Experiments on the biooxidation of sulfidic minerals and elemental sulfur were conducted to determine the structures of the leach residues. Arsenopyrite was oxidized over 5 days with the A ferrooxidans strain in 9K medium with ferrous iron. Fig, 3 demonstrates that the products of arsenopyrite oxidation generated a fraction similar to that of elemental sulfur. To confirm these results, laboratory experiments were per- formed on the biooxidation of pure arsenopyrite and sphalerite (ZnS) After the oxidation of these minerals, the residue was analyzed by XRD. The results are presented in Fig. 4. The main phases identified from Fig. 4 are also shown in Table 2. These data demonstrate that the XRD patterns of these compounds are identical and do not correspond to rhombicelemental sulfur. To confirm the role of bacteria in changing the structure of elemental sulfur, experiments were carried out on the microbial oxidation of rhombic sulfur. The XRD patterns of the original sulfur and the product obtained after biooxidation are also shown in Fig. 4. These results illustrate the significant changes in the structure of rhombic sulfur biooxidation. The crystal structure ofthe pppduct was the same as that of the sphalerite and arsenopyrite biooxidation products. There were no compounds ofiron or arsenic in these products. The product rhombic sulfur biooxidation was identified as a rare variety of elemental sulfur reproducing the B-modification of selenium. A similar compound has been isolated from hot sulfur springs in Portugal (Assuncao and Garrido, 1953); the microorganisms used for biooxidation were possibly present in this envir Thus, prior to sulfur oxidation, the microorganisms change the structure of the mate- rial, independent of the type of mineral. Thus, the initial stage of arsenopyrite biooxidation was accompanied by the formation of elemental sulfur on the surface of arsenopyrite which was the major solid-phase product of oxidation. The crystal structure of sulfur was different from that of its rhombic modification and may be tentatively described as a B-modification; its formation may be affected by the products of microbial metabolism, e.g., phospholipids (Pivovarova et al., 1982) The XRD patterns of the products of arsenopyrite oxidation obtained after the biooxidation of the surface suggest that reaction (1) is more intense when Fe ions originating from microbial