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Orthoquartzite

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An indurated silicate-sandstone that is made up almost exclusively of quartz sand (with or without chert), that is relatively free of or lacks a fine-grained matrix; a hard but unmetamorphosed "pure quartz sandstone", derived by secondary silicification; a quartzite of sedimentary origin, or a pure quartz sandstone. The term generally signifies a sandstone comprising more than 90% to 95% detrital quartz and chert grains that are well-sorted, well-rounded, and cemented primarily with secondary quartz (sometimes with carbonate) in optical and crystallographic continuity with the detrital grains. The rock is characterized by stable but scarce heavy minerals [glossary item missing] (zircon, tourmaline, rutile and magnetite), by lack of fossils, and commonly by prominence of cross-beds [glossary item missing] and ripple marks [glossary item missing]. It commonly occurs as thin but extensive blanket deposits associated with a widespread unconformity (e.g., an epicontinental deposit developed by an encroaching sea), and it represents intense chemical weathering of original minerals and destruction of most, other than quartz, plus considerable transport and washing action before final accumulation (the sand may experience more than one cycle of sedimentation), and stable conditions of deposition (such as the peneplanation stage of diastrophism); e.g., St. Peter Sandstone (Middle Ordovician) of midwestern United States.

See Also: quartzite

Ref: AGI


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