Sample sizes and shapes. Left panel: image of the glassy Mg-rich sample taken with a scanning electron microscope, the scale bar in the lower right is 1 µm. Middle panel: image of the solgel Fe-rich sample E20 taken with a transmission electron microscope, the scale bar is 100 nm. Right panel: black line shows the normalised size distribution measured for sample E30, Mg_0.7 Fe_0.3 SiO₃. The red histogram-like curve shows the size distribution adopted in order to study the impact of the grain size distribution on the optical constants calculation (see Sect. 5).