  We modelled dust grain-size distributions for carbonaceous and silicates
dust, as well as for free-flying iron nanoparticles in the environment of a
$\gamma$-ray burst (GRB) afterglow, GRB 180325A. This GRB, at $z=2.2486$, has
an unambiguous detection of the 2175 \r{A} extinction feature with $R_V=4.58$
and $A_V=1.58$. In addition to silicates, polycyclic aromatic hydrocarbons
(PAH), and graphite, we used iron nanoparticles grain-size distributions for
the first time to model the observed extinction curve of GRB 180325A. We fit
the observed extinction for four model permutations, using 232 sets of
silicates, graphite, carbon abundance in hydrocarbon molecules ($b_C$), and
fraction of iron abundance in free-flying nanoparticles ($b_{\text{Fe}}$).
These four different permutations were chosen to test iron nanoparticles
significance and carbon abundance in hydrocarbons. Our results indicate that
iron nanoparticles contribution is insignificant and there is a degeneracy of
carbon abundances, with the range $(0.0 \leq b_C \leq 0.7)\times10^{-5}$
providing the best-fit to the observed extinction curve of GRB 180325A. We
therefore favour the simplest model of silicates and polycyclic aromatic
hydrocarbons. The silicates are dominant and contribute to the entire
wavelength range of the GRB extinction curve while graphite contributes towards
both the 2175 \r{A} bump and the UV extinction. The afterglow peak luminosity
($1.5\times10^{51}$ ergs/s) indicates dust destruction may have taken place. We
conclude that further investigations into other potential contributors of
extinction are warranted, particularly for steep UV extinction.
