# Physical-Chemical and Thermal Properties of Clays from Porto Santo Island, Portugal
## 4 Results and Discussion
### Chemical and Mineralogical Analysis  
The residual soil samples from Porto Santo Island show SiO\({}_{2}\) concentrations ranging from 41.36% (Sample 14A) to 54.02% (Sample 5) (Table 2); therefore, all samples have a siliciclastic composition, which is coherent with their mineralogical composition (Table 3). The phyllosilicate contents (smectite) are higher than 50% in all samples, with Sample 36A having the highest concentration (82%) and Sample 14A the lowest concentration of phyllosilicates (55%) (Table 3), which is in line with the predominant fine-grained samples (Figure 2). According to [PERSON] et al. (2018) and [PERSON] et al. (2018); [PERSON] et al. (2018), the mineralogical composition of the samples collected on Porto Santo Island shows predominant phyllosilicate contents, above 50%, and in some cases, these contents reach 70% to 90%. Al\({}_{2}\)O\({}_{3}\) varies from 12.52% (Sample 14A) to 17.65% (Sample 5) (Table 2), associated with the hydrated phyllosilicates, specifically smectite, which is present in all samples, and traces of kaolinite also found in Sample 14A (Table 3). The soil samples from Porto Santo Island also have in their mineralogical composition K-feldspars, plagioclase, calcite, Fe-oxides (magnetite/maghemite and hematite), opal, anatase and quartz (Table 3), in accordance with [PERSON] et al. (2018) and [PERSON] et al. (2018); [PERSON] et al. (2018). The concentrations of CaO vary between 4.20% (Sample 36A) and 6.12% (Sample 26) and are related with the presence of calcite (Tables 2 and 3), as is their occurrence in the structure of phyllosilicates as exchange cations. CaO and Na\({}_{2}\)O (which vary from 4.20% to 9.94% and 1.57% to 2.62%, respectively) are present in the structure of the plagioclase, but also in smectite. MgO has concentrations between 3.19% (Sample 26) and 6.01% (Sample 14A) and can be found in the structure of phyllosilicates (smectite) (Table 3) in the octahedral sheet or as exchange cations. Previous studies carried out on Porto Santo Island (2018, 2018, 2018) support the major presence of SiO\({}_{2}\) (45% to 55%), Al\({}_{2}\)O\({}_{3}\) (17% to 19%), Fe\({}_{2}\)O\({}_{3}\) (8% to 12%), CaO (2% to 6%) and MgO (3% to 4%) in the chemical composition of the samples. Figure 2: SEM images (ampliation 150\(\times\) and resolution 15 keV) of Sample 14 (**a**), with different particles sizes and shapes, and Sample 36A (**b**), with a homogeneous morphology. The hydrolysis of aluminous silicates (including feldspars) corresponds to the chemical reaction between water and the mineral with the occurrence of cationic substitutions in the mineral structure by hydrogen ions (H\({}^{+}\) and OH\({}^{-}\)) and is the process responsible for the formation of phyllosilicates (3; 40). Therefore, most residual soil samples that have higher content in phyllosilicates (14, 15A, 36A and 119) present lower content in feldspars and thus a higher degree of maturity than those with higher feldspar contents (5, 14A and 26). The CIA values (Table 2) of the samples are between 49.29% (14A) and 70.33% (36A), which means that the sample with the highest value (36A) is richer in clay minerals and Fe-oxides/hydroxides than the sample with the lowest CIA value (Sample 14A), as evidenced by the mineralogical composition of the samples (Table 3). However, in the case of the sample with lowest CIA value (14A), it is richer in primary minerals such as feldspars. These values are slightly lower when compared to other studies on volcanic soils (29). The presence of magnetite/maghemite and hematite in these samples explains the concentrations of Fe\({}_{2}\)O\({}_{3}\) (8.20-12.50%) (Tables 2 and 3), as well as the reddish colour of the samples. Opal is present in most samples (except in Sample 36A). According to [PERSON] and [PERSON] (2011), the common clays formed by submarine diagenesis do not have quartz in their mineralogical composition, as it can be seen in XRD graphs (Figure 3a,b), although Sample 26 is the only one with minor concentrations of quartz.