The dehydroxylation of chrysotile: A combined in situ micro-Raman and micro-FTIR study

摘要

One of the most important mechanisms releasing water in subducting slabs of oceanic crust is connected to the dehydration of serpentinized oceanic rocks. This study reports on a detailed investigation of the transition from chrysotile-an important serpentine mineral-to forsterite through the release of water. The dehydroxylation of natural chrysotile and the subsequent phase change to forsterite was studied by in situ micro-Raman and micro-FTIR spectroscopy in the temperature range of 21 to 871 °C. Comparisons were made with previously published data of lizardite-1T. Micro-Raman spectra obtained in the low-frequency (100-1200 cm~(-1)) and high-frequency ranges (3500-3800 cm-1) were complemented by micro-FTIR measurements between 2500 and 4000 cm~(-1) to study changes in the chrysotile structure as a function of dehydroxylation progress. In general, room-temperature chrysotile bands lie at higher wavenumbers than equivalent bands of lizardite-1T except of three bands positioned at 301.7, 317.5, and 345.2 cm~(-1). Different band assignments of chrysotile and lizardite-1T Raman spectra from literature are compared. The most striking assignments concern the three aforementioned Raman bands and those lying between 620 and 635 cm~(-1). The present data support a chrysotile- or at least curved TO layer related origin of the latter. Deconvolution of overlapping OH stretching bands at room temperature revealed the presence of five (FTIR) and four (Raman) bands, respectively. A slight change in the ditrigonal distortion angle α during heating and the effects of a radius-dependent dehydroxylation progress can be shown. Furthermore, it was possible to identify a quenchable talc-like phase immediately after the onset of the dehydroxylation at 459 °C. Main bands of this phase are positioned at 184.7, 359.2, and 669.1 cm~(-1) and a single OH band at 3677 cm~(-1), and are thus quite similar to those reported for dehydroxylating lizardite-1T. Their appearance coincides with the formation of forsterite. A maximum in the integral intensity of the talc-like intermediate is reached at 716 °C. At higher temperatures, the intermediate phase breaks down and supports the accelerated growth of forsterite. The lack of OH bands with the concomitant appearance of broad chrysotile-related modes in the low-frequency range after heating the sample to 871 °C indicates the presence of a heavily disordered phase still resembling chrysotile. However, there are no spectral evidences for further Si- and/or Mg-rich amorphous phases during the dehydroxylation and no indications for a relationship between the breakdown of the talc-like phase and the growth of enstatite as previously reported in literature.