mantle and transition zone. The study is published in the journal Phys. Rev. Lett. (DOI: 10.1103/PhysRevLett.117.135701).
Earth is a dynamic planet where convection often occurs and causes deformation in solid rocks by plastic flow. With knowledge of these deformation mechanisms, we can understand and even model the development of anisotropy in Earth’s crust and upper mantle. Zircon (ZrSiO4) is a ubiquitous accessory mineral crust found in a wide variety of sedimentary, igneous, and metamorphic rocks. Zircon can be subject to plastic deformation, through either geodynamical processes or shock metamorphism during the evolution of Earth’s crust and mantle due to age dating.
The diamond-anvil cell (DAC) has long been used to study materials/minerals at high pressure and mostly uses axial diffraction, where the X-ray comes along the loading direction. Radial x-ray diffraction (where the incoming x-ray propagates along the diamond culet) developped in the early 1990s, was used by Geologists and material scientists to investigate materials’ deformation behavior.
“When applying stress on a polycrystalline sample, individual crystals will deform preferentially along the slip planes when the stress exceeds the material’s shear strength. The plastic deformation develops into a lattice-preferred orientation ("texture"). By analyzing the texture, we can understand the deformational process and its mechanism”, said Dr. Chen.
To understand the deformation behavior of zircon-type materials across the zircon-scheelite phase transition, the team performed radial x-ray diffraction experiments on zircon-type GdVO4 as an analogue material because it also exhibits a zircon-scheelite phase transition.
Their results show that the deformed orientations in the zircon and scheelite GdVO4 relate to each other. “The Scheelite phase could inherit a crystalline orientation from the zircon phase during the transformation”, Dr. Hong said.
“The observed textures in the scheelite phase do not evolve upon further compression. Therefore, there is little contribution from further plastic deformation in the experiment”, Prof. Merkel said, one of the principal investigators, from Université Lille 1 in France.
“Our observations show a martensitic mechanism for the zircon-scheelite transformation. This work will help us understand the local deformation history in the upper mantle and transition zone and provides fundamental guidance on material design and processing for zircon-type materials”, Dr. Yue added.
Caption: Texture evolution of GdVO4 through the phase transition. Inverse pole figures of the compression direction are shown up to 23 GPa for both the zircon and scheelite phases. For each case, the experimental pressure and phase proportions are shown in the figure. Pole densities are measured in multiples of a random distribution (mrd). Equal area projections, courtesy of Dr. Binbin Yue.