Abstract
A dense magnesium iron silicate polymorph with a structure intermediate between olivine, ringwoodite, and wadsleyite was theoretically predicted about four decades ago. As this group of minerals constitute the major component of shocked meteorites, constraining their transitional forms and behaviour is of potential importance for understanding impact events on their parent bodies. Here we use high-resolution transmission electron microscopy techniques and single-crystal X-ray diffraction analyses to identify naturally occurring examples of this mineral – recently named poirierite – in shocked chondritic meteorites. We observe nanoscale lamellar poirierite topotactically intergrown within wadsleyite, and additionally within ringwoodite as recently reported. Our results confirm the intermediate structure of poirierite and suggest it might be a relay point in the shear transformations between its polymorphs. We propose that poirierite formed during rapid decompression at relatively low temperature in retrograde shock metamorphism of the meteorites.
Highlights
A dense magnesium iron silicate polymorph with a structure intermediate between olivine, ringwoodite, and wadsleyite was theoretically predicted about four decades ago
Olivine [(Mg,Fe)2SiO4] with Fe/(Mg+Fe) ≈ 0.11 is the most common constituent mineral in the Earth’s upper mantle. It transforms into a spinelloid structure at ~14 GPa and into a spinel structure at ~18 GPa in the mantle geotherm[1]. These high-pressure minerals are considered dominant in the mantle transition zone (MTZ) at depths of 410–660 km
We report further characterisations of the natural ε phase occurrences in three chondritic meteorites (Tenham, Miami and Suizhou) and discuss its formation mechanisms in shock metamorphism based on transmission electron microscopy, single-crystal X-ray diffraction, and firstprinciples calculations. ε-Mg2SiO4 has been approved as a new mineral species by the Commission on New Minerals and Mineral Names, International Mineralogical Association
Summary
A dense magnesium iron silicate polymorph with a structure intermediate between olivine, ringwoodite, and wadsleyite was theoretically predicted about four decades ago As this group of minerals constitute the major component of shocked meteorites, constraining their transitional forms and behaviour is of potential importance for understanding impact events on their parent bodies. Olivine [(Mg,Fe)2SiO4] with Fe/(Mg+Fe) ≈ 0.11 is the most common constituent mineral in the Earth’s upper mantle It transforms into a spinelloid structure (wadsleyite) at ~14 GPa and into a spinel structure (ringwoodite) at ~18 GPa in the mantle geotherm[1]. Microstructures of ringwoodite and wadsleyite in shocked meteorites provide important insights for understanding their deformation and transformation mechanisms of (Mg,Fe)2SiO416–18 These high-pressure phases often show nanometre-scale planar defects observable only under a transmission electron microscope (TEM). Based on the observations of the defect (stacking fault) structures combined with the topological analyses of the crystal structures of olivine, ringwoodite, and wadsleyite, shear mechanisms are posited to promote their polymorphic transformations[17,18]
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