# Properties of the Ignimbrites in the Architecture of the Historical Center of Arequipa, Peru
## 4 Discussion  
Compositionally, there is no significant variability between the white and beige samples. The content of quartz and alkaline feldspars (sandine) and plagioclase (albite) is similar in both in semi-quantitative terms. The quartz phase, about 15% of cristobalite, has a proportion of less than 5% in pink ignimbrites, within the typical tolerance range of these rocks. The pink ignimbrite deposits on the quarry top have a higher density than the Wt and Be ignimbrites. This result is associated with consolidation and compaction processes, which lead to sintering and compression of the pyroclastic material (due to thermal processes of transition to the glassy phase) by quenching [32], which are conditioned by the crystallochemical composition of the glass phase [6]. The mudflows descending the slopes of the volcano would have contributed to compacting the ignimbrite Pk. To this, we must also add the degassing process, which is local and depends on the overlying pyroclastic sediment column; accordingly, beige ignimbrites are characterized by containing more vacuoles. Associated with diagenetic processes, post-sedimentary sheet weathering supports the cationic leaching of unstable phases and directional hardening due to chemical cementation and modifies the porous network, resistance and durability [24; 33]. The ignimbrites are classified as type II, the characteristics of which establish a density of 1.25-1.65 kg/cm\({}^{3}\), porosity of 0.34-0.50 and UCS of 1.8-9.8 MPa [32]. A double process occurs in the non-linear mechanical behavior of ignimbrites, against the compression load. While vacuoles facilitate the spread of cracks, intrusions of fragments of vulcanodetrital contribute to dissipating energy in the manner of ball joints, opposing the continuity of cracks, so the material maintains its cohesion and even reaches an ultimate resistance exceeding the elastic limit until intergranular breakage occurs (Figure 11a). If the correlation of low dry density values, UCS comprised between 1 MPa and 5.5 MPa, higher porosity than 50% and ultrasonic velocities lower than 2000 m/s define the ignimbrites as weak rocks [34], then the ignimbrite Be is characterized in this field, but not the Pk ignimbrites. Although the mechanical resistance of ignimbrites is low, in colonial architecture the masonry is mixed, composed of two leaves, each 20 cm thick with a core filled with pebbles and broken ashlars bonded with coarse mortar (Figure 11b). Therefore, the walls with thickness of 1.50 m to 1.75 m counteract as a whole the thrusts of the semicircular arches and the barrel vaults. The results of the capillary suction (C) and the ultrasonic pulse velocity (UPV) that revealed a slight anisotropy for the Be variety and near isotropy for the Wt and Pk ignimbrites make it possible to assert the lack of directional flows in the quarry. This guarantees the random laying of the ashlars in the masonry and the voussoirs in the vaults, including the carved faces, as a preferential orientation to withstand compression loads was not found. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & & L* & a* & b* \\ \hline \multirow{2}{*}{Wt} & Avrg. & 80.2 & 1.3 & 4.8 \\  & st.dev. & \(\pm\)2.1 & \(\pm\)0.2 & \(\pm\)0.9 \\ \hline \multirow{2}{*}{Be} & Avrg. & 78.4 & 1.8 & 9.3 \\  & st.dev. & \(\pm\)1.6 & \(\pm\)0.2 & \(\pm\)0.8 \\ \hline \multirow{2}{*}{Pk} & Avrg. & 66.3 & 7.4 & 11.8 \\  & st.dev. & \(\pm\)0.9 & \(\pm\)0.4 & \(\pm\)0.5 \\ \hline \multirow{2}{*}{\(\Delta\)E\({}^{*}\)} & Wt-Be & Be-Pk & Wt-Pk & \\ \hline \(\Delta\)E\({}^{*}\) & 4.95 & 13.51 & 16.76 & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Results of color measurement. The low hardness of ignimbrites facilitates carving since the use of hammers, sharp chisels and claw chisels is enough to cut and smooth the lithic intrusions without causing the detachment of the hard grains due to the good adhesion in the matrix (Figure 11c).