## 2 Materials and Methods
### Sample Preparation and Characterization  
The old and abandoned Teliuc tailings pond in Romania was explored with the aim of screening for the presence of certain CRMs (Figure 2). The pond is located at 45\({}^{\circ}\)44\({}^{\prime}\)08\({}^{\prime\prime}\) \(\mathrm{N}\) 22\({}^{\circ}\)53\({}^{\prime}\)01\({}^{\prime\prime}\) E and has an area of approximately 35 ha. The Teliuc pond contains approximately 5.0 \(\times\) 106 m\({}^{3}\) waste, resulting from iron ore processing through flotation. A large area of the Teliuc pond surface (Figure 2) is not covered by vegetation. However, this pond still pollutes the surrounding area, at least with flying dust. Additionally, it pollutes the soil and underground water as heavy metals leach into the environment when it rains. Screening for CRMs in the tailings was suggested by previous literature reports [37; 38]. We collected 40 samples from across the surface of the dump. The full characterization of the tailings is beyond the scope of this study, as we only address an example of how to apply the proposed data analysis model to the evaluation of CRM content in the pond.  
The gross sample of tailings collected weighed approximately 4 kg. The gross sample was crushed, ground, and milled until a powdered state was obtained using home-made laboratory crushers and grinding mills. The fraction that had been sieved using a 1 mm mesh was coned and then quartered, and a subsample weighing approximately 0.5 kg underwent advanced milling using a ball mill (Retsch GmbH, Haan, Germany). The milling process lasted until the residue on the sieve (65 \(\upmu\)m mesh) was less than 10%. The powder that passed through a 65 \(\upmu\)m mesh sieve underwent subsampling through coning and quartering, and approximately 50 g of the powder was selected for XRFS measurements. Scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS) was used to evaluate the tailings' heterogeneity, in terms of both mineralogy and elemental composition. This information is essential for a deeper understanding of the waste condition and to establish the methods for its adequate characterization. The SEM-EDS observations were performed using a Zeiss EVO 50 XVP (Carl Zeiss Microscopy GmbH, Jena, Germany) equipped with an LaB6 electron gun and an EDS accessory from Bruker. The oxide and elemental concentrations were measured through a Xepos ED(P)-XRF spectrometer (SPECTRO Analytical GmbH, Kleve, Germany). The decision to use the ED-XRFS technique was based on its figure of merit, i.e., its cost efficiency ratio. XRFS allows for  
Figure 2: Representative images collected from Teliuc tailing pond, Romania: (**a**) access to the pond; (**b**) edge with boulders and minor rocks; (**c**) marginal grass and lack of vegetation on the pond; (**d**) lunar aspect; (**e**) detail of lunar aspect; (**f**) solid lumpy waste.  
simple, rapid, and cheap sample preparation, while other approaches such as ICP-MS, ICP-AAS, and AAS require a complicated sample preparation process, such as sample dissolution or digestion, that may introduce a number of additional chemicals [38].  
The aliquots for XRFS measurements were prepared as pressed pellets. The pellets contained 7.25 g of tailings. The XRF spectra were processed with the TurboQuant Pellets analytical program.  
The pond contains mostly fine-grained tailings and granular and lumpy minerals (Figures 2 and 3).