## 3 Results and Discussion
### Fixed Bodies  
Table 4 show the mineral quantification of analyzed fired bodies for each temperature. All analyzed samples contain a high percentage of kaolinite and illite. The presence of micaceous minerals along with kaolinite is quite common in many ceramic clays [12]. This is the case for all the analyzed samples that contain illite. Therefore, in this section, we consider the firing transformations experienced by clayed mine spoils. In other words, the formation of mullite and other high-temperature phases were studied from mine spoils that have variable proportions of illite (between 4% and 50%) and/or chlorite (<5%). Cristobalite was not detected as a high-temperature phase in either of the series (OE or AA). Table 4 summarizes the aforementioned results regarding the observations made with FTIR (Figure 3) and XRD (Figure 4). More or less significant amounts of amorphous material and glassy phase were detected in all fired samples. According to the literature [13,14], there is no interaction between the kaolinite and illite particles of clays fired under 1000 \({}^{\circ}\)C. Mullite formation after high-temperature illite breakdown has been studied in phyllosilicate-rich bricks [15]. At T \(\geq\) 900 \({}^{\circ}\)C, allite dehydroxylation is followed by partial melting that triggers the nucleation and growth of mullite [15]. This absence of interaction would be the cause behind the temperature of formation of the spinel phase being separate from the illite content. Therefore, the mullitization process can be explained independently. The creation of  
Figure 4: XRD diagrams of test bodies heated at different temperatures. Legend: Q: quartz; H: hematite; M: mullite. Figure 3: FTIR experimental spectra for fired sample bodies and their calculated FTIR spectra. Temperatures: 995 \({}^{\circ}\)C (**a**) and 1150 \({}^{\circ}\)C (**b**). multite from illite at a lower temperature [15] than the one that leads to the transformation of kaolinite can be attributed to the effect of potassium and other alkaline or alkaline-earth cations present in (or near) the porous hexagonal network of the tetrahedral sheets (containing Si) in 2:1 layers of illite. The effect of K in these positions is similar to the role conducted by this element in glass formation. As the Si-O bonds break, the potassium ions move, keeping the structure open and not allowing atoms to bond back together. As a result, the stability of the tetrahedral layer is significantly reduced compared to its normal stability, which facilitates the structural transformation into mullite at a low temperature. Potassium, therefore, decreases the activation energy required to create mullite. The relative amount of alumina present in illite that is available for the formation of mullite at high temperatures is lower than in kaolinite. As a result, in mine spoils with a high proportion of illite/kaolinite, the amount of mullite that can be created in the fired tile bodies is considerably smaller than in pure kaolinite. The non-existence of cristobalite in clays that are rich in illite can be explained by taking into consideration the chemical composition of illite. By increasing the content in illite, when going from pure kaolinites to mixtures and pure illite, the amount of silica available for the formation of cristobalite also increases slightly, but the content of alkaline metals that create glass increases much more. As a result, the absence of cristobalite is attributed to the presence of glass-forming cations supplied by the illite. At 995 \({}^{\circ}\)C, the formation of a noticeable amount of mullite can be attributed to the high content of illite. Even so, lengthy firing times are needed to be able to observe clearly-defined reflections of mullite in the diffractogram. At 1050 \({}^{\circ}\)C, the 55 \({}^{\circ}\)C rise in firing temperature causes an increase in the percentage of mullite in both samples, and the formation of mullite is much more intense. These samples show the most intense and closest reflections.