The method is based on decomposition of the test portion by 1) sintering with sodium peroxide and treatment with hydrochloric and perchloric acids and treatment with dilute nitric acid, or 2) treatment with hydrochloric, nitric and perchloric acids, evaporation, filtration of silica with any residue, fusion with sodium carbonate and dissolution in hydrochloric and perchloric acids, in both cases evaporation of the solution, filtration of the precipitated silica, ignition, and weighing, treatment of the ignited residue with hydrofluoric and sulfuric acid, ignition, and reweighing. Applies to silicon contents between 1 % and 15 % in natural iron ores, iron ore concentrates and agglomerates, including sinter products.