Petrography

About: Petrography is a research topic. Over the lifetime, 7449 publications have been published within this topic receiving 102018 citations.

Weathering of gneiss in Calabria, Southern Italy

Abstract: The effects of weathering in a Mediterranean climate on the mineralogy and microfabric of Paleozoic gneiss of the Sila Massif, Calabria, southern Italy, have been studied. Field observations show highly weathered rock forms a residual soil. Micromorphological and mineralogical properties of bedrock and saprolite show that the weathering process is characterized by at least two major stages, having two distinct rock microfabrics. In the first stage, the morphological features of the original rock are preserved and weathering is manifested mainly by microfracturing, and large portions of the rock remain unaltered. The second stage of weathering involves further development of microcracks and progressive chemical attack on the minerals. This latter stage occurs along both compositional and microstructural discontinuities, with etch pitting of feldspar, and neoformation of clay minerals and ferruginous products replacing feldspar, biotite, and iron-bearing garnet. The determination of quantitative petrographic indices provides a measure of the various stages of weathering.

13 SEM Observations on Ion-milled Samples of Devonian Black Shales from Indiana and New York: The Petrographic Context of Multiple Pore Types

Abstract: Middle Devonian Marcellus Shale and late Devonian New Albany Shale samples were argon-ion milled and studied by scanning electron microscope for petrographic features and pore development. Petrographic observations revealed that a finite number of pore types exist in spite of considerable variability in composition, depositional setting, and compaction history. The four major pore types are framework pores, framework shelter pores, dissolution pores within inorganic grains, and organic matter pores. Framework pores occur in open spaces of the grain fabric. The most common framework pores are defined by phyllosilicate (clays, micas) and carbonate grains (biogenic, diagenetic) and also occur in areas with abundant diagenetic silica. Framework pores associated with carbonate grains can be well developed where abundant skeletal debris provides shelter porosity. Phyllosilicate framework pores increase in abundance with increasing clay content, but more critically, their abundance in compacted shales hinges on the presence of pressure shadows generated adjacent to mechanically competent grains (quartz, feldspar, dolomite, calcite, pyrite) that resist compaction. Dissolution pores in inorganic grains were predominantly encountered in association with calcite and dolomite grains. In places, other minerals, such as pyrite, also can show dissolution effects, but do not contribute significantly to overall porosity. Dissolution pores probably reflect decreased pH associated with the formation of carboxylic and phenolic acids at elevated temperatures (about 80 to 120° C). At low carbonate contents (a few percent), dissolution pores constitute only isolated porosity, but in shale intervals that contain abundant carbonate, or where carbonate grains were concentrated into laminae, this pore type may be an important facilitator of gas storage and transmission. Pores within organic matter are maturity dependent and restricted to amorphous organic matter (bituminite/amorphinite) in thermally mature samples (>0.6%Ro). Grain and fabric shrinkage during core storage and sample processing produced pore artifacts as well (false pores), and their correct identification is critical for accurate petrographic pore assessment.

Source‐rock evaluation and basin modelling in ne egypt (ne nile delta and northern sinai)

Abstract: The area comprising the NE Nile Delta and adjacent northern Sinai is an important hydrocarbon province in Egypt, but the origin of the hydrocarbons and their migration is not fully understood. In this paper, we evaluate organic source facies and assess the results of basin modelling in order to improve our understanding of the regional petroleum system. Organic-geochemical and petrographic analyses of 800 rock samples ranging in age from Early Jurassic to Pleistocene were used in this study. Most of the samples studied had moderate to high TOC contents, and kerogen was a mixture of Types II and III with a wide range of petroleum generation potentials from very poor to very good. Thermal and burial history models indicate that the source rocks entered the early-mature/mature stage in very recent times (Pleistocene to Recent). The top of the oil window ranges in depth from 1,800 m to 4,500 m, whereas the bottom of the oil window was not reached by most of the studied wells. In combination with numerical modelling results, structure contour maps for key horizons reveal possible migration trends and prospective areas. We tentatively identify prospective targets for hydrocarbon exploration, both in the Tertiary sequence and in the Cretaceous succession, especially where carbonate build-ups are present.

A data brief on the Smithsonian Abyssal Volcanic Glass Data File

Abstract: [1] It is well known that the compositions of glasses quenched by contact with seawater are a good guide to unmodified erupted liquid compositions We present here an updated version of the Smithsonian Abyssal Volcanic Glass Data File (AVGDF) of the major and minor element compositions of such glasses mainly from seafloor spreading centers determined by electron microprobe analyses These data help define the major and minor element compositions of the most abundant terrestrial volcanic rock The AVGDF contains 9050 individual glass analyses along with the sample donor, latitude, longitude, depth, tectonic setting, ship-cruise-dredge, and the major and minor elements SiO2, Al2O3, FeO*, MgO, CaO, Na2O, K2O, TiO2, P2O5, and MnO (for some samples) The samples are from 664 Atlantic, 28 Caribbean, 89 Indian, and 1304 Pacific localities These include dredge, submersible, DSDP and ODP sites The data serve to (1) map out and model processes associated with magma generation and fractionation in the asthenosphere and lithosphere beneath seafloor spreading centers and (2) as an educational aide for exercises in the use of such data at the graduate and undergraduate levels in geochemistry, geophysics, and igneous petrology The extensive polished sample collection is available on loan for additional petrographic (in reflected light) or microbeam analyses

Significance of the unequilibrated ordinary chondrites

Abstract: Olivine compositions of thirteen olivine-hypersthene and olivine-bronzite chondrites, chosen for their apparent freedom from recrystallization, were determined with an electron microprobe. A parallel petrographic study of these samples, and five others in which Keil and Fredriksson found variable olivine compositions, was used to relate the extent of variation in each sample to textural characteristics. Large variations in olivine composition occur in all of the chondrites examined in which (1) intergrowth between chondrules and matrix is absent or very slight and (2) clear, brown mafic glass occurs in the matrices of barred and porphyritic chondrules. Smaller variations occur in samples which show slightly greater intergrowth of the chondrules and matrix and in which microcrystalline material occurs in place of glass. Dispersion of olivine compositions is absent from samples which contain petrographic evidence of substantial recrystallization. Variations in olivine composition and the presence of mafic glass appear to be characteristic of the least-altered chondrites and are probably characteristic of the material from which the chondrites formed. These properties disappear early during metamorphism, before recrystallization becomes petrographically obvious. Thus Bjurbole, which appears only slightly recrystallized on petrographic grounds, has recrystallized sufficiently to homogenize its olivine. These conclusions conflict with the present models of Wood and of Fredriksson, and they impose a new restriction on modifications of these models as well as on the formulation of other models.