POLYSOME-WIDTH VARIATION IN PALYGORSKITE-SEPIOLITE MINERALS: A TEM AND AFM INVESTIGATION OF STRUCTURAL VARIATION AND TRANSFORMATION

Transmission electron microscopy (TEM) investigations of palygorskite-sepiolite samples indicate that widths of the 2:1 layer ribbon modules (polysomes) commonly vary. Polysomes observed in one sepiolite sample correspond to double-, triple-, and quadruple-tetrahedral chain widths and a similar variation was observed in yofortierite, Mn-rich palygorskite. Differences in polysome width may explain variations in a number of properties of palygorskite-sepiolite minerals, such as exchange properties involving large organic molecules and chemical composition. Investigation of the microfabric of a natural palygorskite-smectite assemblage from the Meigs Member of the Hawthorne Formation, southern Georgia shows a transformation from palygorskite to smectite. This transformation may be responsible for the lower abundance of palygorskite in Mesozoic and older sediments. Atomic force microscopy (AFM) indicates that some samples of palygorskite show altered fibers that exhibit an oriented overgrowth of another mineral phase displaying a platy morphology. This latter mineral forms along the length of the palygorskite crystals with an interface parallel to the {010} faces of the palygorskite. The resulting grain structures have an elongate “wing-like” morphology. TEM imaging of this sample shows lattice fringes indicating that 2:1 layers of smectite are intergrown with polysomes of the palygorskite. These features indicate an epitaxial overgrowth of smectite on palygorskite and illustrate the structural relationship between platy overgrowths on fibers observed in AFM data. The epitaxial relationship is described as {010} [001] palygorskite ║ {010} [001] smectite. Polysome widths of palygorskite fibers involved in these textures commonly vary and polysome widths are consistent with double tetrahedral-chains (10.4 Å), triple tetrahedral-chains (14.8 Å), quadruple tetrahedral-chains (21.7 Å), and quintuple tetrahedral-chains (24.5 Å). This work was supported by the National Science Foundation under grants EAR-0001122 and EAR-0001251.