# Mineralogical Transformations in Granitoids during Heating at Fire-Related Temperatures
## 3 Results
### Visual Differences after Heating  
Visible changes were observed on the samples depending on the temperature, mainly as colour changes and fissuring [14]. The four granitoids showed a visible reddening in all samples heated to 400 \({}^{\circ}\)C and above, as well as some new fissures or a widening of the existing ones.  
ER showed a whitening of quartz due to visible cracks in samples heated to 600 \({}^{\circ}\)C and above. The overall colour did not change much; whereas the cracking partially crumbled the sample heated to 800 \({}^{\circ}\)C and completely crumbled the sample heated to 1000 \({}^{\circ}\)C.  
G was the granitoid that held together better, with only some slight reddening and thin but visible fissures in samples heated to 1000 \({}^{\circ}\)C. While both noted granitoids have a low proportion of mica, which diminishes mineral thermal expansion mismatch, ER has a quartz/feldspar ratio close to 1 (which leads to higher micro-stress) than G with a quartz/feldspar ratio of 0.37 [36].  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Mineral (\%)**} & \multicolumn{5}{c}{**Grain Size (mm)**} & **Porosity (\%)** \\ \hline  & **Q 1** ** & **AF** & **P** & **M** & **Q** & **AF** & **P** & **M** & **Rock Average** & \\ \hline ER & 48 & 40 & 7 & 5 & 3 & 6 & 2 & 1 & 5 & 0.56 \(\pm\) 0.03 \\ G & 25 & 35 & 32 & 8 & 8 & 18 & 9 & 2 & 11 & 0.68 \(\pm\) 0.03 \\ GS & 47 & 20 & 20 & 13 & 4 & 4 & 4 & 2 & 4 & 3.68 \(\pm\) 0.06 \\ NG & 18 & 8 & 29 & 24 & 5 & 5 & 5 & 2 & 5 & 0.53 \(\pm\) 0.01 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Summary of the stones’ main features. Grain size was measured from macroscopic samples. Thus, some mineral aggregates have been included as a single mineral.  
GS exhibited a noticeable change, associated mainly to clay minerals oxidation. The quartz \(\alpha\) to \(\beta\) phase transition at 573 \({}^{\circ}\)C did not lead to visible cracking in samples heated to 600 \({}^{\circ}\)C, probably due to a too short period at the target temperature. However, the cracking was obvious in samples heated to 800 \({}^{\circ}\)C and aggravated in samples heated to 1000 \({}^{\circ}\)C.  
NG showed an aesthetical decay; mica and amphibes became golden-brownish and, even if the sample heated to 1000 \({}^{\circ}\)C remained unbroken, the volume increase and the newly created fissures were clearly visible. The physical integrity of these two last granitoids was better than in the previous ones due to their higher mica proportion that accommodated the stresses caused by the mineral thermal expansion mismatch [14; 36].  
The physical differences before and after heating were evident under SEM observations (Figure 3).  
Figure 2: Microscope images of the selected granitoids.  
As the ER sample disaggregated completely after the 1000 \({}^{\circ}\)C test, the observations were performed in samples heated to 800 \({}^{\circ}\)C. The alkali feldspar of ER was the lesser altered of the stone, even when intra and intercrystalline fissuration was evident. Quartz developed wide fractures (inter-, intra-, and trans-) as observed in Figure 3e.  
Mica experienced a great change in all the granitoids, although it can absorb large stress without cracking, the high thermal expansion and contraction during heating and cooling produced open intergranular cracks and sheet opening (Figure 3f-h). Accessory minerals such as zircons or magnetite on NG also produced wide boundary cracks due to the differential expansion.