Performance of analytical techniques (SWIR imaging, XRD, EPMA) for the identification of minerals frequently formed during natural and technological geothermal processes

This study compares the performance of three analytical methods, hyperspectral imaging in the short-wave infrared range (SWIR), X-ray powder diffraction (XRD), and electron-microprobe analysis (EMPA) for the identification of minerals frequently formed during natural and technological geothermal processes. The samples from three geothermal sites in Indonesia contain a spectrum of minerals, which are characteristic for geothermal activities (alunite, jarosite, cristobalite, tridymite) and/or are involved in scaling-forming processes in geothermal plants (barite, celestine, common opal, calcite). We show and discuss the strengths and weaknesses of the individual techniques with respect to properly identifying these minerals and to approximate their relative proportions. The SWIR camera permits an unambiguous identification of only a small selection of the geothermally relevant minerals, which restricts its employment as routine tool in the context of geothermics. XRD and EPMA are more powerful techniques and deliver complementary information. In case that SWIR inactive species occur, (b) the entire spectrum of minerals present in a sample has to be unambiguously identified, (c) amorphous substances are present, (d) accurate mineral compositions are required, and (e) detailed information on minerals occurring in low abundance and grain size is warranted, the combined use of XRD and EMPA is mandatory.