Mineral fibers have a chemical composition including the following constituents, as weight percentages: SiO2 30% to 50%, Al2O3 10% to 20%, CaO+MgO 20% to 35%, Na2O+K2O 1% to 10%, wherein the mineral fibers include a content of total iron, expressed as Fe2O3, of from 5% to 15% and a redox, which corresponds to the weight ratio between the content of ferrous iron, expressed as Fe2O3, and the total content of iron, expressed as Fe2O3, of less than 0.6.