NOTE STRUCTURE DEFECTS IN PALYGORSKITE

It is well established that palygorskite has an amphibole-like, double-chain structure. The linked tetrahedral groups forming continuous heets are divided into ribbons of various widths due to alternation of the direction of tetrahedra which run parallel to the fibre axis (Brindley, 1980). This alternation of the chains leaves a series of holes of well-defined size which are filled with water molecules (channel water). Based on microscopic observations of cleavage surfaces (Bailey, 1980), the mineral appears to be built up of regular stacking of planes parallel to (h00) and (hkO) which form predominantly the surface of the laths parallel to the c-axis. This is further supported by the enhanced X-ray reflections from these planes. Some degree of structural disorder might be expected to result from the progressive limination of the channel water on heating. In addition, the weak interchain bonding in palygorskite is likely to result in a shift of the layers parallel to the fibre axis. According to the model of Drits & Sokolova (1971) and Preisinger (1963), which is similar to the orthorhombic version of Bradley (1940), this mineral would be expected to show a shift of c/3 parallel to the fibre axis (Bailey, 1980). In view of these facts it was decided to carry out a systematic study of these two defects which are likely to occur at differen