Microinfrared reflection spectroscopic mapping: Application to the detection of hydrogen-related species in natural quartz

A new method of microinfrared reflection spectroscopy and mapping analysis is briefly introduced. It was used to detect distributions and structures of hydrogen-related species (e.g. H₂O, SiOH and SiH) in plastically deformed natural quartz. We used a Fourier transform-infrared spectrometer with a microscopic imaging system fully automated for all microscope functions (e.g. focusing, aperture, stage motion and measurements). Mapping can be made in thin sections with a thickness of 50 μ at room temperature and low temperatures (77 K) using a liquid N₂ cooling system. Infrared reflection spectra were obtained by five scans for each point with a range from 4000 to 400 cm _-1. The spectra were measured five times within about 2.5 s at each position. The scanning interval was 100-150 μ using a 100 × 100 μ² aperture. All obtained spectral data were stored in computer memory to construct two-dimensional mappings of infrared absorption. From the comparisons between infrared mapping images and deformation microstructures, in addition to the molecular H₂O around 3600-3400 cm^-1, the hydrogen-related point defects (i.e. SiOH and SiH) around 970-900 cm ^-1 within quartz grains and between grain boundaries increased with decreasing grain sizes (increasing plastic strain). The method can detect the SiOH and SiH along grain boundaries that enhance the hydrolytic weakening of natural quartz.