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
(CO32-) 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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