About 40 samples of argillaceous sediments and sedimentary rocks collected from some formations of Pleistocene to Oligocene age in the Kakegawa-Omaezaki region, Shizuoka Prefecture, were mineralogically examined. On the basis of X-ray diffractograms obtained from the <2-μm clay fraction of the samples, major clayminerals were identified and their relative abundances were estimated.
Smectite is the most dominant clay mineral in some formations, while chlorite in others. Illite occurs in significant amounts in every sample and kaolinite is always less than 10% or absent. Irregularly mixed-layered illite/smectite and chlorite/vermiculite phases were observed only in highly consolidated mudstone.
The low abundance of kaolinite and high abundance of smectite and chlorite are in harmony with the fact that these sediments are marine. Estimated clay mineral compositions show no systematic variation from Pleistocene Furuya Formation to Middle Miocene Saigo Group. However, there is a gradual variation between abundance of mixed-layer phases and that of discrete smectite in the sedimentary rocks older than Late Middle Miocene; mixed-layer phases increase from Kurami Group to Oligocene Setogawa Group with the corresponding decrease in discrete smectite. This variation can be ascribed to the burial diagenesis.

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Electron microscopic investigations of spherical kaolinite artificially produced, by means of Pt-C shadowing, gold decoration replica and lattice imagery techniques have revealed that the spherical morphology is due to radiating columnar growth of kaolinite crystals, and not due to concentric layering of curved platy crystals. Thus spherical kaolinite is regarded as a type of spherulite, which is a representative morphology of crystal aggregations appearing under higher supersaturation conditions.

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Infrared absorption spectra in the 4000-400 cm^-1 range of natural trioctahedral chlorites of the Mg-Fe series (orthochlorites) are presented with some discussions on the relationship to chemical composition. The octahedral composition can be estimated by the natures of the bands at 3680-3615 cm^-1, in the 700-600 cm^-1 range, and at 470-410 cm^-1. The bands in the 3620-3400 cm^-1 range, in the 1100-900 cm^-1 range, and at about 750 cm^-1 give approximate tetrahedral composition. A composition determinative graph has been prepared. In addition, the infrared method is shown to be useful occasionally for identification of mixtures of chlorites having different compositions.

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Embryonic halloysites were found in the Ap horizon of a paddy soil derived from volcanic ash. These halloysites were nearly X-ray amorphous to poorly crystallized. They showed infrared absorption bands, more or less poorly defined but characteristic of halloysite, and appeared as “open” spherical to oval particles withdiameters of 15 to 60 nm and as thinly spread “amorphous” materials. About half of halloysites were dissolved incongruently by 0.2M oxalate-oxalic acid (pH3.0) and congruently by hot M/3 sodium citrate. Embryonic halloysites were associatedwith goethite and ferrihydrite containing Al and Si respectively, and abundant diatoms, and considered to form from volcanic ash in a Si-rich environment.

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The OH stretching bands of nacrite were resolved into four bands with decreasingtemperature. These bands were assigned to 3 inner surface OHs and one inner OH from the OD bands of the nacrite after selective deuteration. The 3 inner surface OH stretching bands shifted toward lower frequency with decreasing temperature, which indicated an decrease in the length of the hydrogen bond in the interlayer of nacrite at low temperature. This shorter hydrogen bond was estimated also from the increasing frequencies of the OH in-plane bending bands at a low temperature. From the unit cell parameters of the nacrite, some structural deformations were estimated as increases of the layer shift along X-axis and the tetrahedral SiO₄ rotation angle at low temperature.

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