The year 2003 showed a renewed interest in the use of vibrational spectroscopy and, in particular, the application of infrared and Raman spectroscopy to study clay minerals. At the workshop held at the 2003 Clay Minerals Society/Mineralogical Society of America conference in Athens, Ga., a rather unknown technique called infrared emission spectroscopy (IES) was discussed in detail as a means to study thermal behavior of clay minerals. So, how can we use vibrations of molecules to study, for example, phase transitions?

Infrared emission spectroscopy

To study clay-mineral changes such as dehydration and dehydroxylation, samples are usually heated to a certain temperature and rapidly cooled down; spectroscopic analysis then takes place at room temperature. Although this procedure often provides useful information, we cannot guarantee that the mineral structure does not change during the cooling process before the spectral measurement. Smectites, for example, are known to quickly rehydrate when exposed to moist air. Infrared emission spectroscopy, therefore, offers a useful technique that can be applied in situ during the heat treatment. The measurement of discrete vibrational frequencies emitted by thermally excited molecules, known as Fourier Transform Infrared Emission Spectroscopy (FTIRES or IES) has not been widely used for the study of mineral structures. The major advantages are that the samples are measured in situ at elevated temperatures and IES requires no sample treatment other than making the sample of submicron-particle size.

IES technique

The infrared spectrometer has an emission cell consisting of a modified atomic absorption graphite rod furnace. A platinum disk acts as a hot plate to heat the sample and is placed on the graphite rod. In this example, an insulated thermocouple was embedded inside the platinum plate. A proportional temperature controller attached to the thermocouple held the operating temperature of the sample at about 2 degrees Celsius.

The cell's design is based on an off-axis paraboloidal mirror mounted above the heater, which captures the infrared radiation and directs the radiation into the spectrometer. The spectrometer has been modified by removing the source assembly and mounting a gold-coated mirror at an angle of 45 degrees, enabling the infrared (IR) radiation to be directed into the FTIR spectrometer.

The single-beam reference spectrum obtained during the recording of a normal infrared transmission spectrum is actually the emission spectrum of the instrument source, modified by the instrument response function. Transmission spectra are obtained by ratioing the signals in the absence and presence of the sample. Similarly, emission spectra are obtained by ratioing the emission signal of the sample to that of a reference, usually a blackbody source that emits radiation according to Planck's radiation formula.

In the normal course of events, three sets of spectra are obtained: First, the blackbody radiation (actually greybody radiation) over the temperature range is selected at various temperatures; second, the platinum plate (sample holder) radiation is obtained at the same temperatures; and third, the spectra from the platinum plate covered with the sample are collected. The emittance spectrum at a particular temperature is calculated by subtracting the single beam spectrum of the platinum plate from that of the platinum-plus sample, and the result is ratioed to the single beam spectrum of an approximate blackbody (graphite).

An example

In the 2002 Geotimes Highlights issue, Ray Frost described the preparation and characterization of hydrotalcite. IES can analyze such a hydrotalcite (Mg₆Al₂ (OH)₁₆(CO₃)4H₂O), determining its residual soil components. In the region below 1,000 cm^-1 (wavenumbers), magnesium/aluminum-hydrotalcite showed a major change around 350 degrees-400 degrees Celsius. The bands disappeared at 772, 923 and 1029 cm^-1, corresponding to the aluminium hydroxyl-like out-of-plane deformation and doublet vibrational deformation modes. New bands were observed at 713, 797 and 1075 cm^-1. The first and last band, accompanied by the band at 545 cm^-1, indicated the formation of spinel (MgAl₂O₄). The band around 713 cm^-1, however, is also very close to is very close to one of the characteristic vibrations of magnesium oxide at 717 cm^-1. X-ray diffraction of the heated hydrotalcite confirmed spinel and MgO as breakdown products.

The 1,341 and 1,398 cm^-1 bands, associated with the interlayer carbonate (CO₃^2-) anions show a shift toward higher wave numbers at 300 degrees-350 degrees Celsius, followed by either a disappearance or a decrease in wave numbers between 600 degrees-650 degrees Celsius. Above 300 degrees, the 1,341 cm^-1 band has disappeared, whereas the other band remains visible although it strongly diminishes in intensity. This suggests the presence of dissimilar carbonate species reacting differently upon heating. One possibility is that the interlayer carbonate ions are located near different cations in the hydroxide sheets. Alternatively, the carbonate ions are located at similar positions near the hydroxide surface, but one as a true interlayer anion and the other as an adsorbed anion on the outer surface of the crystal.

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