SECTORS IN POLYGONAL SERPENTINE - A MODEL-BASED ON DISLOCATIONS

Based on coexisting rolled chrysotile and polygonal serpentine fibers with 15 or 30 sectors each, a crystallographic model for polygonizatio n of chrysotile is proposed. It is based on an assumed chrysotile-toli zardite transition. Polygonization of chrysotile requires more likely 15 partial dislocations per turn, as required by polytype translationa l operators for serpentines. The observed number of sectors correspond s to the two most elastically stable arrays of dislocations. Homogeneo us shear of the layer stacking arising from intersector kinking result s in a cyclic distribution of twins and/or different polytypes. This m akes the fiber axis a five-fold symmetry axis and consequently polygon al serpentine and chrysotile to be both forms of serpentine with local fivefold symmetry. This model is alternative to the recent crystallog rapic model by Chisholm (1991, 1992).