Slope evaluation for the mixtures studied in this work. Panels A, B, C, and D: IR spectra normalized at 1.5 µm of basaltic samples [BAS] at different grain sizes (<50, 50–200, 200–500 and 500–1000 µm) mixed in different proportions (from 1 to 50%) with synthetic amorphous carbon [SAC], spectra are limited in the range 1.4–2.6 µm. Panels E, F, G, and H: values of the slope against percentage of SAC dark component for each mixture with errors. The black cross, plus, and triangle markers are the value of the slope for mixture at 50%, 70%, and 90%, respectively, from Poggiali et al. (2022). The black star marker is the value of the slope of pure SAC. Note: the samples at 50%, 70%, and 90% have a grain size that is comparable to that of sample SAC-A (<50 µm).