The Boda Claystone Formation (BCF) is an extended sedimentary sequence formed in a shallow-water salt lake under semi-arid to arid climatic conditions during the middle Permian period. The rock was formed predominantly from denuded and altered products of three primary felsic sources, the Mórágy Metagranite Complex, the Baksa Metamorphic Complex and the rhyolitic Gyűrűfű Formation, resulting in the recent dominant sheet silicate components, illite and chlorite. BCF has been considered a potential host rock for high-level nuclear waste, too. Thus, it has been characterized by several powerful methods so far (X-ray diffraction (XRD), transmission electron microscopy (TEM), etc.). ⁵⁷Fe Mössbauer spectroscopy may provide a unique additional tool to study iron-bearing minerals. Iron is dominantly present in a ferrous form in minerals of the fresh parent rocks (in the biotite group and amphibole), and in a ferric oxide, hematite, in altered Gyűrűfű Formation. During transformations of biotite group minerals and amphibole, the partial release of ferrous iron or its conversion to ferric form takes place with the stabilization of recent illite and chlorite, while the original layered structure is still preserved. Mössbauer spectroscopy revealed the dominant presence of ferrous iron located in cis-M2 octahedral sites both in parent biotite group minerals and in the final illite, as well as chlorite in both stages. The proportion of ferrous iron in biotite group minerals was halved during the stages of evolution by conversion to ferric iron still in sheet silicate illite or by segregation into separate hematite inclusions. The transformation process of biotite group minerals and amphibole of the source rocks is connected only to the iron-bearing smaller fraction of sheet silicates in the BCF clay mineral assemblage. Determination of Fe²⁺/Fe³⁺ ratios in sheet silicates was also pertinent in two sections of BCF. Namely, in samples from the Gorica region, Fe³⁺ was dominant, siting in illite, whereas Fe²⁺ was also present in significant portions in chlorite in samples from the Western Mecsek Anticline. The interpretation is deduced in correspondence with results of extended XRD, and high-resolution TEM studies.

Advantageous properties of Mössbauer spectroscopy allow the method to be used for the examination of industrial catalysts as well. The method is confined for catalysts containing Mössbauer nuclei (⁵⁷Fe and ¹¹⁹Sn in most practical cases), on the other side the obtained information is rather unique. Some further limitations emerge in studies for industrial catalysts with this technique in comparison to common catalyst studies, namely the extreme operating conditions, i.e. the elevated temperature and pressure. Thus the in situ conditions for studies cannot be completed easily. To circumvent this difficulty the states of catalysts prior and after the usage can be compared (ex situ conditions). Further, since real catalysts are used for long time period in the industry, their studies can be replaced by accelerated studies performed under ‘conditions relevant to real processes’. Beside catalysis, recycling and reuse of former industrial waste is also an important field for application of the method. A personal selection from reports from recent years on application of the method in representative large scale industrial processes is presented in the overview.

Acetone-stabilized Au- and Sn-solvated metal atoms (SMAs) were used as to obtain Au- and AuSn-supported catalysts by simple impregnation on a reducible (TiO₂) and a non-reducible (Al₂O₃) metal-oxide. Their catalytic behaviour was investigated for cyclohexane oxidation to cyclohexanol and cyclohexanone (KA oil), and their morphological and physical properties were studied by TEM, STEM-EDS and ¹¹⁹Sn-Mössbauer spectroscopy. The catalytic results firstly demonstrated that the bare supports played a role on the reaction mechanism, slowing down the formation of the oxidation products and directing the radical formation. Hereinafter, the comparison between the monometallic Au-supported catalysts and the corresponding bimetallic Au-Sn catalysts allowed for the understanding of the potential role of Sn. ¹¹⁹Sn-Mössbauer characterization analyses showed the presence of SnO_2, which was recognized to favour the electrons’ exchange to form radicals, interacting with oxygen. Such interaction, in particular, could be favoured by the co-presence of Au. Moreover, the same metal composition on the catalyst surface resulted in a different catalytic behaviour depending on the support.

Mössbauer spectroscopy can be advantageous for studying catalysts. In particular, its use in in situ studies can provide unique access to structural features. However, special attention must be paid to the interpretation of data, since in most studies, the samples are not perfectly homogeneous. Balance and compromise should be found between the refinement of evaluations by extracting and interpreting data from spectra, while also considering the presence of possible inhomogeneities in samples. In this review, examples of studies on two types of catalysts are presented, from which, despite possible inhomogeneities, clear statements can be derived. The first example pertains to selected iron-containing microporous zeolites (with ⁵⁷Fe Mössbauer spectroscopy), from which unique information is collected on the coordination of iron ions. The second example is related to studies on supported PtSn alloy particles (with ¹¹⁹Sn probe nuclei), from which reversible modifications of the tin component due to interactions with the reaction partners are revealed.

Electric explosions of amorphous ribbons (Fe₄₅Co₄₅Zr₇B₃ HITPERM, Fe_73.5Si_15.5B₇Nb₃Cu₁ FINEMET, and bulk amorphous Fe_71.6Mn_0.6Si_3.4C_12.3B_12.2) were studied in water and for the bulk amorphous alloy in ethylene glycol, in order to study whether the amorphous state can be preserved in the process. Formed products were collected and analyzed by Scanning Electron Microscopy, X-ray diffraction, Mössbauer spectroscopy, and High-Resolution Transmission Electron Microscopy. Chemical reactions have taken place in large extent between melted and evaporated components of ribbons and the decomposed cooling media. The oxidation reactions removed quickly the glass-forming elements (Zr, B, Si) from the metallic particles. Oxides were formed both on surface of globules and in separate phases from evaporated components. The amorphous state was partly retained in FINEMET, and to a greater extent in bulk amorphous alloy. Chemical interactions were more limited in ethylene glycol and carbon atoms formed from the decomposed coolant contributed to stabilization of amorphous phase. Graphical abstract