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Stresses and pressures at the quartz‐to‐coesite phase transformation in shear deformation experiments
Author(s) -
Richter B.,
Stünitz H.,
Heilbronner R.
Publication year - 2016
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1002/2016jb013084
Subject(s) - coesite , quartz , phase (matter) , deformation (meteorology) , materials science , overburden pressure , shear (geology) , shear stress , geology , mineralogy , composite material , geotechnical engineering , chemistry , organic chemistry , paleontology , eclogite , subduction , tectonics
Coesite was found in quartz aggregates, experimentally deformed at confining pressures of 1.0–1.5 GPa and temperatures between 600°C and 900°C. The confining pressure ( P c ) and, in most cases, the mean stress ( σ m ) of the experiments were below those of the quartz‐to‐coesite phase transformation. Yet coesite formed when the maximum principal stress ( σ 1 ) was within the P‐T range of the coesite stability field. In one sample, the euhedral coesite grains were corroded indicating that coesite started to transform back to quartz. It is inferred that this sample started to deform with σ 1 above the quartz‐to‐coesite phase transformation and, with ongoing deformation, σ 1 decreased to values in the quartz stability field due to strain weakening. In all cases, σ 1 triggered the quartz‐to‐coesite reaction as well as the reverse reaction, suggesting that σ 1 is the critical parameter for the quartz‐to‐coesite transformation—not P c or σ m . With progressive deformation, the coesite laths rotated toward the shear plane as more rigid particles with the sense of shear. In case of back reaction, new quartz grains exhibit no systematic crystallographic relationship with respect to old coesite. The experiments cover different degrees of pressure “overstepping,” different temperatures, and different experimental durations at P and T , and deformation always enhances the reaction kinetics. The observation that σ 1 is critical for a pressure‐dependent phase transformation (also for reversals) poses questions for the thermodynamic treatment of such phase transformations.