Interlayer Structure in Sodium Micas

抄録

In this paper we present our recent results with respect to the crystal structures of two trioctahedral sodium micas, aspidolite and wonesite, and discuss the nature of the interlayer structure in sodium micas. High-resolution transmission electron microscopy (HRTEM) and electron diffraction analyses of these micas indicate the existence of a large layer offset, i.e., lateral shift between the two tetrahedral sheets across the sodium-bearing interlayer region. The amounts of the layer offset are about 0.9Å(aspidolite) and 1.25 Å(wonesite), and their direction is one of [100], [110] and [110]. These directions are occasionally disordered. By combination of the intralayer shift in the 2: 1 layer and the layer offset, ordered aspidolite has monoclinic ([100] layer offset, C2/m) and triclinic ([110] or [110] layer offset, C1) cells with one-layer periodicity. Both structures were identified in powder X-ray diffraction patterns and probably the triclinic structure is more common. This is the first report that structural variations are generated in micas by the combination of the intralayer shift and layer offset. The layer offset in wonesite is close to [110] and the structure is one-layer triclinic (C1).
These results give us an insight that sodium micas (preiswerkite, paragonite, aspidolite, wonesite, etc.) can possess various amounts of the layer offset, depending on the cavity space in the tetrahedral sheet that is primarily determined by the ditrigonal rotation angle. Therefore the composition in the 2: 1 layer can drastically change the crystallography (e.g., powder diffraction patterns) of the sodium micas. In conclusion, sodium micas are not the simple analogue of potassium micas but belong to a group with a very unique character.