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Minerals, rocks, discontinuities and rock mass
Published in Ömer Aydan, Rock Mechanics and Rock Engineering, 2019
Chert is a fine-grained, silica-rich cryptocrystalline sedimentary rock. It varies greatly in color from white to black but most often manifests as gray, brown, grayish brown and light green to rusty red.
Sediments and Sedimentary Rocks
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
Chemical precipitation can also create secondary minerals in rocks and sediments. Figure 8.65 shows jasper (a variety of inorganic chert), made of SiO2 stained by red iron oxide, formed when groundwater deposited silica in preexisting rock nearly 3.5 billion years ago. Chert, which can also form through organic processes, includes many distinct varieties, including flint (black or gray because of included organic matter), jasper (red or yellow because of included iron oxides), petrified wood (preserved by silica), and agate (silica with concentrically layered rings of distinctive colors).
Sedimentary rocks
Published in W.S. MacKenzie, A.E. Adams, K.H. Brodie, Rocks and Minerals in Thin Section, 2017
W.S. MacKenzie, A.E. Adams, K.H. Brodie
Cherts are rocks composed of authigenic silica – that is silica formed either by precipitation from water or as a secondary mineral within the sediment. Silica is usually in the form of fine-grained quartz. Primary cherts comprise mostly the remains of organisms which secrete siliceous hard parts such as some sponges and the microfossils radiolaria and diatoms. Figure 163 and Figure 164 show a radiolarian chert. The sample shows the spherical radiolarian tests and a few thin spines set in a matrix masked by red-brown iron oxide. The fine-grained nature of the quartz making up the radiolaria is evident in the crossed polars view (Figure 164).
Future of photovoltaic materials with emphasis on resource availability, economic geology, criticality, and market size/growth
Published in CIM Journal, 2023
G. J. Simandl, S. Paradis, L. Simandl
Silicon is a nonmetallic element in Group 14 (carbon family) of the periodic table with atomic number 14. It is the second most abundant element in the earth’s crust by weight (31.14%) after oxygen (Rudnick & Gao, 2014). It can be found in a wide variety of minerals and elemental compounds. Silicon dioxide (SiO2) or silica is one of the most common compounds, forming all quartz polymorphs and varieties, agate, opal, and chert. Quartz is one of the main rock-forming minerals and the main constituent in high-purity sand, sandstone, and quartzite. It is commonly the main constituent of cores of pegmatites and mineralized or barren hydrothermal veins. Silica materials are available on all continents and satisfactory for most common applications, including ferrosilicon and metallurgical-grade silicon (MG-Si). However, in most cases, the silica content of these rocks is too low and the impurities content is too high for direct transformation to solar- or electronic-grade Si.
From intrabasinal volcanism to far-field tectonics: causes of abrupt shifts in sediment provenance in the Devonian–Carboniferous Drummond Basin, Queensland
Published in Australian Journal of Earth Sciences, 2019
K. Sobczak, S. E. Bryan, C. R. Fielding, M. Corkeron
A major change in sandstone composition is observed for Cycle 2 formations (Figures 14–16). All analysed Cycle 2 sandstones have >60 vol% quartz, and most of the Telemon and Mt Hall formations contain >80 vol% quartz (Figure 15), with few lithic grains and even less detrital feldspar. Most samples plot within the recycled orogen field. Quartz occurs as both monocrystalline and polycrystalline varieties (including chert). The majority of the quartz grains (in some samples—all grains) show weakly undulose extinction and contain fluid inclusion trails (Figure 16b), contrasting with Cycle 1 sandstones. Mineral inclusions such as apatite, are present inside some large quartz grains. There is also a small number of inclusion-free quartz grains with straight extinction. Feldspar, where present, is dominated by plagioclase and usually sericitised to various extents. Mica grains are commonly present, with muscovite more abundant than biotite; zircon and apatite are common detrital heavy minerals. The grain composition is consistently quartz-rich in the south and most of the northern parts of the basin, although samples collected from the northernmost locations (Dandenong and Campaspe DDH-1) show a slight increase in the percentage of feldspar and lithic grains, and consequently less quartz (Appendix 2). Unlike Cycle 1, Cycle 2 quartz grains are consistently well rounded across most of the basin. The grains become more angular in the Campaspe DDH-1 drill core, as noticeable for all clast types in both Mt Hall and Raymond formations intersected in the core.
Alteration and mineral zonation at the Mt Lyell copper–gold deposit, Tasmania
Published in Australian Journal of Earth Sciences, 2018
Glen Lyell is a large 700 m × 250 m NNW-oriented alteration zone at the southern end of the Mt Lyell field. At the surface, it is a zone of intensely cleaved quartz–muscovite schist surrounded by chlorite–muscovite-altered rocks. Surface and drill SWIR analysis and HyMap survey results (see below) indicate that the alteration is an advanced argillic assemblage of pyrophyllite, topaz, alunite and barite. There is only minor muscovite associated with this alteration and no cryptocrystalline quartz (‘chert’). Numerous attempts have been made to drill this alteration assemblage at depth, but faulting, combined with the very schistose nature of the alteration, resulted in many drilling difficulties. However, 14 holes have been completed, and this shows that at 500 m depth the alteration assemblage remains unchanged, the shape of the alteration zone is subvertical, and only very low tenor Cu, Au and Ag results are recorded in the advanced argillic alteration. Minor Cu intersections are recorded in the chlorite–muscovite rocks. The alteration assemblage and low Cu tenor at Glen Lyell are similar to those seen in the Prince Lyell hole PLD 0124 (pyrophyllite, zunyite) and also at the top of the Prince Lyell (see HyMap discussion below) and Western Tharsis (Huston & Kamprad, 2001) deposits.