Posterior probability distribution of the composition of interstellar dust from GRB 221009A from fitting a limited parameter space with a refractive index ranging from Mg₂SiO₄ to Fe₂SiO₄. The 1, 2, and 3σ intervals are indicated with increasingly lighter shades. While MgFeSiO₄ is consistent with these constraints, a composition dominated by iron (i.e. Fe₂SiO₄) is disfavoured, while an iron-free dust composition is excluded. This seems to be due to the imaginary component of the refractive index, k, being too low to be consistent with the data without the presence of a good fraction of some heavier elements, i.e. iron, in the dust (Fig. 7).