Abstract
Detailed mineralogical studies were made of sericites in the Roseki ores which are distributed in the western part of Japan.
It was confirmed from the X-ray diffraction patterns that the structural scheme of these sericites was of the muscovite type. Therefore, a classification of these sericites was attempted according to their average interlayer spacings: 10.0Å, 10.05Å, 10.2Å, and 10.5Å. As the average basal spacing increased, the disordering in the layer-stacking also increased.
The variation in the chemical composition of these sericites indicated an increase of the ratio, Si/Al, for the tetrahedral layers associated with a decrease in the alkali content at the interlayer sites. The increase of the ratio, Si/Al, results in increasing disorder of the layer-stacking. An excess decrease of the alkali ions in disordered sericites as revealed by the calculation based on the composition of ideal muscovite is compensated partly by an introduction of ammonium ions between the layers.
Weight loss curves upon heating indicated that the weight loss below 350°C is due to the dehydration of interlayer water and that between 350°C and 600°C to the liberation of the interlayer ammonium ions. This was confirmed also by the behavior, at high temperatures, of the infra-red absorption bands at 1620cm-1 and 1410cm-1 which correspond to the bending vibrations of H2O and NH4+, respectively.
From these experimental results, it was concluded that the variation in the physical properties of these sericites can be correlated to a structural change from the muscovite structure to montmorillonitic ones.
The presence of interlayer H3O+ is suggested by an excess weight loss of some sericites, but in the lack of direct experimental evidences it is still open to question.
