# Mineralogical Transformations in Granitoids during Heating at Fire-Related Temperatures
## 3 Results
### Chemical and Mineralogical Composition  
The three major minerals: quartz, alkali feldspar, and biotite, were analysed before and after heating by \(\upmu\)-XRF in the ER, G, and GS granitoids. Plagioclase was not analysed because of its small size and weathering degree that could give misleading results. Muscovitic was also analysed in GS granitoid. Biotite, titanite, and amphibole were the selected minerals in NG granitoid. Table 2 shows the oxide composition of each mineral for each stone in the fresh state and the absolute difference after heating at 1000 \({}^{\circ}\)C. Before heating, quartz composition in ER and GS was purely SiO\({}_{2}\), whereas in G, quartz showed impurities of feldspar. After heating, quartz showed a more heterogeneous composition, with up to 10% of its total composition mainly of Al and K oxides. This was particularly noticeable in the purest specimens before heating. Only G and ER showed variations in alkali feldspar after heating, consisting of a reduction in SiO\({}_{2}\). Specifically, G showed an increase in K\({}_{2}\)O in this mineral, and ER an increase of 10% of MgO and the appearance of Fe\({}_{2}\)O\({}_{3}\). Biotite showed variations in all granitoids, but it was not possible to establish a clear trend: for example, in GS SiO\({}_{2}\) increased and MgO decreased, whereas in NG SiO\({}_{2}\) increased and CaO decreased. Figure 3: Scanning electron microscope images of the selected granitoids after heating. Q: quartz; AF: alkali feldspar; P: plagioclase; B: biotite; Ms: muscovite; Am: amphibole; Px: pyroxene; Ap: apatite; Ti: titanite. Photos (**a–d**) correspond to granites before heating. Photo (**e–h**) showed the crack development of the granites after heating. XRD analysis was carried out in powdered samples. The temperature was increased progressively up to 1000 \({}^{\circ}\)C and then cooled down to initial values for a final measure. The variation in the peaks may show two different trends. If the peaks move laterally, that means a variation in the internal structure. A shift to higher diffraction angles reflects a decrease in the network spacing, that is, a contraction. If the peak changes in intensity, the mineral phase may experience a change, even becoming a different species. Figure 4 shows XRD spectra for each granitoid subjected to an increase in temperature followed by a cooling down. An easily recognizable mica peak is found at approximately 9\({}^{\circ}\), corresponding to the (001) plane. This peak moves towards higher angles with temperature, which indicates a contraction. In ER and GS, micas also exhibited a compositional change, recognised by a change of the peak intensity at the highest temperature, almost disappearing from the pattern. However, after cooling down, mica peaks appeared again, which implied the recovery of the original composition. Quartz and feldspars also showed a movement of their peaks towards an expansion. The mineral that experienced a higher dilation was plagioclase, followed by Q and finally by alkali feldspar. NG showed a variation in the hornblende peak. The mineral hardly dilated although it showed a decrease in intensity, almost disappearing at high temperatures. As in the case of mica, this mineral recovered with a return to initial conditions. XPS analysis of the whole rocks shows different proportions of potassium, iron, and titanium, which are the key elements in the transformation by heating as observed in previous results. XPS allows us to observe the chemical changes in some of the key elements, especially the iron, which is sensitive to high temperatures [14]. Regarding the iron content and the ratio of Fe/O, there is a general decrease in all the samples. That implies an increase in oxygen that corresponds to an oxidation. The region of the spectrum corresponding to high resolution iron 2p XPS transitions for fresh and 800-1000 \({}^{\circ}\)C heated rocks can be clearly deconvolved in three main peaks. XPS data analysis showed two main peaks on the spectra deconvolution corresponding to iron oxides.