Chalcedony cement

Quartz can occur as detrital grains or as chalcedony cement. Chalcedony cement or particles in samples can be particularly difficult to detect using optical microscopy because the material is anisotropic in nature and similar in appearance to microcrystalline francolite. Quartz can also occur as opaline varieties in sedimentary phosphate rocks. 

Chert is defined here, following Folk (1980, p. 79), as a chemically precipitated sedimentary rock, essentially monomineralic and composed chiefly of microcrystalline and/or chalcedonic quartz, with subordinate megaquartz and minor amounts of impurities. Chalcedony, which consists of sheath-like bundles of thin, radiating fibers of SiOi, is rare in most Precambrian chert. Precambrian chert occurs as distinct beds or lenses, particularly in Archean rocks, as nodules or silicified laminae in carbonate rocks, or as a sihceous end-member, as granules or cements in iron formation. 

Chalcedony replaces bioclasts, particularly echinoderms, and calcite cement in some samples of shelf arenites near the base of SI (up to 10%). It occurs as... 

The siliceous clay raw material is a source of both aluminum and silica. It may contain one or more types of clay minerals. Other sources of silica are quartz, chalcedony, opal, feldspar, etc. Table 1 shows the types of clay minerals and their suitability as raw mixes for cement manufacture. ... 

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