Bioleaching processes in combination with electromagnetic separation can substantially improve the quality of feldspar raw materials. Bacterial destruction of silicates is necessary for releasing intergranular spaces of silicate grains. These are often impregnated with iron minerals which can be subsequently removed by electromagnetic separation. Bioleaching is also effective for the removal of surface layers of fine iron minerals. After the bioleaching, a new mineral, weddellite (calcium oxalate dihydrate), was identified in the fine grain fraction in samples. Weddellite is formed upon the destruction of Ca-plagioclase and its formation indicates that oxalic acid was one of the metabolites involved in the bioleaching process. Bioleaching resulted in a 35-41% decrease in Fe content of raw material from Strazov and the subsequent electromagnetic separation decreased the Fe content up to 69-74% of Fe. The second raw material was granite from Poproc and bioleaching removed 60% of Fe in spite of a high content of Fe2O3. The removal was increased up to 75% of Fe in combination with electromagnetic separation. Iron removal by biological leaching and electromagnetic separation ensures high quality and white color of raw material. (c) 2005 Elsevier Ltd. All rights reserved.