Characterisation of minerals using evolved gas analysis and infrared spectroscopy.

Several methods have been used for the characterisation of minerals in powders and in rocks. Particular emphasis has been directed towards the use of chemical probes to elucidate the surface mineralogy in order to provide detection profiles suitable for both qualitative and quantitative chemometric analysis. The first probe evaluated was cyclohexylamine (CHA). Ni[2+]- and Al[2+]-exchanged montmorillonite (SWy-2 -Wyoming, USA), were exposed to cyclohexylamine vapour (CHA). The samples were characterised by X-ray diffraction (XRD), thermogravimetric analysis (TGA) and evolved gas analysis (EGA), which included TGA, real time gas phase FTIR (RT-GP-FTIR), real time mass spectrometry (RT-MS) and organic trap module-gas chromatography-mass spectrometry (OTM-GC-MS). CHA decomposed along different routes via interrelated mechanisms that were cation dependent. Over Al[3+]-SWy-2 hydrodenitrogenation via the Hofmann degradation was the predominant route whereas ring dehydrogenation to aniline either directly from CHA or possibly via cyclohexylimine was significant over Ni[2+]-SWy-2. Very little unmodified cyclohexylamine was detected in the evolved gases which means that the desorption of CHA cannot be used as a quantitative measure of the acidity of cation-exchanged clays. Nonetheless, as a qualitative probe, CHA has the potential to distinguish the difference between Ni[2+] and Al[3+]-SWy-2.The second probe evaluated was 3-cyanopyridine (3CYP). TGA has shown that temperature programmed desorption (TPD) of 3CYP is capable of distinguishing between the Ni[2+]- and Al[3+] exchanged forms of SWy-2. 3CYP was found to be unsuitable for a variety of reasons highlighted by its ability to condense within the EGA system and the long incubation times necessary to produce detectable thermal events.The most suitable probe used so far is Dimethylformamide (DMF), being able to intercalate quickly and showing no evidence of modification. Principal component analysis (PCA) and partial least squares (PLS) modelling techniques were applied to the EGA DMF detection profiles of four clay mineral groups. Each group was successfully characterised and provided valuable information about intra and inter group relationships. The use of RT-MS detection profiles for ions m/z =18 and 73 (DMF parent ion) were shown to have the most potential for more sensitive quantitative applications.Application of quantitative PLS modelling to mixed mineral standards has shown that it is possible to estimate the clay mineral constituents in both powders and rocks. The use of the ion m/z = 73 provided an R[2] value of 0.98. Use of ion m/z =18 (H[2]O) has perhaps shown most potential, due to its ability to produce training sets with excellent correlation coefficients of 0.95 for SWy-2, 0.97 for KGa-2 and 0.95 for CCa-1. However, because SWy-2 and 1-Mt (illite) demonstrate overlapping detection of DMF (where uptake of DMF by 1-Mt is comparatively small), it is impossible to directly separate their relative contributions at the concentration levels under study. The use of mass spectrometry has shown that it is possible to detect to levels of 1% with considerable scope for detecting smaller levels.Environmental scanning electron microscope (ESEM) and energy dispersive X-ray analysis (EDX) of cation exchanged SWy-2 and a sandstone rock exposed to 2-bromopyridine (2BPY) have shown the potential of using the bromine functional group as a tag to locate the presence of swelling minerals in rocks by EDX.