Observation of dislocations and subboundaries in the optically active crystal Bi12GeO20 with reflection birefringence topography

The birefringence topography of a Bi₁₂GeO₂₀ single crystal which has a strong natural optical activity is presented. The natural optical activity of the crystal may be completely compensated by reflection polarised light microscopy so that the birefringence images of dislocations and subboundaries are observed. The Burgers' vectors of dislocations were found to be of the types 1/2 1 1 1, 1 0 0 and 1 1 0 and dislocations which form subboundaries are also of these types.