Nine specimens of regularly interstratified dioctahedral mica-smectites of hydrothermal origin mainly from roseki deposits were studied mineralogically, together with corresponding four synthesized materials.
Natural specimens are rich in SiO₂, Al₂O3 and H₂O which are accompanied by considerable amounts of Na₂O, K₂O and CaO. The expansible layers have a beidellitic composition, the layer charge being largely due to tetrahedral substitutions. Interlayer cations of the non-expansible layers are composed not only of Na and K but also of Ca.
The basal spacings obtained by X-ray diffraction under various experimental conditions are similar to those of rectorite, although they differ slightly from each other. There is a positive correlation between d (060) and d (001). The interlayer composition of mica layer affects the basal spacing of interstratified mica-smectite; a positive correlation is shown between the K content and d (001).
In the infrared absorption measurements, specimens rich in Ca have double peak absorptions around 930-900 cm^-1 and 480cm^-1, which are due to considerable Al-for-Si substitution in the tetrahedral sheet as in margarite.
The morphology of interstratified mica-smectite varies from clearly rectangular, rhombic and hexagonal shapes to quite irregular one. Rectangular shape like rectorite develops in some specimens. The lattice images of the Mitsuishi specimen by high resolution electron microscopy show that the mineral is composed of an alternation of 10Å layers and 9.3Å layers, the latter corresponding to dehydrated smectite.
Interstratified mica-smectite is synthesized in the presence of alkali and alkaline earth elements whose ionic radii are not less than those of Na and Ca. The synthetic experiments suggest that the interstratified mica-smectite of hydrothermal origin might have been formed through an embryo of smectite-like phase at the initial stage of crystallization.