Margaret D. Foster

Bio: Margaret D. Foster is an academic researcher from United States Geological Survey. The author has contributed to research in topics: Montmorillonite & Kaolinite. The author has an hindex of 10, co-authored 14 publications receiving 808 citations.

The Relation Between Composition and Swelling in Clays

Abstract: The phenomenon of swelling is associated with the hydration of clay; however, all clays do not swell when hydrated. Steps in the mechanism of hydration and swelling of different types of clays as observed and interpreted by several investigators and some theories proposed as to the cause of hydration and swelling are reviewed. The concept of clays as colloidal electrolytes that dissociate to a greater or less extent when dispersed in water seems to explain most satisfactorily the significant relation between the degree of swelling on hydration and the composition of the clay minerals. In the kaolinite group, in which there are generally no replacements, the small number of exchangeable cations associated with the clay structure are presumed to be held by broken bonds on the edges of the sheets. Even though kaolinite, as shown by Marshall, is more highly ionized than montmorillonite, this greater ionization, because of the small number of cations present and their location on the edges of the sheets, cannot pry the units apart or leave the sheets sufficiently charged to cause the mineral to exhibit the phenomenon of swelling. In the montmorillonite structure, on the other hand, isomorphous replacements, most commonly of magnesium and ferrous iron for aluminum in the octahedral layer, and, to a slight degree, replacement of aluminum for silicon in the tetrahedral layer, give the structure a net residual charge of 0.7 to 1.10 milliequivalents, which is neutralized by cations held electrostatically and located, for the most part, between the sheets. On hydration such a structure tends to ionize, the degree of ionization depending on (a) the nature of the exchangeable cation and (b) the kind and extent of isomorphous replacements. The characteristically great swelling of sodium montmorillonite as compared with calcium montmorillonite can be correlated with its much greater ionization. The differences in swelling of different montmorillonites have been correlated with the nature and extent of octahedral substitution and are attributed to the effect of these replacements on the anionic strength of the structural unit and its consequent degree of ionization as influenced by the changes in polarization throughout the structure caused by these replacements. Hydrous mica, with the same structure as montmorillonite, is characterized by even a greater degree of isomorphous replacements and, consequently, a greater charge. However, a large part of this charge is neutralized by fixed, nonexchangeable and nonionizable potassium, and ionization of the exchangeable cations is unable to overcome the effect of this fixed potassium. It is probable that the greater replacements in the hydrous mica structure, as in the montmorillonite structure, have a depressing effect on ionization. The result is that hydrous micas are characterized by a very low degree of swelling.

Distribution of the Elements in Some Major Units of the Earth's Crust

Abstract: This paper presents a table of abundances of the elements in the various major units of the Earth's lithic crust with a documentation of the sources and a discussion of the choice of units and data.

The velocity of compressional waves in rocks to 10 kilobars: 1.

Abstract: The velocity of compressional waves has been determined by measurement of travel time of pulses in specimens of rock at pressures to 10 kilobars and room temperature. Most of the samples, mainly igneous and metamorphic rocks, furnished three specimens oriented at right angles to one another. The present paper gives experimental details, modal analyses, and numerical tables of velocity as function of direction of propagation, initial density, and pressure. Discussion of various aspects of the measurements is reserved for part 2.

The genesis of basaltic magmas

Abstract: This paper reports the results of a detailed experimental investigation of fractionation of natural basaltic compositions under conditions of high pressure and high temperature. A single stage, piston-cylinder apparatus has been used in the pressure range up to 27 kb and at temperatures up to 1500° C to study the melting behaviour of several basaltic compositions. The compositions chosen are olivine-rich (20% or more normative olivine) and include olivine tholeiite (12% normative hypersthene), olivine basalt (1% normative hypersthene) alkali olivine basalt (2% normative nepheline) and picrite (3% normative hypersthene). The liquidus phases of the olivine tholeiite and olivine basalt are olivine at 1 Atmosphere, 4.5 kb and 9 kb, orthopyroxene at 13.5 and 18 kb, clinopyroxene at 22.5 kb and garnet at 27 kb. In the alkali olivine basalt composition, the liquidus phases are olivine at 1 Atmosphere and 9 kb, orthopyroxene with clinopyroxene at 13.5 kb, clinopyroxene at 18 kb and garnet at 27 kb. The sequence of appearance of phases below the liquidus has also been studied in detail. The electron probe micro-analyser has been used to make partial quantitative analyses of olivines, orthopyroxenes, clinopyroxenes and garnets which have crystallized at high pressure.