Quartz sand containing mineral impurities was subjected to bioleaching with Bacillus spp. strains and subsequent elutriation. Bioleaching of the sample removed visible Fe-bearing minerals that coated quartz grains. During the bioleaching, elements were dissolved from silicate minerals. Poorly crystalline Fe-oxides that sealed siderite nodules were released from intergranular spaces and formed a fine-grained (<0.1 mm) fraction. Voltammetry of the fine-grained fraction indicated reductive dissolution of Fe3+. However, chemical analysis showed a slight increase in Fe2O3 content and a decrease in the FeO content following three months bioleaching in the Bacillus culture. X-ray diffraction analyses showed the loss of Fe-bearing minerals and mica following the bioleaching and the elutriation of the fine-grained fraction. Transmission electron microscopy and energy-dispersive analysis of X-rays showed nucleation of fine-grained Fe,Al-silicates on bacterial surface after three months of bioleaching. (C) 2003 Elsevier Ltd. All rights reserved.