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Chert (/ tʃ ɜːr t /) is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, [1] the mineral form of silicon dioxide (SiO 2). [2] Chert is characteristically of biological origin, but may also occur inorganically as a chemical precipitate or a diagenetic replacement, as in petrified wood.
Here radiolarian cherts overlie spilites and volcanic rocks. Radiolarites are also found in the nearby Southern Uplands where they are associated with pillow lava. The Scottish radiolarites are followed by deposits in Newfoundland from the Middle and Upper Ordovician. The red Strong Island Chert for instance rests on ophiolites.
3 Composition. 4 Occurrence. 5 Types of concretion. ... those in limestones are commonly an amorphous or microcrystalline form of silica such as chert, flint, ...
When opal silica accumulates faster than it dissolves, it is buried and can provide a diagenetic environment for marine chert formation. [8] The processes leading to chert formation have been observed in the Southern Ocean, where siliceous ooze accumulation is the fastest. [8] Chert formation however can take tens of millions of years. [7]
Flint, occasionally flintstone, is a sedimentary cryptocrystalline form of the mineral quartz, [1] [2] categorized as the variety of chert that occurs in chalk or marly limestone. Historically, flint was widely used to make stone tools and start fires. Flint occurs chiefly as nodules and masses in sedimentary rocks, such as chalks and limestones.
In geology and particularly in sedimentology, a nodule is a small, irregularly rounded knot, mass, or lump of a mineral or mineral aggregate that typically has a contrasting composition from the enclosing sediment or sedimentary rock. Examples include pyrite nodules in coal, a chert nodule in limestone, or a phosphorite nodule in marine shale.
Composition [ edit ] Ooids are most commonly composed of calcium carbonate ( calcite or aragonite ), but can be composed of phosphate, clays, chert , dolomite or iron minerals, including hematite .
The layers have very distinct banded successions that are made up of iron rich layers that alternate with layers of chert. Iron formations are often associates with dolomite, quartz-rich sandstone, and black shale. They sometimes grade locally into chert or dolomite. They can have many different textures that resemble limestone.