Abstract
A continuous or nearly continuous octahedral sheet places important constraints on the geometry and topology of the attached tetrahedra in modulated 2:1 layer silicates. A plot of the average radius size of the tetrahedral cations vs. the average radius size of the octahedral cations shows the distribution of structures with respect to misfit of tetrahedral and octahedral (T-O) sheets, with modulated structures forming when misfit is large. It is tentatively concluded that such misfit is a requirement for modulated structures to form. It is evident that misfit of T-O sheets is a limiting factor for geometrical stability. On the basis of tetrahedral topology, modulated 2:1 layer silicates are divided into two major categories, islandlike (e.g., zussmanite, stilpnomelane) and striplike (e.g., minnesotaite, ganophyllite) structures. Bannisterite, which is more complex, most closely approximates islandlike structures. The plot involving cation radii is useful as a predictive tool to identify modulated structures, and parasettensite and gonyerite are identified as possible new members of the group. Diffraction data suggest that parsettensite has a variation of the stilpnomelane structure with smaller islandlike regions and gonyerite has a modulated chlorite structure.
Chemical mechanisms to eliminate misfit of T-O sheets usually involve A1 substitutions. Because modulated 2:1 layer silicates commonly occur in Al-deficient environments, structural mechanisms are usually a requirement to eliminate misfit of component sheets. Besides a modulated tetrahedral sheet, structural mechanisms include (1) a limited corrugation of the interface between the T-O sheets that allows for a continuously varying structural adjustment, (2) distortions of the octahedral-anion arrangement located at strip or island edges, and, at least in one case, (3) cation ordering.
Near end-member compositions, modulated 2:1 layer silicates usually have A1 in tetrahedral coordination, when A1 is present, rather than in octahedral sites. It is concluded that a continuous edge-sharing octahedral sheet with large cations has a constraining influence on the size of possible substituting cations. This effect may explain the lack of octahedral ordering schemes in hydroxyl-rich micas where a small cation might occupy the M(l) site and a large cation might occupy the M(2) sites that surrounded M(l).
