Preferred Mineral Orientation Research Articles

Seismic Anisotropy in the Lower Oceanic Crust

HESS1 suggested that upper mantle compressional wave velocities near the Mendocino and Molokai fracture zones of the Pacific vary with azimuth and this anisotropy is the result of preferred mineral orientation. Recent seismic refraction evidence indicates that in many regions the Earth's upper mantle is anisotropic2–4 and the observed Anisotropy in the upper mantle correlates well with laboratory studies of velocity Anisotropy in dunites and peridotites5,6. Although anisotropy complicates seismic refraction investigations its importance cannot be overestimated; it places severe limitations on probable upper mantle compositions and provides a means of estimating the stress fields which existed during the formation of preferred mineral orientation in the upper mantle.

Clay-Ironstone Concretion Preserving Fabrics Due to Progressive Compaction

Fabrics arising from preferred orientation of clay minerals are presented in a sequence of samples taken from the center of a diagenetic siderite concretion into the enclosing shale sediment. Compactional histories, as deduced directly from estimated porosity values before cementation with siderite, and as deduced from the rotation of mineral flakes to produce the observed fabric, confirm each other. It is concluded that the particular carbonate concretion studied started to grow within the pore space of a flocculated clay sediment soon after deposition. An early period of rapid growth is distinguished from a second, protracted phase which extended effectively until the sediment achieved its present degree of compaction.

Intrusion features of some hypabyssal south African kimberlites

Kimberlite in certain dykes and in the deepest parts of some diatremes show textural and other features which contrast with those in the breccia diatremes. Some hypabyssal kimberlite intrusions show relatively high-temperature contact phenomena including baking of country-rock sediments and sedimentary xenoliths, and contrasting with the brecciated texture of most diatreme-facies kimberlites, in the hypabyssal kimberlites are numerous examples of preferred orientation of inequidimensional minerals (? trachytic flow texture), and rapid mineralogical gradients from the contact towards the dyke centres that may be attributable to flowage differentiation. In the Benfontein sill (Kimberley area) there is well-developed horizontal banding due to gravitational settling, and pseudo-sedimentary structures are also present. The accumulated evidence indicates that kimberlite existed as a relatively hot fluid up to depths of 2–3 km below the land-surface at the time of intrusion; above this level, gas release caused diatreme formation, brecciation and adiabatic cooling. These views are contrary to those of geologists who postulate eruption of kimberlite as a cold breccia directly from the mantle or deep within the crust, but accords with the views of many Russian geologsts who accept the existence of kimberlite magma, the extrusive equivalent of which is the ultrabasic lava meimechite.

Preferred Mineral Distributions of Polymineralic Rocks Related to Non-Hydrostatic Stresses as Expressions of Mechanical Equilibria

Recrystallization of a mineralogically heterogeneous rock under non-hydrostatic stress conditions should result in a preferred mineral distribution as well as preferred mineral orientations that are related to the stress direction. A stress-strain model is developed wherein calculations show that different mineral distributions provide different strain energies per unit stress for the rock. The maximum strain energy per unit stress is achieved if the minerals are arranged to give the modal composition of the rock in the direction parallel to the maximum compressive stress direction and monomineralic layers parallel to the direction of applied shear stress. These mineral distributions or arrays provide the maximum work per unit stress and the maximum decrease in the mechanical potential energy for the system and are regarded as the equilibrium arrays. However, these arrays do not necessarily provide the minimum chemical potentials of the constituents of the strained minerals. It is concluded that the equilibrium form for the system is the mechanical equilibrium mineral array, and the results of strain are described by Kamb's (1959, 1961) treatment of chemical potentials of strained systems.

Elastic strain and preferred orientation in monoclinic crystals

Microcline phenocrysts in a monzonite gneiss, and hornblende and actinolite crystals in a series of amphibole schists, form strong lineations of elongate crystals perpendicular to axes of small folds. The crystallographic c axes of the amphibole and microcline crystals form strong maxima parallel to the A fabric axis of the rocks; the b axes of microcline form a strong maxima parallel to the C fabric axes and the b axes of the amphiboles form maxima parallel to the B fabric axis. If the folds and foliation planes are products of inelastic shear strain, these orientations correspond to the maximum elastic strain energies per unit stress for these crystals and agree with the prediction of preferred orientation by MacDonald (1960) . A predicted effect of positive and negative shear strain was not observed in these crystals. Preferred mineral orientations and recrystallization may be interpreted by use of elastic strain theory to give additional insight into the problems of rock deformation.

The Origin of Crossed-Girdle Orientations of Optic Axes in Deformed Quartzites

In the narrow metamorphic aureole of the Papoose Flat granitic pluton, Inyo Mountains, California, progressive flattening of stratified sedimentary rocks is observed; it is accompanied by recrystallization and development of foliation, lineation, and preferred orientation of minerals. The field and petrographic evidence indicate that these phenomena occurred concomitantly during a single episode of deformation and metamorphism associated with the emplacement of the pluton into previously unmetamorphosed rocks. The type of strain in the rocks is reflected in distortion of fossils and well-developed boudins. Preferred orientation in recrystallized quartzites is shown to be related to this episode, and the purpose of the paper is to correlate the preferred orientations with the strain and stress insofar as they are specified by the structures noted. The quartz axes are typically oriented in two nearly perpendicular planes that are symmetrically inclined to the foliation and intersect in the foliation perpend...

Clay Fabric and its Relation to Structural Properties in Mid-Continent Pennsylvanian Sediments

ABSTRACT Many structural properties of argillaceous sediments correlate closely with clay fabric; the degree of preferred orientation of clay particles. Shaly structure is associated with a significant degree of clay particle orientation, massive structure with a more random clay particle arrangement. Fissile structure occurs in shales where there is a high degree of orientation of clay minerals and other flaky constituents. It is best developed in shales containing a high percentage of organic material and where organic material is concentrated in bands. Organic material and in some cases, carbonate minerals, exert a secondary influence on fabric and structural properties. In ancient sediments that contain a significant percentage of organic material, the preferred orientation of clay minerals generally increases as the percentage of organic matter increases. Sediments with more than 10 percent carbonate minerals commonly show a poor degree of clay particle orientation. Little relation appears to exist between fabric or structural properties and clay mineral composition, except perhaps in sediments with appreciable montmorillonite. Poor particle orientation has been shown to exist in sediments in which montmorillonite is the major clay mineral component. No apparent relation was found between fabric or structural properties and total clay content (above 20 to 30 percent), geologic age, or depth of burial. Variations in orientation of clay particles, from essentially parallel to nearly random, occur within a few vertical inches in some thin stratigraphic sections. This, and the close relation of particle orientation to structural properties suggest that the orientation of clay minerals (fabric) in argillaceous sediments is not dominantly controlled by compaction caused by the weight of overlying sediments. These relations appear to reflect physicochemical conditions in the environment of deposition that influence the mode of clay mineral sedimentation, and/or geochemical factors that may promote particle rearrangement soon after deposition.

Shear wave velocities in metamorphic rocks at pressures to 10 kilobars

Shear wave velocities are reported at pressures to 10 kb for a variety of metamorphic rocks. The velocities were found to vary with propagation and displacement directions. Anisotropies at high pressures correlate with preferred mineral orientation. Calculated shear velocities from modal analyses are within a few per cent of the mean observed velocities. Elastic constants calculated from the shear wave velocities and previously reported compressional wave velocities are given for isotropic elastic theory. It is suggested that the elastic properties of micaceous rocks are similar to materials with hexagonal symmetry.

Experiments on preferred orientation of platy minerals

Polycrystalline aggregates of phlogopite, talc, and brucite have been grown hydrothermally from their constituent oxides at 300–600° C, 3–5 kb, and compressed 10–30% in short-term experiments (typically 30 minutes). Under hydrostatic conditions, approximately random orientation of crystals results. When the specimen is strained at high temperature, either during or after growth of the minerals, a preferred orientation of basal planes normal to the axis of compression results. Since a similar result is obtained by straining at room temperature after growth of the minerals, the mechanism of orientation is probably mainly mechanical rotation after formation. Microscope examination showed that the preferred orientation is most marked in coarser grains of the aggregates. A second kind of foliation is defined in some specimens by closely spaced, narrow domains within which coarse grains are slightly rotated. These domains occur in conjugate sets symetrically oriented at about 45° to the axis of compression. They are interpreted as shear domains and are geometrically similar to incipient strain-slip cleavage in foliated rocks. The experiments may represent likely behavior in geological situations where the temperature or time scale precludes recrystallization during deformation, but they are probably not directly revelant to cases of axial-plane cleavage where reorientation through an influence of stress or strain during recrystallization is believed to have occurred. No unequivocal indication of the latter process was obtained in the experiments.

Compressional wave velocities in metamorphic rocks at pressures to 10 kilobars

Velocities of compressional waves have been measured at pressures of 100 to 10,000 bars for various metamorphic rocks which are believed to be important constituents of the earth's crust. Detailed petrographic and petrofabric analyses are reported from, thin sections cut from the specimens. The principal factors contributing to the velocities of compressional waves in igneous and metamorphic rocks are discussed. Particular attention is given to the dependence of velocity on porosity, mineral orientation, and mineral composition. The principal conclusions are: (1) the initial changes in velocity with pressure are related to the arrangement and shape of pore spaces in a rock, (2) variation of velocity with propagation direction in metamorphic rocks is a consequence of preferred mineral orientation, and (3) velocities calculated from single-crystal data agree with most of the measured velocities.

The Nunarssuit intrusive complex South Greenland. Part I. General description

The Nunarssuit intrusive complex South Greenland. Part I. General description

A note on the thermodynamic theory of nonhydrostatically stressed solids

In a recent paper printed in this journal, Kamb [1961] reviewed the thermodynamic theory of nonhydrostatically stressed solids. I agree with his view cited as follows: ‘Since the Gibbs free energy is only a secondarily defined quantity in ordinary hydrostatic thermodynamics, energy and entropy being the primary quantities of physical significance, it is of course also permissible in nonhydrostatic thermodynamics to attempt to define a Gibbs free energy in a suitable way’ (p. 265). However, I should like to comment on his discussion. Kamb presented the theoretical basis both for the preferred orientation of minerals and for phase transformation under the nonhydrostatic stress (Appendix 2, p. 269). His derivation of the equilibrium condition does not seem to be explicit. His equation [49] (hereafter equations that appeared in Kamb's [1961] paper are denoted by [ ]) must be:

Die r�ntgenographische und optische Gef�geanalyse von Erzen, insbesondere mit dem Z�hlrohr-Texturgoniometer

Quantitative determinations of preferred orientation of opaque minerals have been made only in a few cases by measuring cleavage faces (galena,D. Schachner-Korn). In this paper several methods are investigated critically which are likely to allow the determination of pole figures of ore minerals in a reasonable time. In the first part the special difficulties are discussed which occur in measurements of preferred orientation with X-ray pole figure goniometers (reflection method byL. G. Schulz) when the specimen consists of more than one mineral, when the mineral investigated is coarse-grained, and when the specimen is small. A critical discussion of instrumental errors is included. This method can now also be applied to the determination of preferred orientation in rocks, ceramic materials, etc. In the second part optical methods are given which include the construction of pole figures from measurements of bireflection and polarization figures of uniaxial minerals in reflected light. Examples of pole figures of pyrrhotite, hematite, pyrite, sphalerite, and chromite are given.

Deformation of Rocks.: III. Cleavage and Fissility

Previous articleNext article FreeStudies for StudentsDeformation of Rocks.: III. Cleavage and FissilityC. R. Van HiseC. R. Van Hise Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Journal of Geology Volume 4, Number 4May - Jun., 1896 Article DOIhttps://doi.org/10.1086/607530 Views: 65 Citations: 3Citations are reported from Crossref PDF download Crossref reports the following articles citing this article:Spencer R. Pelton Provisioning the High Country: A Distributional Analysis of Ground Stone Tools from the Colorado Front Range, Plains Anthropologist 62, no.243243 (Mar 2017): 247–268.https://doi.org/10.1080/00320447.2017.1291468Ivan S. Zagorčev The geometrical classification of folds and distribution of fold types in natural rocks, Journal of Structural Geology 15, no.3-53-5 (Mar 1993): 243–251.https://doi.org/10.1016/0191-8141(93)90123-RW. D. Means, M. S. Paterson Experiments on preferred orientation of platy minerals, Contributions to Mineralogy and Petrology 13, no.22 (Jan 1966): 108–133.https://doi.org/10.1007/BF00518022