This predominance of clay minerals such as kaolinite (aluminium
silicate) and mica (pyllosilicate) is also observed in the chemical
analysis data where the ratio between silica and alumina
concentration (SiO2/Al2O3) is about 2 in agreement with other
studies. (Souza et al., 2005). The low percentage of alkaline oxide
(Na2O+K2O) with respect to the alkaline-earth indicates a minor
presence of fluxing agents. In general, the K2O content in clay is
higher than the content of Na2O, since micaceous minerals are
more resistant to weathering (Souza et al., 2011). On the other
hand the presence of CaO and MgO, derived from calcite (CaCO3)
and dolomite (CaMg(CO3)2), are typical components of clays for
bricks. The considerable amounts of iron and calcium oxides are characteristics of the clays of ferruginous–calcareous type responsible
for the development of the brick red color after firing