The core in a large Bi12SiO20 crystal as well as the regions free of optical inhomogenities was examined by X-ray double-crystal diffraction topography. It was established that the observed defects in the central core are two-dimensional. The absorption of some impurities as well as the composition changes observed in this area by other authors, should be considered as a consequence of the formation of these defects. (C) 2002 Elsevier Science Ltd. All rights reserved.