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.

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Haradaite, SrVSi₂O₇, is orthorhombic, space group Amam, with four formula units in the cell, a=7.06, b=14.64, c=5.33Å. The structure contains Si₄O₁₂ chains which are joined together by vanadium to form a sheet closely related to those in lamprophyllite, bafertisite and astrophyllite. These minerals may conveniently be called quasi-layer silicates.
There are two kinds of bridge bonds in the chain: one is a nonlinear bond (∠Si-O-Si 150°) of length 1.664±0.008Å, the other is a linear bond of length 1.578±0.009Å. The non-bridge bonds have a length 1.59₈±0.03Å. A general survey on the Si-O bond lengths in silicates shows that, for nonlinear cases, the mean value of bridge bonds is 1.641Å and that of non-bridge bonds 1.605Å, and for linear cases, these values are respectively 1.59Å and 1.63Å.

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The refinement of the structure of α-Ag₂S was undertaken together with an accurate determination of the lattice constants. The final structure gave the R-factor of 16.8% including unobservable reflections.
The crystals found in nature are almost invariably twinned, and a theoretical consideration on the modes of twinning led to the conclusion that twenty-eight twin laws are possible.
Heat treatments of the specimens were carried out in order to know the details of the progress of twin formation premonitory to the α-β transition. The specimens remain substantially unaltered up to 152-156°C, the only change being observed in the β -angle. Above this temperature range, individuals of twins in various orientations appear, and the X-ray reflections take an appearance of trains of spots. At about 165°C these spots along each line fuse into one another, giving an arc-like intensity distribution. Some nodes of strong intensities along these arcs are converted into sharp reflections of the high-temperature form at the transition temperature, 173°C.
As the rise of temperature, therefore, the structure of Ag₂S makes the first approach to the β-form by giving itself more maked cubicity through the change in the β-angle, at the next step simulates the symmetry of the high-temperature form by twinning, and finally at the transition temperature rearranges into parallel orientation the twin individuals which have been deranged owing to their obliquity.

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Truscottite from the Toi mine was found in one of the auriferous quartz veins cutting Miocene pyroclastic rocks. The chemical composition is SiO₂ 63.56, TiO₂ 0.08, Al₂O₃ 0.81, FeO 0.66, MgO 0.70, MnO 2.41, CaO 26.24, H₂O(+) 4.31, H₂O(-) 0.48, total 99.25%, which is recalculated as (Ca_1.77Mn_0.18Mg_0.06Fe_0.03)_1.99(Si_4.00Al_0.06)_4.06O_9.18(OH)_1.81 based on O=11, and the cell constants are a₀=9.720Å and c₀=18.84Å respectively calculated from the X-ray powder data. The mineral occurs as aggregates of small flakes in the quartz with small amounts of adularia and rhodochrosite.
The temperature of formation of the ore deposits is estimated from the known results of the synthetic studies of this mineral carried out by previous investigators.

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