# Exploring Residual Clays for Low-Impact Ceramics: Insights from a Portuguese Ceramic Region
## 3 Results and Discussion
Samples B1-2 and E1 showed higher KI values, possibly related to strong diagenesis and poor crystallinity, typical of the diagenetic zone, with sample B1 exhibiting the most advanced diagenesis. Group A falls within the anchorage, transitional between diagenesis and metamorphism, with the highest Al/(Fe + Mg) ratios. Chemical differentiation based on the I(002)/I(001) reflection intensity ratios revealed that illites from the samples of groups A and D, as well as B1 and C1, were aluminum (ratios > 0.3) [46]. In contrast, samples B2, C2, and E1 contained ferromagnesian (biotitic) illite (ratios < 0.3), which is not common and was probably inherited from the parent rock. The chemical composition of these illites also indicates distinct mica-group mineral affinities: group A and sample D1 exhibited compositions close to muscovite (KAl\({}_{2}\)(AlSi\({}_{3}\)O\({}_{10}\))(OH)\({}_{2}\)); B1, C1, and D2 were compositionally closer to phenige (KAl\({}_{1.5}\)(Mg,Fe)\({}_{0.5}\)(Al\({}_{0.5}\)Si\({}_{3.5}\)O\({}_{10}\))(OH)\({}_{2}\)); and B2, C2, and E1 resembled a mixture of biotite (K(Fe\({}^{2+}\)/Mg)\({}_{2}\)(Al/Fe\({}^{3+}\)/Mg/Ti)([Si/Al/Fe]\({}_{2}\)Si\({}_{2}\)O\({}_{10}\))(OH/F)\({}_{2}\)) and muscovite [47]. The samples' chemical compositions (Table 3) showed that alumina (Al\({}_{2}\)O\({}_{3}\)), closely associated with clay minerals such as kaolinite and illite, ranged from 17.72 (D1) to 27.68% (B2). Samples B1-2, with the highest alumina content (>27%), can be related with the kaolinite and illite minerals present (Table 2), confirming the classification as aluminous clays with high plasticity and ceramic reactivity. Samples A1-2 presented relatively high alumina (20-23%) and moderate Fe\({}_{2}\)O\({}_{3}\) (9-10%), supporting their kaolinite-smectite nature and reddish coloration. Samples C1-2 showed intermediate alumina (22%) and relatively low Fe\({}_{2}\)O\({}_{3}\) (8-9%), consistent with their beige-to-yellowish tone and balanced clay mineralogy. Samples D1-2 and E1 displayed lower alumina content (18-21%), higher Fe\({}_{2}\)O\({}_{3}\) (up to 14.7% in D1), and MgO (up to 5.3% in D2), with higher Na\({}_{2}\)O (up to 2.6% in D1), which may reflect the presence of Na-rich smectites (e.g., montmorillonite, (Na,Ca)\({}_{0.33}\)(Al,Mg)\({}_{2}\)(Si\({}_{4}\)O\({}_{10}\))(OH)\({}_{2}\)\(\cdot\)n\({}_{2}\)O) or Na-feldspars. The chemical composition supported the identified mineral phases, with higher smectite content and greater plagioclase abundance in these samples. The Fe\({}_{2}\)O\({}_{3}\) content in D1 and D2 suggested the influence of Fe-bearing phases such as siderite and pyrite, even if in low concentrations. These results, along with higher loss of ignition (LOI) values, indicative of carbonates and volatile-bearing clays, were consistent with the less plastic, carbonate-influenced materials, often used as structural fillers or in low-shrinkage ceramic formulations. The K\({}_{2}\)O present was related to a higher illite content or K-feldspar, \begin{table}
\begin{tabular}{l c c c c c c c c} \hline
**Mineral** & **A1** & **A2** & **B1** & **B2** & **C1** & **C2** & **D1** & **D2** & **E1** \\ \hline kaolinite & 31 & 26 & 55 & 50 & 40 & 43 & 7 & 15 & 8 \\ illite & 10 & 3 & 30 & 39 & 15 & 2 & 5 & 2 & 61 \\ smectite & 59 & 71 & 15 & 11 & 45 & 55 & 88 & 83 & 31 \\ \hline \end{tabular}
\end{table} 
Table 2: Mineral phases identified in <0.002 mm fraction (in %). which can contribute to early-stage fluxing behavior in ceramics [48; 49]. Similarly, the relatively high MgO and Fe\({}_{2}\)O\({}_{3}\) levels in D1-2 and E1 were related to the presence of ferromagnesian minerals, identified by I(002)/I(001) \(<\) 0.3, possibly inherited from the parent material. The higher LOI values in samples B1-2 were consistent with the clay mineral content, particularly kaolinite and smectite, which release structurally bound water during dehydrodxylation >500 \({}^{\circ}\)C [6], with the contribution of organic matter expected to be minimal based on the sample color and mineralogy.