SAED and HRTEM Investigation of Palygorskite

The microstructure of palygorskite from Longwang Mountain of Xuyu County, Jiangsu Province, was studied by energy dispersive X-ray analysis (EDX), selected-area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). The average composition of the palygorskite studied is (Si7.38Al0.62) (Al0.96Fe3 0.62Mg2.86□0.56)Ca0.03K0.06O20(OH)2(OH2)4, which is rich in Mg. Several SAED patterns from a single crystal of palygorskite were obtained with different zone axes. The polymorphs (monoclinic and orthorhombic) are unequivocally distinguished by distant interplanar angles, even though they possess similar sets of d-values. High-resolution images of three principal zones (010], 100] and 110]) were obtained. The lattice fringes on HRTEM images along 010] have spacings of 0.319 nm. These fringes are interpreted as periodic alterations of two tetrahedral (T) sheets and one octahedral (O) sheet (-TT-O-TT-O-). We have directly observed trioctahedral and dioctahedral individual palygorskite particles along 100]. They are all presented as dark lines along 001], but the width of dark lines corresponding to trioctahedral crystals (0.913 nm) is twice that of the dioctahedral ones (0.456 nm). This is because the trans-sites are occupied by cations in trioctahedral palygorskite. The width of dark lines along 110] is 1.024 nm, a bit thinner than the theoretical spacing (1.044 nm). This is because water molecules quickly leave the structure upon the irradiation by the electron beam.