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

Abstract

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 during heating and the effects of a radius-dependent dehydroxylation progress can be shown. Furthermore, it was possible to identify a quenchable talclike 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.

Kinetics of the chrysotile and brucite dehydroxylation reaction: a combined non-isothermal/isothermal thermogravimetric analysis and high-temperature X-ray powder diffraction study

“…The multiple natures of peak maxima in DTG curves of chrysotile are typical for a reaction scenario with (metastable) reaction intermediates as inferred by Viti (2010) and experimentally demonstrated by MacKenzie and Meinhold (1994), Gualtieri et al (2012) and Trittschack and Grobéty (2013). The broad temperature range in which chrysotile dehydroxylates is related to the radius-dependent dehydroxylation temperature and the appearance of an H 2 O-containing talc-like intermediate phase, which breaks down at even higher temperatures (Trittschack and Grobéty 2013).…”

“…The broad temperature range in which chrysotile dehydroxylates is related to the radius-dependent dehydroxylation temperature and the appearance of an H 2 O-containing talc-like intermediate phase, which breaks down at even higher temperatures (Trittschack and Grobéty 2013). The much simpler DTG graphs of brucite seem to be indicative for a direct dehydroxylation and subsequent formation of periclase MgO without intervening (metastable) phases as demonstrated by XRPD studies (Bearat et al 2002;Nahdi et al 2009).…”

“…The outer diameter of the chrysotile fibres was determined from high-resolution TEM images and gave values ranging between 23 and 85 nm, more than 75 % between 30 and 60 nm. A detailed description of the dehydroxylation reaction is provided by Trittschack and Grobéty (2013).…”

“…the rate-limiting step(s) changes with α. The nature of possible rate-limiting steps has been identified by in situ HT-XRPD, Raman and FTIR spectroscopy as well as ex situ TEM analyses (Gualtieri et al 2012;Trittschack and Grobéty 2013). The nanotube structure of chrysotile dehydroxylates from the outer, less curved sheets inwards.…”

“…Both reactions have been studied comprehensively in the past (e.g. Martin 1977;Datta et al 1987;Datta 1991;MacKenzie and Meinhold 1994;Butt et al 1996;Halikia et al 1998;McKelvy et al 2001;Bearat et al 2002;Cattaneo et al 2003;Yue et al 2005;McKelvy et al 2006;Nahdi et al 2009;Viti 2010;Gualtieri et al 2012;Trittschack and Grobéty 2013) and offer, therefore, good possibilities to compare assessed data. Mechanistic interpretations are performed by comparing modelfree and reaction-progress-resolved values of the apparent activation energy (E aα ) with the ones plotted into the two independent master plot graphs.…”