## 2 Materials and Method
### Data reduction, Quantification, and Visualization  
#### 2.6.1 Data Reduction  
In a first step, undesirable spectral features have to be separated from the Raman bands. This preprocessing step has to be done with care, because it is easy to introduce but not necessarily to detect artifacts in a Raman image [31]. All spectra were corrected for (i) the wavelength-depended instrumental sensitivity (white light correction), (ii) a possible spectrometer shift by using the \"internal\" Ne standard, (iii) cosmic spikes, and (iv) background contributions that at high-temperature are mainly composed of black body radiation and in some case of continuous fluorescence signals. As a first step, the white-light spectrum measured by the manufacturer was used to correct every spectrum of an image for the wavelength-dependent instrumental response or sensitivity. Then, the spectra were corrected for a possible spectrometer shift during long-time image acquisition using the position of the intense Ne line at 1707.06 cm\({}^{-1}\) of each spectrum [32]. For this, a Gaussian function was fitted to this Ne spectral line and the line position was then used to correct for any frequency shift during long-time imaging. With such an internal standard, the propagated precision of the frequency of a Raman band, obtained by least-squares fitting, was usually between \(\pm\) 0.05 and \(\pm\) 0.10 cm\({}^{-1}\) (2\(\sigma\)) for RT spectra. The  
Figure 3: Comparison between the fast scanning mode (SWIFT® mode) with 0.6 s per spectrum (recording time 50 min) and the point-by-point method with 1 s times 10 accumulation per spectrum (recording time 8 h 43 min). Note that the acquisition time has to be faster than the mineral reactions under investigation. Grain boundaries are slightly different when using the fast scanning mode that is negligible when comparing only images recorded with the same acquisition time. A semi-quantitative statement is still possible. accuracy is thereby coupled to the precision of the Ne line wavelength determination, which is better than \(\pm 10^{-5}\)% [26]. As a next step, all spectra collected in the SWIFT(r) mode, excluding the spectra taken at RT, were corrected for cosmic spikes by the spike correction function implemented in the LabSpec software. This procedure identifies all signals with a full width of half maximum (FWHM) smaller than 6 cm\({}^{-1}\) as cosmic signals, which are automatically removed. The analyzed spectral range was then reduced to the range between 100 and 1200 cm\({}^{-1}\) to simplify the following band fitting procedures. This area covers the main rocking and bending (between 100 and 700 cm\({}^{-1}\)) and stretching vibrations (between 700 and 1200 cm\({}^{-1}\)) of all mineral phases of interest. For quantification and image visualization, the spectra had to be further corrected for background contributions. Two fundamental background contributions often occur within this frequency range that are related to sample fluorescence and black body radiation which is electromagnetic radiation emitted by the sample and the furnace material at temperatures above \(\sim\)800 \({}^{\circ}\)C [33]. Fluorescence can be caused by the presence of specific fluorescing cations (e.g., Cr\({}^{3+}\), Fe\({}^{2+}\), etc.) [33] that are incorporated in the crystal structure. Fortunately, fluorescence effects are more significant at lower temperatures. The Raman spectrum of natural wollastonite, for instance, recorded at RT shows strong fluorescence signals (Figure 4), but at temperatures between 900 and 1000 \({}^{\circ}\)C, the Raman spectrum of wollastonite has the lowest overall background. Above 1100 \({}^{\circ}\)C the background is mainly influenced by the black body radiation that increases with increasing wavenumber (Figures 4 and 5). Significant portions of the black body radiation are in the region of Raman-active vibrations and therefore partly mask the Raman signals [34].