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Semi-precious stones
Published in Francis P. Gudyanga, Minerals in Africa, 2020
Agate is a cryptocrystalline variety of silica which occurs in various kinds of rock but is associated with volcanic rocks and common in certain metamorphic rocks [954]. Most agates may be found as nodules in volcanic rocks or ancient lavas, in former cavities produced by volatiles in the original molten mass. These may have been filled by siliceous matter deposited in regular layers upon the walls. Veins or cracks in volcanic or altered rock underlain by granitic intrusive masses may be filled with agate giving rise to banded agate, riband agate and stiped agate.
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.
Minerals and rocks
Published in A.C. McLean, C. D. Gribble, Geology for Civil Engineers, 2017
Chert and flint are varieties of cryptocrystalline silica which can be used as aggregate in concrete if they are weathered. If chert or flint is fresh, it may be alkali reactive and therefore unsuitable to use with Portland cement. Cherts occur as bands or nodules within limestone sequences.
Laboratory investigations on fine aggregates used for concrete pavements due to the risk of ASR
Published in Road Materials and Pavement Design, 2021
Daria Jóźwiak-Niedźwiedzka, Aneta Antolik, Kinga Dziedzic, Katalin Gméling, Karolina Bogusz
Gibas et al. (2019) has shown that a concrete made with a Portland cement CEM I 42.5R with Na2Oeq=0.58%, and the crushed coarse aggregate is classified as innocuous when combined with reactive siliceous river sand exhibited substantial expansion during the performance testing. The crucial reactivity of natural sand clearly contributed to the increase of the concrete prism length. The significance of the siliceous sand reactivity, which corresponds to the sand S2, analysed in the above research, was demonstrated and its potential to alkali-silica reaction is given in Figures 7 and 8. The SEM analysis confirmed that siliceous river sand S2 provokes alkali-silica reaction, and thin section analysis in XPL-G (Figure 7) revealed that the sand grains contained micro- and cryptocrystalline quartz (fine-grained chert) which contributed to the alkali-silica reaction.
The geological history and hazards of a long-lived stratovolcano, Mt. Taranaki, New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2021
Shane J. Cronin, Anke V. Zernack, Ingrid A. Ukstins, Michael B. Turner, Rafael Torres-Orozco, Robert B. Stewart, Ian E. M. Smith, Jonathan N. Procter, Richard Price, Thomas Platz, Michael Petterson, Vince E. Neall, Garry S. McDonald, Geoffrey A. Lerner, Magret Damaschcke, Mark S. Bebbington
Basalts contain phenocrysts of plagioclase, clinopyroxene, titanomagnetite and olivine in a groundmass of brown, cryptocrystalline glass with microcrystalline plagioclase, clinopyroxene and titanomagnetite ± olivine ± orthopyroxene. Apatite is a common accessory phase and hornblende and orthopyroxene are minor constituents, with phlogopite very rare (Gow 1968; Neall et al. 1986; Stewart et al. 1996; Zernack et al. 2012). Basaltic andesites are petrographically similar to the basalts but span two end memberw from pyroxene-basaltic andesites to amphibole-basaltic andesites (the latter containing up to 30% amphibole). The andesites are dominated by plagioclase and amphibole with <20% of samples containing significant modal clinopyroxene. Higher-silica andesites, (e.g. the summit dome) also contain biotite in a groundmass of cryptocrystalline (<0.05 mm), plagioclase, pyroxene and titanomagnetite.
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.