Water-rich coesite in prismatine-granulite from Waldheim/Saxony

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Marco E. Ciriotti
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Water-rich coesite in prismatine-granulite from Waldheim/Saxony

Messaggio da Marco E. Ciriotti » lun 09 gen, 2023 12:29

Referenza:
▪ Thomas, R., Davidson, P., Rericha, A., Rechnagel, U. (2022): Water-rich coesite in prismatine-granulite from Waldheim/Saxony. Veröffentlichungen Museum für Naturkunde Chemnitz, 45, 67-80.

Abstract:
In this paper, we studied very rare coesite-like crystals in prismatine granulite rock from Waldheim/Saxony with Raman spectroscopy. This coesite shows a clearly different Raman spectrum characterized by very strong bands at 272 and 339 cm-1, in addition to the strong band between 521 and 528 cm-1. In addition to these bands, a further strong band at about 960 cm-1 is always present. The high-frequency range of the Raman spectrum is characterized by intense bands at 3272, 3446, and 3581 cm-1, corresponding to ν4, ν3, and ν1 OH-stretching bands in the FTIR spectra. These strong bands demonstrate an unusually high water content of 8.5%. There is also an unusually high Al and a moderate B content. At pressures around 9 GPa, the incorporation of water and Al in the weight-% level is not possible. Therefore, we assume a primary crystallization as ultra hydrous stishovite at very high pressure (~30 GPa) and a transformation of this stishovite into coesite during decompression during extremely rapid transport towards the surface. As a result of this process, the Al and OH initially incorporated into the stishovite passed over into a new lattice of the new coesite phase, probably in a metastable form. In this process, corundum-like groups are formed in the coesite lattice. Because of the significantly different Raman characteristics, we refer to this new phase as coesite-W – W for Waldheim, as we are unaware of any similar discovery, including synthetic analogues.
Marco E. Ciriotti

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