Proto-imogolite allophane from a buried volcanic ash layer has been subjected to shearing forces in the gap between a cone-shaped rotor and the stator plate of a programmable viscometer. Its strength, as measured by shear stresses developedunder increasing rates of shear, decreased rapidly during shearing. Plots of shear stress against shear rate showed hysteresis between rotor acceleration and subsequent deceleration. Resting a sheared allophane in the rotor gap for up to 1 week at constant moisture, produced only a slight recovery of strength.
Scanning electron micrograph of freeze-dried allophane revealed a cellular mesh structure that coalesced at surfaces. Shearing action changed this structure to globular clusters of adhering particles.
Drying allophane from its natural moisture content of 250% to 100% H₂O (w/w) enhanced strength over a range of shear rates but further drying induced a substantial loss of strength at higher rates of shear. Drying to 60% H₂O (w/w) coalesced the mesh structure to form adhering globular particles but at 0% H₂O these allophane particles acquired sharp edges. Rewetting after drying to 60% H₂O (w/w) enabled shear strength to recover at low rates of shear but after drying to 0% H₂O the rewet allophane had little strength.

抄録全体を表示

Aggradational conversion of Fe-rich trioctahedral smectite to interstratified chlorite/smectite in acidic pyroclastic sediments has been examined in terms of the structural and chemical changes. The studied specimens were obtained from the drillhole HT-42 of the Ohyu District, Akita Prefecture, Japan. In the sediments of about 1000m thick, expandability in chlorite/smectite decreases downward discontinuously from 100% to 80%, from 50% to 40%, and from 15% to 10%. Three phases of chlorite/smectite with about 80% expandable layers, corrensite, and swelling chlorite with about 15% expandable layers coexist at deeper places ranging from-700m to-900m. The Si content decreases and the Al content increases in the tetrahedral position of chlorite/smectite with decreasing expandability, while the Mg, Al, and Fe contents in the octahedral position increase. The exchangeable interlayer cations consist mainly of Ca regardless of the expandability. The percentages of Na and K in the exchangeable interlayer cations decrease and that of Mg increases with decreasing expandability. Based on these observations, the conversion of smectite to chlorite is divided into two steps of reactions from smectite to corrensite and from corrensite to chlorite. The two reactions overlap in a certain P-T range. The pattern of conversion may be caused by that corrensite packet produced in the parent smectite layers tends to segregate from the smectite layers with advancing conversion and that corrensite is a stable phase in considerably wide P-T range. Such a pattern of conversion of chlorite/smectite is clearly distinguished from that of dioctahedral smectite to illite previously reported by Inoue and Utada (1983).

抄録全体を表示

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.

抄録全体を表示