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Basin Analysis: A Synthesis
Published in Supriya Sengupta, Introduction to Sedimentology, 2017
Having obtained a broad picture of the basin architecture and the overall nature of the basin fill, one should undertake a petrographic study of the rock samples collected from the field. Modal analysis of sandstone samples by the point-counting method provides quantitative information on the mineral proportions. The sandstones are broadly classified into arenites and wackes on the basis of matrix content. The proportions of quartz, feldspars and rock fragments are then plotted on a QFR diagram for further subdivision of the sandstone type (see Fig. 4.3). Proportions of undulose, non-undulose and polycrystalline varieties of quartz, plotted on the Indiana diamond diagram (see Fig. 2.3), give clues to the nature of the non-sedimentary source rock. The heavy mineral assemblage (see Table 2.2), degree of sorting and rounding (see Fig. 3.9) of the clastic particles provide insight not only into the possible provenance, but also into the transportational history of the sediments. Grain-size data may be utilised in a limited way for interpreting palaeo-environment and palaeohydraulics (Chapter 3).
Natural aggregate sources and production
Published in Mark Alexander, Sidney Mindess, Aggregates in Concrete, 2005
Mark Alexander, Sidney Mindess
Once a representative sample (within the error limits achieved during the sampling process) has been obtained, it can be analysed physically or chemically. For petrographic analysis, it is necessary to report the results on a mass or volume proportion basis. This requires either separating the sample by hand into its discrete particles (if different minerals are represented by liberated particles), or counting constituent grains within a sub-sample by eye or under a microscope, depending on grain size. For a quantitative estimation of composition, the sample is screened into closely spaced fractions. Where minerals are mixed in individual grains, thin or polished sections are used for microscopic examination. A point counting technique is then used, and the error depends on the number of points counted and the proportion of minerals present. Point counting on thin or polished sections is usually done using an 'automatic' point counter. A separate key is assigned to each constituent mineral and the slide is mechanically advanced one interval along a grid counting line each time a key is pressed, the key depending on which mineral is under the cross-wires. The total count for each key is automatically recorded and used to calculate the mineral composition. The pre-set grid interval (generally 0.2-1.0 mm) depends on the grain size of the rock. A minimum of 1600 points is usually considered representative. This specialist technique must be undertaken by a competent geologist
Fragment size measurements and statistics
Published in John A. Franklin, Takis Katsabanis, Measurement of Blast Fragmentation, 2018
Mineral grains in rock, and also powders, can be measured optically by microscope, using point-counting or line-scanning methods. Either a micrometer stage is moved beneath the crosshairs of the eyepiece, or the hair of a filar micrometer in the eyepiece is moved across the image of the particle.
Experimental Study of Dry Desliming Iron Ore Tailings by Air Classification
Published in Mineral Processing and Extractive Metallurgy Review, 2019
Steven P. Suthers, Pravan Pinto, Venkata Nunna, Anh V. Nguyen
Point counting analysis was performed on mounted polished sections of various size fractions of the prepared classifier feed sample to determine the proportions of the mineral components. The overall composition determined by point counting was 7.9% hydrohematite, 38.8% vitreous goethite, 22.4% earthy goethite, 25.3% ochreous goethite, 3.5% kaolinite, and 2.1% quartz. The normalized weighted distributions of the mineral components in each size fraction were calculated relative to the bulk sample and are shown in Figure 4. The figure shows that a substantial portion (74%) of the total ochreous goethite in the classifier feed is distributed in particles smaller than 25 µm, as well as 23% of the total quartz and 22% of the total earthy goethite, while the majority of the hydrohematite, vitreous goethite and kaolinite is distributed in the coarser size fractions between 106 and 2000 µm. This indicates that the tailings sample under study constitutes two distinct tailings streams that were combined before being disposed of.
Paleosols and weathering leading up to Snowball Earth in central Australia
Published in Australian Journal of Earth Sciences, 2021
Point counting of individual beds shows abundant dolomite grains throughout each bed, but little depletion of rock fragments and feldspar near the surface (Figure 7). Clay enrichment toward the surface of beds is abruptly truncated below sharp grainsize discontinuities with overlying siltstone and sandstone, and clay is confined to intervals only 5–15 cm thick. Dolomite is depleted near the top of the profiles but is very abundant at the base of beds. The dolomite rhombs are rounded, have darkened edges, and are separated by intervening red clay. Dolomite grains in red siltstones are not interlocking and clean of clay like dolomite in stromatolitic units within the Johnnys Creek Formation (Klaebe et al., 2017; Southgate, 1989).
Zebra rock and other Ediacaran paleosols from Western Australia
Published in Australian Journal of Earth Sciences, 2021
Point counting of individual beds shows surface depletion of rock fragments and feldspar in proportion to degree of destruction of bedding, especially marked within the thin zebra rock beds (Figure 7). Clay enrichment is commonly abrupt below sharp grainsize discontinuities with overlying sandstone and confined to intervals only 5–15 cm thick, unlike alteration by symmetrical hydrothermal or diffuse metamorphic alteration (Kelka et al., 2017; Wallace & Hood, 2018). This is best explained as the result of hydrolytic weathering to clay of feldspar within soil profiles (Retallack, 2013, 2019). These abrupt contacts are well-developed soils overlain sharply by fluvial sandstones.