IR spectra 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 of 7.5–9.5 µm to highlight the CF position, shown in panels A, B, C, and D. The average value for each grain size is represented by the vertical dashed black line. Panel E: Position of each mixture with different percentage of dark material. Dashed horizontal lines and the label on the right show the average value of the CF position for each BAS sample series.