# Mineralogical Transformations in Granitoids during Heating at Fire-Related Temperatures
## 4 Discussion  
### Mineral Thermal Expansion  
As noted, and observed above, one of the main parameters influencing the damage in granitoids by temperature is the differential mineral expansion due to their heterogeneous composition. For this reason, a deeper discussion of each mineral was developed independently. Quartz: The experiments carried out by several authors with thermal dilatometers, X-ray diffraction, and neutron scattering reflected a progressive increase in volume up to 500 \({}^{\circ}\)C. From 500 \({}^{\circ}\)C to almost 600 \({}^{\circ}\)C, quartz abruptly increased the volume due to the beginning of the alpha-beta transition, and after that, the volume stabilized or decrease slightly [40; 41; 42]. The volume of 1 Kg of quartz was calculated considering its density and the cell parameters obtained with XRD (1). The calculated results for quartz in each of the studied rocks are compared with [PERSON] data [41] (Figure 9). \[\varrho\mathrm{t}=n\times\mathrm{A}/\mathrm{V_{c}}\times\mathrm{N} \tag{1}\]  
where \(\varrho\mathrm{t}\) is the theoretical density in \(\mathrm{g}/\mathrm{cm^{3}}\); n is the number of atoms per unit cell, \(\mathrm{V_{c}}\) is the volume of the unit cell in \(\mathrm{cm^{3}}\) calculated from the XRD results, and \(\mathrm{N}\) is the Avogadro number. The approach from XRD results gave the same trend up to 600 \({}^{\circ}\)C, although with a volume change lower than the reference. The measurements carried out revealed a contraction along the a- and c-axis for all the quartz of the selected stones, whereas some authors [43] found a negative expansion in the c-axis for temperatures above 600 \({}^{\circ}\)C. This beta negative expansion was explained in terms of asymmetrical forces in the Si-O-Si tetrahedral bonds, in which bending is energetically more favourable than compressing Si-O bonds [44]. In addition, alpha-beta transition was observed not exactly at 573 \({}^{\circ}\)C but at approximately 625-650 \({}^{\circ}\)C as a gradual change in which dilation and contractions alternated, that is, alpha and beta structures appeared alternately with time or even coexisted [45]. The trends obtained in this study are close to the bibliography ones, with the implication that the calculations from XRD data revealed a volume expansion lower than that measured in single crystals, which may influence that after the phase transition, the four granitoids showed  
Figure 8: Biotite cell expansion adapted from software. Figure 9: Quartz volume expansion of the granitoids compared to previous studies (* [PERSON] 1971). the recovery to the initial dimensions at approximately 800 \({}^{\circ}\)C (Figure 10a). This quartz volume recovery in the studied granitoids may explain the creation of intergranular cracks around quartz that produce a loss of integrity of the rocks. ER is the granitoid that broke completely at 800 \({}^{\circ}\)C, whereas the other three resisted up to 1000 \({}^{\circ}\)C. Quartz content in this granitoid is approximately 50%, similar to GS. The main difference between them that can explain the better integrity of GS is its finer grain size and the mica rich composition that facilitate the absorption of the stresses associated with quartz volume changes. Feldspars' pre-existent microcracks propagate through weakness planes such as cleavages, twin planes, and perthites. In addition, plagioclase exhibited commonly secondary mineral phases in their nucleus. All these characteristics favoured microcrack development during heating [46]. Feldspar showed a contraction in the b- and c- axis and a expansion in the a-axis, for alkali feldspar as well as for plagioclase. In Figure 10b, the peaks of the (002) plane from oligoclase and albite corresponding with the c-axis show the contraction with temperature. Both minerals are triclinic, so the differences between their 2 Theta angles and the length change are due mainly to the potassium or sodium proportion of the solid solution and zoning within the crystal.