# Physical-Chemical and Thermal Properties of Clays from Porto Santo Island, Portugal
## 4 Results and Discussion  
### Morphology and Grain Size Analysis  
The seven residual soil samples from Porto Santo Island show different grain size distributions (Table 1). The results indicate that Samples 5, 26, 36A and 119 contain 81%, 76%, 67% and 50%, respectively, of silt + clay fraction (<63 \(\upmu\)m), which means fine granularity and better profitability compared to Samples 14, 14A and 15A, which contain sand fraction concentrations above 50%. The concentrations of fine particle fraction (silt + clay) in Samples 5, 26, 36A and 119 are lower than the samples of Porto Santo Island studied by [PERSON] et al. (2013), which in their case was 90%. According to [PERSON] et al. (2015) and [PERSON] (2017), samples suitable for application in thermal treatments and dermocosmetics must have a high fine-particle fraction concentration. Studies carried out in the Spanish Thermal Centre show that in most of these places, the healing clays applied in pelotherapy have around 57-70% of particles sized between 2 and 20 \(\upmu\)m ([PERSON] et al., 2018).  
Moreover, the samples should have specific surface area values greater than 10 m\({}^{2}\)/g ([PERSON] et al., 2013), also found in residual soils from Porto Santo Island, which present specific surface area values ranging between 39 and 90 m\({}^{2}\)/g (Table 1). Samples 14, 15A, 26 and 36A show the highest values of the specific surface area from 89 m\({}^{2}\)/g to 90 m\({}^{2}\)/g (Table 1). These samples have similar values to the samples studied by [PERSON] et al. (2013) in same region, which in their case was 79 m\({}^{2}\)/g, although these values are significantly lower when compared to [PERSON] et al. (2013) and [PERSON] et al. (2013), who obtained 119 m\({}^{2}\)/g. In addition, the specific surface area values are also within the range of values obtained from previous studies about thermal treatments and dermocosmetics ([PERSON] et al., 2018; [PERSON] et al., 2018). These values are due to the highest clay particle fraction, and thus the presence of clay minerals. Samples 26 and 36A have a high ratio between the content of fine particles fraction and specific surface area, although it is advisable that these samples undergo a maturation process or mechanical grinding to obtain a finer and more regular grain in order to be suitable for therapeutic applications.  
The SEM images of residual soil samples from Porto Santo Island show two different morphologies with different shapes and sizes (Figure 2a,b). Samples with less than 50% silt-clay fraction (14, 14A and 15A) exhibit a heterogeneous morphology with different particle sizes, with some grains more rounded and others more elongated (Figure 2a). Furthermore, the samples with a higher fine-particle fraction concentration (5, 26, 36A and 119) exhibit a more homogeneous morphology, with the fine particles displaying a scaly arrangement (Figure 2b).  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Soil Samples** & **\% Clay** & **\% Silt** & **\% Sand** & **Soil Texture** & **Specific** \\
**(\(<2\)\(\upmu\)m)** & **(\(<63\)\(\upmu\)m)** & **(\(>63\)\(\upmu\)m)** & **(USDA, 2017)** & **Surface Area** \\ \hline
5 & 14 & 67 & 19 & Silt loam & 83 \\
14 & 6 & 30 & 64 & Sandy loam & 90 \\
14A & 2 & 20 & 78 & Loamy sand & 39 \\
15A & 8 & 30 & 62 & Sandy loam & 90 \\
26 & 15 & 61 & 24 & Silt loam & 90 \\
36A & 11 & 56 & 33 & Silt loam & 89 \\
119 & 5 & 45 & 50 & Sandy loam & 76 \\ Smectite * & - & - & - & - & 150–800 \\ Pelotherapy & - & - & - & - & \textgreater{}10 \\ Applications ** & & & & & \\ \hline \end{tabular}  
* not defined.  
\end{table}
Table 1: Grain size distribution and specific surface area of residual soil samples from Porto Santo Island as well as theoretical values for smectite and pelotherapy applications.