Bentonite Mineralogy Part 1: Methods of Investigation - Posiva

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3.2 Differential thermal analysis {DTA)
Differential thermal analysis is used for registration the temperatures of endothermic
and exothermic reactions. It is often coupled with TGA, thermogravimetric analysis that
registers changes in sample weight. The furnace heating rate can be chosen. Post and
Borer (2002) used heating rate 7°C/min and noticed, that the endotherm temperature
correlated with the interlayer cation of mixed-layer illite/beidellites and beidellites.
DTA gives information about the thermal stability of clay minerals but effects of longterm
exposition to elevated temperatures cannot be simulated. For instance hydration
water is lost with heating to 11 0°C if the heating is prolonged over 24 hours. At normal
heating rate in DT A, 1 0°C/min, some hydration water still exist in clay minerals heated
to 300°C, even if most of it is lost below 200°C. Structural water is lost between 500
and 700°C.
The thermal stability of dioctahedral smectites consisting of cis-vacant 2:1 layers are
characterized by dehydroxylation temperatures of 150-200°C higher than those for the
same mineral consisting of trans-vacant 2:1 layers, ea 700°C vs ea 550°C, respectively
(Drits et al., 1995). In case distinction between these two types is considered necessary,
the use ofDTA is recommended.
Recommendation:
Use of DT A/TGA is recommendable only if the distinction between cis- and transvacant
varieties is considered as necessary.
3.3 Fourier transform infrared spectroscopy {FTIR)
Infrared spectroscopy is best suitable to investigation of compounds with simple
chemical formula. Clay minerals are chemically related and IR spectra of a mixture of
them are difficult to interpret. Bentonites are mineralogically simple, although
chemically complicated, and FTIR can be used especially for comparing samples with
each other. Because the amount of sample used for pressing the pellets is very small (1 -
3 mg of sample/1 00 - 300 mg of KBr) clay fraction should be used. It also contains less
detrital minerals like quartz and feldspars. If it is necessary to record the IR spectra of
bulk samples they have to be homogenized carefully. In a series of papers on bentonite
mineralogy, no information is given on the fraction used for FTIR. Apparently, bulk
material has been used. It is possible that some information can be obtained of the
amorphous compounds present.
Madejova & Komadel (2001) recommend the use of two different sample/KBr ratios for
pressing the pellets: 2 mg of sample and 200 mg ofKBr to record optimal spectra in the
region of 4000-3000 cm- 1 and 0,5 mg of sample and 200 mg of KBr to record optimal
spectra in the region 4000-400 cm- 1 • Discs for the 4000-3000 cm- 1 were heated
overnight at 150°C to minimize the water absorbed on KBr and the clay sample. A list
ofiR-bands in two Wyoming bentonites are given in table 3-4.