Left: comparison of the calculated absorption (dashed blue line), scattering (solid black line), and extinction (solid red line) efficiencies at hν = 7120 eV (near Fe K-edge) for olivine. The two green regions indicate the size range investigated in this work: in particular (0.005 ≤ a ≤ 0.25) μm (filled green) and (0.02 ≤ a ≤ 1) μm (hatched green). Right: comparison of the calculated absorption, scattering, and extinction efficiencies per unit of radii as a function of the grain size. The absorption Q-factor per grain size remains roughly constant along the two size ranges analysed in this work (the green areas). Instead, the scattering Q-factor per radii unit increases, achieving the maximum value at ~3 μm.