Figure 5

FTIR imaging and the distribution of the hydrous components in a garnet from Cazadero. (a) Optical image of the examined sample. (b) Distribution of the hydrous components resulting from a grid of 40 × 40 μm² single spots. (c) High-resolution image collected as a grid (reported on the image) of 15 FPA spots, each covering 170 × 170 μm²; the image clearly shows that the hydrous components are strictly related to the included fibrous minerals, while the garnet host is anhydrous. Both images were obtained by integrating the signal in the OH-stretching 3700–3400 cm⁻¹ range. The intensity of the absorption is proportional to the colour scale on the left, where blue = zero and red = maximum. (d) Selected single spectra (plotted with the same absorbance scale), collected with a 40 × 40 μm beam in a fibrous-rich area toward the crystal core (red line), and in a clean area in the garnet host (black line). The spectrum collected in the garnet host is totally flat, indicating the sample to be totally anhydrous; the spectrum collected in the hydrous zone shows a convoluted absorption (see Methods); the peaks of amphibole (Amp) and phyllosilicate (Chl, a phase close in composition to a chlorite) are evidenced. Spectra collected in the NIR (6000–4000 cm⁻¹) range in the hydrated core (not shown) display only a weak band at 4170 cm⁻¹ while no absorption occurs at wavenumbers >5000 cm⁻¹; this indicates the presence of OH-groups only as hydrous component in the sample.