TY  - JOUR
AU  - Koch-Mueller, Monika
AU  - Lathe, Christian
AU  - Wunder, Bernd
AU  - Appelt, Oona
AU  - Bhat, Shrikant
AU  - Ebert, Andreas
AU  - Farla, Robert
AU  - Roddatis, Vladimir
AU  - Schreiber, Anja
AU  - Wirth, Richard
TI  - The coesite–stishovite transition of hydrous, Al-bearing SiO<sub>2</sub> : an in situ synchrotron X-ray study
JO  - European journal of mineralogy
VL  - 36
IS  - 6
SN  - 1617-4011
CY  - Göttingen
PB  - Copernicus Publications
M1  - PUBDB-2025-00527
SP  - 1023-1036
PY  - 2024
AB  - We examined the influence of Al<sub>2</sub>O<sub>3</sub> and H<sub>2</sub>O on the position of the coesite–stishovite transition by means of in situ X-ray diffraction measurements with the large-volume press at the PETRA III synchrotron in Hamburg. The position of the transition was determined by several reversal experiments and was found to be shifted almost in parallel by about 1.5 GPa to lower pressures compared to results for the pure SiO<sub>2</sub> system reported by Ono et al. (2017). Two further reversal experiments with either additional Al<sub>2</sub>O<sub>3</sub> or additional H<sub>2</sub>O added to SiO<sub>2</sub> showed smaller changes compared to the results of Ono et al. (2017), indicating the effect of the coupled Al and H incorporation in coesite and stishovite on their transition. Further investigations of the solid quenched products and of products from additional multi-anvil experiments performed at the GFZ Helmholtz-Zentrum für Geoforschung in Potsdam were done by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), electron probe micro-analysis (EPMA), and Fourier transform infrared (FTIR) and Raman spectroscopy. Generally, the recovered samples of the in situ experiments contained less stishovite than expected from the last in situ XRD pattern before quenching. Thus, these investigations clearly show that hydrous, Al-rich stishovite that formed at high pressure (P) and temperature (T) could, at least partly, not be quenched to room conditions and transformed to coesite with unusually high (Al, H) contents. As result of this, conventional quench experiments would lead to erroneous results of the transition in the (Al, H)-bearing SiO2 system. We observed two kinds of coesite in the experiments: one relatively Al-poor coesite transformed under equilibrium conditions at P and T from stishovite over a certain time frame and an Al-richer one, sometimes pseudomorphically replacing former stishovite during the decompression process to room conditions. Within both types of coesite, nanometre-sized kyanite inclusions and relicts or remnants of stishovite were observed by TEM. These observations resemble those of Yang et al. (2007) on ophiolites with identical textures and phases and were interpreted as result of a stishovite transition back to coesite during retrograde metamorphism. Our results clearly indicate that the coesite–stishovite transition is sharp but can considerably vary in depth by the addition of Al and H to the SiO<sub>2</sub> system. This has consequences for the assignment and interpretation of the depth variation of the seismic X discontinuity.  
LB  - PUB:(DE-HGF)16
UR  - <Go to ISI:>//WOS:001379934000001
DO  - DOI:10.5194/ejm-36-1023-2024
UR  - https://bib-pubdb1.desy.de/record/622847
ER  -