Catalyzes the oxidation of ferrous iron(II) to ferric iron(III) and stores iron in a soluble, non-toxic, readily available form (PubMed:11323407, PubMed:15201052). Important for iron homeostasis (PubMed:11323407, PubMed:15201052). Iron is taken up in the ferrous form and deposited as ferric hydroxides after oxidation (PubMed:11323407, PubMed:15201052). {ECO:0000269|PubMed:11323407, ECO:0000269|PubMed:15201052}. This is the function of FTMT (ferritin mitochondrial, Ensembl gene identifier ENSG00000181867).