MODULATED CRYSTAL-STRUCTURES OF GREENALITE AND CARYOPILITE - A SYSTEMWITH LONG-RANGE, INPLANE STRUCTURAL DISORDER IN THE TETRAHEDRA SHEET

High-resolution transmission electron microscope (TEM) images confirm that greenalite and caryopilite are modulated 1:1 phyllosilicates. The octahedrally coordinated Fe (greenalite) and Mn (caryopilite) form tr ioctahedral sheets. Six-member rings of tetrahedra link to form triang ular islands four or five tetrahedra across, with each island coordina ting to one octahedral sheet. Adjacent islands are inverted and link t o the neighboring octahedral sheet, which results in a triply-intersec ting corrugation for the tetrahedral sheet. Islands vary in numbers of tetrahedra about a mean dicated by the octahedral sheet dimension. Is land separations range about a mean distance within the X-Y plane, wit h island alignment fluctuating as a function of lattice vectros define d by the octahedral sheet. The tetrahedra thus show limited short-rang e order (spanning to five octahedra), but long-range disorder. Linkage s of tetrahedra between islands are apparently completely disordered. Because of this disorder, there is no definable unit-cell. Fourier cal culations involving non-repeating structures cannot use unit-cell frac tional coordinates and Miller indices. We calculated diffraction patte rns by finding the real-space coordinates of every atom in the model r elative to a defined origin. The reciprocal space variable, d, is sam pled at intervals of 0.005 Angstrom to build the continuous Fourier tr ansform of the model. Discrete polytypes of 1T and 1M for greenalite a nd caryopilite, respectively, were identified. Where grains contain mi xtures, the relative abundance of each polytype is related to composit ion, with the dominant polytype based on minimizing misfit between the sheets of octahedra and of tetrahedra. Stacking in greenalite and car yopilite is defined by the relative positions of adjacent octahedral s heets and, therefore, limits on the displacements of neighboring domai ns of silicate rings within (001) are possible. Domain boundary linkag es, however, cannot be determined precisely by using either diffractio n or imaging data.