Hans Ramberg

Natural and Experimental Boudinage and Pinch-and-Swell Structures

Examples of boudinage and pinch-and-swell structures are described and the mechanisms of formation discussed. One-dimensional plastic compression and compensating one- or two-dimensional expansion of layered cakes of putty, plasticene, and cheese result in structures strikingly similar to the natural boudinage and pinch-and-swell structure. Studies of boudinage and related structures give information about relative plasticity and relative strength of rocks. The commonness of boudinage structures in gneiss-crystalline schist regions shows that one-dimensional compression and compensating elongation in other dimensions is a very common mode of deforming crystalline rocks. The tension which this kind of plastic flowage creates in competent rock layers not only produces boudinage, or pinch-and-swell structure, but is also important in localizing the growth of minerals during metasomatism. Mathematical analysis of the boudinage evolution shows that field measurements combined with data for tensile strength of rocks and their viscosity are sufficient to estimate pressure gradients and rate of flowage connected with the formation of boudins.

The Distribution of

The Mg/(Mg + Fe) ratios of coexisting olivines and orthopyroxenes from different types of rocks have been determined by means of indices-of-refraction measurements. Some data from literature have also been added. The distribution function of Mg and Fe in olivine-pyroxene pairs is discussed. It is shown that this function may be used as a geologic thermometer when some additional thermochemical data are obtained. The distribution function also indicates that orthopyroxene is a nonideal mixed crystal, the intermediate members of the series being more stable than one would expect. This nonideal character of the orthopyroxene series is indicated by deviation from linearity of the indices of refraction and the unit-cell dimensions when these quantities are plotted against the mol fraction of enstatite in the pyroxenes. The a and b parameters of the unit cell of different orthopyroxenes are measured. The aluminum content in orthopyroxenes is discussed briefly.

Fe^{++}

The formation of quartzo-feldspathic rocks-principally pegmatites-is discussed from the viewpoint of thermodynamic stability of minerals and mineral associations. Most quartzo-feldspathic rock bodies, migmatites, and pegmatitic veins encountered in folded chains exhibit mineral associations which indicate a formation at temperatures below which silicate melts cannot exist. Data from a granulite facies area in western Greenland show that, even at the relatively high P-T conditions that prevailed there, the granitic component in the system was unable to form melts. Furthermore, pegmatites and quartzo-feldspathic massifs usually possess mineral associations in harmony with the degree of metamorphism prevailing in the surrounding region. In high-grade areas the pegmatites-independent of their bulk chemical composition-show high-temperature mineral assemblages; in low-grade areas low-temperature assemblages occur in the pegmatites. Because the melting curve of a magma is definitely determined by the bulk composition of the system (at constant pressure), the logical conclusion must be that most pegmatites, veined gneisses, migmatites, and several acid massifs were formed by some metamorphic-metasomatic processes in which silicate melts were unable to co-operate.

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In the absence of free-energy data, the heats of formation of crystalline compounds and the heats of reaction among such solids are valuable indicators of the relative stabilities of crystalline compounds such as minerals. With that in mind, the writer has discussed several general rules which relate heats of reaction to various fundamental properties of ions and atoms, such as the ionic radius, number of charges on the ion, ionization potential, and polarizability. Some of these rules are well known among physical chemists, others are less known, and few of them have been discussed and employed in geochemical literature. The recent compilations of thermochemical data have also made it possible to check the validity of the rules and to explore some details not studied previously. The general stability rules discussed in the first sections of the paper are utilized in a discussion of the oxyphile-sulfophile or lithophile-chalcophile grouping of elements in geochemistry, to which the second half of the paper is devoted. It is shown that the oxyphile character of elements often is very different from their lithophile character. In other words an elements affinity for oxygen in oxides is often very different from its affinity for silicate anions or other oxysalt anions. Elements of geochemical significance are arranged in an oxy-sulfo potential sequence and in a silicate-sulfo potential sequence based upon thermochemical data. The silicate-sulfo potentials of the elements are in excellent agreement with their natural geochemical distribution.

Mg^{++}

Because gravity influences chemical equilibrium, consideration of the thermodynamics of our globe requires understanding of chemical processes and stability in the gravitational field. The influence of gravity on stability of mixed condensates (crystals or magmas) and of stoichiometric crystals is analyzed. The theoretical analysis supports the hypothesis of granitization and metasomatism by upward diffusion of some elements.

in Coexisting Olivines and Pyroxenes

The thermal stabilities of simple hydrous solids are discussed from a crystal-energy point of view. It is shown that the relative thermal stabilities of such solids are related to the field strength of the cation to which the OH-group is attached. If the field is only moderately strong, then the stability of the hydrous compounds increases with decreasing field strength around the cation which is attached to OH. In other words, the relative thermal stabilities of the hydrous solids are opposite to what one would expect on the basis of the strength of the metal-OH bonds. The reason for this is discussed in some detail. If the field around the ion at which OH is attached is stronger than a certain limit (coinciding with the field around

Relationships between Heats of Reactions among Solids and Properties of the Constituent Ions, and Some Geochemical Implications

B_{3+}

A Theoretical Approach to the Thermal Stabilities of Hydrous Minerals. I. General Principles as Revealed by Studies of Hydroxides and Oxyacids

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