The optical system of an IR microscope equipped with a FPA is an apertureless imaging system, whose ultimate spatial resolution is comparable (although never better) than that attainable by confocal microscopes. Commercially available FPA arrays are typically composed of 1024 × 1024 FTIR detectors a few tens of microns or less in dimension, and allow the acquisition of thousands of IR spectra simultaneously generating mid-IR (MIR) images with a high resolving power. FTIR-FPA imaging offers a very powerful tool to visualize directly the distribution of the target molecule across the sample, however its use in Earth Sciences is still relatively rare ( Della Ventura et al., 2010, 2014 Marxer and Novak, 2013). For instance, diffusivities determined using bulk analyses may be orders of magnitude greater that those obtained from diffusion profiles ( Zhang and Cherniak, 2010) the former may be affected by the presence of cracks (e.g., Radica, 2015), extended defects or additional pathways that may be underestimated. A major problem in diffusion studies is the lack of reliable microchemical data for the target molecules this problem is exacerbated when dealing with C-O-H arrangements, due to the well-known difficulty of quantifying light-elements, their speciation and distribution across the sample.
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A particular case is provided by those rock forming minerals that are structurally characterized by cavities or channels which leave free space to the molecular diffusivity: notable examples are the zeolites, several sorts of feldspathoids, and ring silicates such as beryl and cordierite there is increasing evidence that diffusion along these connected structural cavities is typically faster than the volume diffusion (e.g., Radica, 2015). Intra-crystalline diffusion is the slower mechanism, whereas grain boundaries and planar defects act as fast pathways for the traveling of ions or molecular species (e.g., Zhang et al., 2006). Diffusion may occur in different ways, including: (i) intra-crystalline (volume), (ii) grain boundary ( Fukuda et al., 2009 Demouchy, 2010a, b), (iii) dislocation ( Yund et al., 1981), or (iv) planar defects ( Zhang et al., 2006) diffusion. Combination of TOF-SIMS and FPA data shows a significant depletion of type II H 2O along the hourglass boundary, indicating that water diffusion could be controlled by the distribution of alkali cations within channels, coupled to a plug effect of CO 2.ĭiffusion of hydrogen-oxygen species in minerals as a function of T and P has been addressed by numerous experimental studies (e.g., Ingrin and Blanchard, 2006 Watson and Baxter, 2007 Farver, 2010 and references therein) these processes have in fact extremely important consequences in Earth Science systems. This piece of information is mandatory when the study is aimed at extracting diffusion coefficients from analytical profiles. Therefore, FTIR imaging provides evidence that different diffusion mechanisms may locally combine, suggesting that the distribution of the target molecules needs to be carefully characterized in experimental studies. High-resolution synchrotron-light FTIR imaging, in addition, also allows enhancement of CO 2 diffusion along the hourglass boundary to be distinguished from diffusion along fractures in the grain. The hourglass zone boundary may be thus considered as a structural defect possibly due to the mismatch induced by the different growth rates of each sector. Polarized FPA-FTIR imaging, on the other side, revealed that the chemical zoning acts as a fast pathway for carbon dioxide diffusion, a feature never observed so far in minerals. High-resolution FESEM images revealed that the hourglass boundary is not correlated to physical discontinuities, at least at the scale of tens of nanometers. The sample was treated at 800☌, 500 MPa, in a CO 2-rich atmosphere. In this work we investigate the strongly inhomogeneous distribution of CO 2 and H 2O in a synthetic beryl having a peculiar hourglass zoning of Cr due to the crystal growth. 6Institut für Mineralogie, Leibniz Universität, Hannover, Germany.
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