# Exploring Residual Clays for Low-Impact Ceramics: Insights from a Portuguese Ceramic Region
## 3 Results and Discussion
Although the bright orange hues in samples B1 and B2 may suggest elevated Fe content, the mineralogical analysis revealed only trace amounts of hematite (Fe\({}_{2}\)O\({}_{3}\)) and goethite (Fe\({}^{3+}\)O(OH)), and a low presence of siderite (FeCO\({}_{3}\)) and pyrite (Fe\({}_{5}\)) in B1, suggesting that the col  
Figure 2: Samples’ relative granulometric distribution. oration may be influenced by finely dispersed or amorphous Fe phases not detected by XRD [43]. The beige tone of sample C1, despite a moderate phyllosilicate content and no detectable hematite or goethite, may be attributed to the higher presence of quartz and plagioclase, along with low concentrations of Fe-bearing minerals. Samples D2 and E1, darker in color, contained low Fe-bearing phases (siderite, pyrite in D2) and may reflect post-depositional transformation. The clay minerals identified in the <0.002 mm fraction were kaolinite (26-31%; Al\({}_{2}\)Si\({}_{2}\)O\({}_{5}\)(OH)\({}_{4}\)), little (K\({}_{0.65}\)Al\({}_{2.0}\)(Al\({}_{0.65}\)Si\({}_{3.35}\)O\({}_{10}\))(OH)\({}_{2}\)), and smectite (e.g., montmorillonite (Na,Ca)\({}_{0.33}\)(Al,Mg)\({}_{2}\)(Si\({}_{4}\)O\({}_{10}\))(OH)\({}_{2}\cdot\)nH\({}_{2}\)O) (Table 2). Samples A1-2 presented high smectite (59-71%), and kaolinite (26-31%) content, suggesting high plasticity and water retention capacity. These results support their classification within [PERSON]'s zone 3, although the smectite content may require careful drying to prevent cracking. Samples B1-2 exhibited a mixed assemblage with high kaolinite (50-55%) and illite (30-39%) contents and low smectite (59-71%) content. \begin{table}
\begin{tabular}{l c c c c c c c c c} \hline \hline
**Mineral** & **A1** & **A2** & **B1** & **B2** & **C1** & **C2** & **D1** & **D2** & **E1** \\ \hline quartz & 17 & 17 & 10 & 8 & 20 & 20 & 6 & 10 & 8 \\ phyllosilicates & 61 & 60 & 77 & 81 & 38 & 39 & 46 & 41 & 52 \\ K-feldspar & 4 & 2 & 1 & 2 & 3 & 7 & 4 & 6 & 3 \\ plagioclase & 2 & 7 & 2 & 2 & 21 & 15 & 16 & 14 & 13 \\ calcite & - & 2 & - & 1 & 1 & tr & 9 & tr & 2 \\ dolomite & 2 & 3 & 1 & 3 & 3 & 4 & 2 & tr & 2 \\ opal C/CT & 2 & 5 & 2 & 2 & 10 & 8 & 12 & 10 & 11 \\ zeolite & 3 & - & - & - & 1 & 2 & - & - & 2 \\ goethite & tr & tr & tr & tr & - & - & - & - & - \\ anhydrite & 2 & 1 & 1 & tr & 2 & 2 & 1 & 1 & 1 \\ siderite & 2 & 1 & 5 & tr & tr & 1 & 1 & 3 & 2 \\ pyrite & 1 & 1 & tr & tr & tr & 1 & 1 & 7 & tr \\ anatase & 3 & - & tr & tr & 1 & - & - & - & - \\ hematite & 1 & 1 & 1 & tr & - & tr & - & - & 1 \\ amphibe & - & - & - & 1 & - & 1 & 2 & 8 & 3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mineral phases identified in <0.063 mm fraction (in %; tr—trace). Figure 3: Samples projected in the Winkler diagram with suitable fraction mixtures for manufacturing: zone 1—solid bricks; zone 2—perforated bricks; zone 3—roofing tiles and masonry bricks; and zone 4—hollow brick wall and pavement [39; 40; 41]. tite (11-15%) content, suggesting good workability with some reduced swelling potential. The illite content in B2 can enhance dry strength but may reduce plasticity compared to pure kaolinite clays [44]. Samples C1-2 displayed balanced proportions of kaolinite (40-43%) and smectite (45-55%), with low illite (1-15%) content, and is thus expected to have good results in ceramic processing, combining good plasticity with moderate shrinkage and a good firing response. Samples D1-2 contained significantly more smectite (83-88%) and less kaolinite (7-15%), while E1 presented 61% illite, which enhances mechanical strength and thermal resistance but results in lower plasticity. The abundance of smectite in D1-D2 can contribute to plasticity but also raises concerns about volumetric instability unless managed via tempering. The crystallinity of illite was evaluated using the Kubler Index (KI), with results suggesting that samples from groups C and D exhibited the lowest KI values, indicative of a higher structural order [45]. These samples can be associated with the epizone, a low-grade metamorphic zone where illite recrystallizes toward muscovite.