## 3 Results  
### Characterization of Construction Materials  
The three schist samples (weakly, moderately and strongly weathered) can be described as micaceous quartz schist with marked foliation, composed of micaceous minerals (biotite, muscovite and chlorite), quartz and accessory minerals (Figure 3, Table 3). Thin-section analysis of the weakly weathered schist showed that the biotite was formed by crystals of a tabular-laminar habit (0.1-0.8 mm) with brown-yellow pleochromism and frequent pleochroic halos; no alteration processes were observed (Figure 3a). Muscovite, with crystals of 0.06 to 0.15 mm, was interwoven with biotite. Isolated chlorite crystals (0.10-0.45 mm) were scarce; however, chlorite blasts (1.2 \(\times\) 0.7 mm)-(1.6 \(\times\) 1.0 mm) were frequent, constituted in turn by aggregates of chlorite crystals, quartz crystals, biotite and some iron ore. Quartz occurred in allotriomorphic crystals that tended to be elongated and occupied the intercrystalline positions of micas. These crystals varied in size from 0.05 to 0.25 mm, although larger crystals were also observed (0.5 \(\times\) 0.10 mm). Accessory minerals were frequent, particularly apatite in rounded and elongated crystals (0.05-0.15 mm), zircon in small, rounded crystals (0.03 mm) as inclusion in biotite, opaque iron minerals in small sections of sizes from 0.05-0.01 mm and feldspars forming part of the chlorite blasts. Thin-section analysis of the moderately weathered schist showed that the biotite was formed by crystals of a tabular-laminar habit (0.10-0.25 mm) with distinct pleochroism (from brown-yellow to white) bordering the quartz crystals. No obvious alteration processes were observed in biotite crystals. Muscovite occurred in colourless tabular crystals associated with biotite. The crystals varied in size from 0.12 to 0.20 mm. Chlorite appeared in tabular crystals ranging from 0.12 mm to 0.25 mm, forming aggregates of chlorites or chlorite blasts (of sizes from 1.0 \(\times\) 0.9 mm-2.4 \(\times\) 1.6 mm) made up of aggregates of chlorite and quartz crystals. Rock foliation tended to deform around the grains. The following accessory minerals, which were relatively frequent and represent a source of metamorphic gramitic-pegmatitic materials, were detected: apatite, in rounded and elongated crystals (0.08-0.15 mm); tourmaline, in sub-rounded crystals (0.05-0.15 mm); zircon, in small, rounded crystals (0.03-0.05 mm); iron minerals, in small sections (0.02-0.08 mm); and, feldspars forming part of the chlorite blasts. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Mineral**} & \multicolumn{3}{c}{**Degree of Weathering of Schist**} \\ \cline{2-4}  & **Weak** & **Moderate** & **Strong** \\ \hline Quartz & 31.6 \(\pm\) 4.1 & 37.0 \(\pm\) 2.8 & 30.3 \(\pm\) 5.0 \\ Biotite & 19.7 \(\pm\) 2.8 & 26.1 \(\pm\) 2.4 & 37.9 \(\pm\) 3.6 \\ Muscovite & 28.9 \(\pm\) 2.2 & 17.4 \(\pm\) 1.8 & 18.2 \(\pm\) 1.5 \\ Chlorite & 14.5 \(\pm\) 1.1 & 13.0 \(\pm\)1.0 & 10.6 \(\pm\) 0.9 \\ Accessory minerals & 5.3 \(\pm\) 0.3 & 6.5 \(\pm\) 0.6 & 3.0 \(\pm\) 0.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Mineral composition of the schist by modal analysis. The values are expressed as mean \(\pm\) error (in percentage) of three independent thin sections from three subsamples from each category. Figure 3: Thin sections of the weakly (**a**) and strongly (**b**) weathered schist samples. ‘Q’, ‘C’, ‘B’, and ‘M’ denote the quartz, chlorite, biotite and muscovite grains, respectively, and the pores and foliation appear in black. Thin-section analysis of the strongly weathered schist (Figure 3b) showed that the biotite was formed by crystals of a tabular-laminar habit (0.12-0.5 mm) with brown-yellow pleechroism; no alteration processes were observed, only the slight segregation of iron oxyhydroxides. Muscovite crystals of 0.10 to 0.20 mm were interwoven with biotite. Chlorite appeared as well-developed tabular crystals (0.15-0.40 mm) generally oriented transversely to the foliation.