## 3 Crack Evolution  
The influence of temperature on crack development in granite specimens was revealed through the collected data. Microcracks in the rock are generally classified as intragranular (contained within a single grain), intergranular (along the grain boundaries), and transgranular (affecting more than one grain) [19]. This study examined the intragranular cracks of feldspar, quartz, and biotite minerals at various temperatures and the intergranular cracks at the boundaries between minerals. Samples of the analyzed microscopic images are provided in Figure 1, where red lines indicate the intergranular cracks. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Temp, \({}^{\circ}\)C**} & **Feldspar** & **Quartz** & **Biotite** & **Other** & **Elastic** & **Porsity,** & **Density** & **UCS,** & **Tensile** & **P-Wave** \\  & **Content,** & **Content,** & **Content,** & **Minerals,** & **Moallus,** & **GPa** & **\%** & **g/cm\({}^{5}\)** & **MPa** & **Strength,** & **Velocity,** \\  & **\%** & **\%** & **\%** & **\%** & **\%** & **GPa** & **\%** & **MPa** & **MPa** & **km/s** \\ \hline
25 & 50 & 40 & 5 & 5 & NA & 0.2 & 2.63 & NA & 9.03 & 4.71 \\
100 & 75 & 20 & 5 & 0 & 12.96 & NA & 2.64 & 134.1 & 7.62 & 4.04 \\
200 & 72.21 & 11.89 & 15.9 & 0 & 22.25 & NA & NA & 149 & NA & 3.57 \\
300 & 59.85 & 11.12 & 21.56 & 7.47 & 11.12 & 0.68 & 2.63 & 102.04 & 59.85 & 3.52 \\
400 & 83.51 & 9.04 & 0 & 7.45 & 7.36 & NA & NA & 72.51 & 4.85 & 3.03 \\
500 & 60.59 & 34.09 & 5.32 & 0 & 20.5 & 1.43 & NA & 187 & NA & 3.09 \\
600 & 63.59 & 27.72 & 4.94 & 3.75 & 5.69 & NA & 2.61 & 54.88 & 1.51 & NA \\
700 & 29 & 50 & 15 & 6 & 1.82 & NA & NA & 22.43 & 8.69 & NA \\
800 & 48.34 & 41.78 & 4.29 & 5.39 & 6.96 & NA & 2.49 & 38.07 & NA & 0.49 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: A sample of the dataset. The gathered data and observations indicate that three clear patterns emerge as the temperature increases [18; 23; 33; 34; 35; 36; 37; 38; 39; 40]. From room temperature to about 200 \({}^{\circ}\)C, there are generally no visible intergranular cracks in any of the minerals (quartz, feldspar, and biotite). The minerals exhibit a close arrangement, devoid of initial pores or fissures [41]. However, some instances show intergranular cracks between quartz and feldspar minerals when the temperature reaches 200 \({}^{\circ}\)C. The initiation of grain boundaries is first observed at the triple junctions of quartz and feldspar minerals, which are typically considered the weakest grain boundaries due to their longer and weaker intergranular bonds. Intergranular cracks between two quartz minerals at 200 \({}^{\circ}\)C were also detected. Therefore, this temperature can be regarded as the threshold for the initiation of intergranular cracks in granite. Upon increasing the temperature to 400 \({}^{\circ}\)C, more intergranular cracks appear between two quartz minerals and between quartz and feldspar minerals. More specifically, the intergranular boundary between biotite and feldspar minerals only weakens at 400 \({}^{\circ}\)C. These intergranular cracks are caused by strong bound water loss, dihydroxylation loss of constitution water, and solid mineral expansion between 100 and 500 \({}^{\circ}\)C, as suggested by previous studies [34]. In contrast, biotite has shown a supporting effect, since the boundaries of biotite with quartz or feldspar did not result in intergranular cracks at 400 \({}^{\circ}\)C.  
The increase in the temperature to 600 \({}^{\circ}\)C leads to an increase in the widths of intergranular cracks between two quartz minerals, quartz and feldspar minerals, and biotite and feldspar minerals. The width of the crack could reach up to 30 \(\upmu\)m [34]. However, the intergranular cracks between quartz and biotite minerals initiate at 600 \({}^{\circ}\)C. Finally, when the temperature was increased to 800 \({}^{\circ}\)C, the width of intergranular cracks between all minerals also increased.