Fourier transform infrared spectroscopy (FTIR) is a spectroscopic technique that has been used for analyzing the fundamental molecular structure of geological samples in recent decades. As in other infrared spectroscopy, the molecules in the sample are excited to a higher energy state due to the absorption of infrared (IR) radiation emitted from the IR source in the instrument, which results in vibrations of molecular bonds. The intrinsic physicochemical property of each particular molecule determines its corresponding IR absorbance peak, and therefore can provide characteristic fingerprints of functional groups (e.g. C-H, O-H, C=O, etc.). In geosciences research, FTIR is applied extensively in the following applications:
Analysing the trace amount of water content in Nominally anhydrous minerals (NAMs) Measuring volatile inclusions in glass and minerals Estimating the explosion potential in volcanic setting. Analysing chemotaxonomy of early life on earth Linking biological affinities of both microfossils and macrofossils These applications are discussed in details in the later sections. Most of the geology applications of FTIR focus on the mid-infrared range, which is approximately 4000 to 400 cm−1.
Instrumentation
The fundamental components of a Fourier transform spectrometer include a polychromatic light source and a Michelson Interferometer with a movable mirror. When light goes into the interferometer, it is separated into two beams. 50% of the light reaches the static mirror and the other half reaches the movable mirror. The two light beams reflect from the mirrors and combine as a single beam again at the beam splitter. The combined beam travels through the sample and is finally collected by the detector. The retardation (total path difference) of the light beams between the static mirror and the movable mirror results in interference patterns. The IR absorption by the sample occurs at many frequencies and the resulting infereogram is composed of all frequencies except for those absorbed. A mathematical approach Fourier Transform converts the raw data into spectrum.
Advantages
The FTIR technique uses a polychromatic beam of light with a wide range of continuous frequencies simultaneously, and therefore allows a much higher speed of scanning versus the conventional monochromatic dispersive spectroscopy. Without the slit used in dispersive spectroscopy, FTIR allows more light to enter the spectrometer and gives a higher signal-to-noise ratio, i.e. a less-disturbed signal. The IR laser used has a known wavelength and the velocity of the movable mirror can be controlled accordingly. This stable setup allows a higher accuracy for spectrum measurement.
Sample characterization Transmission FTIR, attenuated total reflectance (ATR)-FTIR, Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and reflectance micro-FTIR are commonly used for sample analysis .
Applications in geology
Volatiles diagnosis
The most commonly investigated volatiles are water and carbon dioxide as they are the primary volatiles to drive volcanic and magmatic processes. The absorbance of total water and molecular water is approximately 3450 cm-1 and 1630 cm-1. The peak height of the absorption bands for CO2 and CO32− are 2350 cm−1 and 1430 cm−1 respectively. The phases of volatiles also give different frequency of bond stretch and eventually produce a specific wavenumber. For example, the band of solid and liquid CO2 occurs in between 2336 and 2345 cm−1; and the CO2 gas phase shows two distinctive bands at 2338 cm−1 and 2361 cm−1. This is due to the energy difference under vibrational and rotational motion of gas molecules. The modified Beer-Lambert Law equation is commonly used in geoscience for converting the absorbance in the IR spectrum into the species concentration:
ω = A M l ε ρ {\displaystyle \omega ={\frac {AM}{l\varepsilon \rho }}}
Where ω is wt. % of the species of interest within the sample; A is the absorbance of the species; M is the molar mass (in g mol−1); ϵ is molar absorptivity (in L mol−1 cm −1); l is sample thickness (in cm); ρ is density (in g mol−1) There are various applications of identifying the quantitative amount of volatiles by using spectroscopic technology. The following sections provide some of the examples:
Hydrous components in nominally anhydrous minerals Nominally anhydrous minerals (NAMs) are minerals with only trace to minor amounts of hydrous components. The hydrous material occurs only at crystal defects. NAMs chemical formulas are normally written without hydrogen. NAMs such as olivine and orthopyroxene account for a large proportion in the mantle volume. Individual minerals may contain only a very low content of OH but their total weight can contribute significant as the H2O reservoir on Earth and other terrestrial planets. The low concentration of hydrous components (OH and H2O) can be analyzed with Fourier Transform spectrometer due to its high sensitivity. Water is thought to have significant role in affecting mantle rheology, either by hydrolytic weakening to the mineral structure or by lowering the partial melt temperature. The presence of hydrous components within NAMs can therefore (1) provide information on the crystallization and melting environment in the initial mantle; (2) reconstruct the paleoenvironment of early terrestrial planet.
Fluid and melt inclusions
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