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Classification of Chert Sedimentary Rocks

Chert is one the major sedimentary rock types; it occurs as a common surface rock and is a major component of gravel beds and bars in rivers and streams. Chert is left behind after dolomite erodes away into soil. American Indians preferred the hard but easily chipped stone in their manufacture of arrowheads, knives, spear blades, and tools. It was also used in the firing mechanisms of flintlock rifles.

Sedimentary Rock
- Rocks are classified by geologists according to how they were formed; chert is a type of sedimentary rock. The many different processes that formed sedimentary stone complicates their classification, but the main categories of sedimentary rock are clastic or siliciclastic rock, biochemical rocks and chemical rocks. Chert, rock salt and gypsum are categorized as chemical rocks formed in association with marine environments. The chief component of sedimentary chert is silicon dioxide (SiO2).
Formation
- Chert falls into the category of siliceous chemical rock made of the compressed skeletal remains of microscopic marine organisms. The shells of diatoms, radiolarians, glass sponges and flagellates fall to the ocean bottom to set the stage for chert formation. Chert isn't classified as a biochemical rock like coal, as it forms by the chemical recrystallization of silica into chert. Chert is made when seawater evaporates, causing the dissolved mineral silicon dioxide to precipitate into solid form.
Chert Properties
- Chert is classified as a microcrystalline variety of quartz like chalcedony, agate and jasper; microcrystalline quartz contains microscopic, highly compacted crystals. Some geologists classify chert as a cryptocrystalline species of quartz. Chert is extremely weather-resistant and splits in glass-like conchoidal fractures along fault lines in the stone. It's usually white or light gray, but is found in a range of colors from red, yellow, brown and tan to dark gray. Its colors come from chemicals and minerals, such as iron oxide, in the sediment surrounding the chert.
Chert Types
- Chert is classified into two major varieties -- nodular chert and bedded chert. Bedded chert is composed of layers of the stone interbedded with shale. Nodular chert grows within a softer sediment matrix of limestone or dolomite that erodes away over time. The nodules form by a process called diagenesis that involves the migration of silica to solidify sediment into rock. Chert nodules are solid-colored or banded with different colors. Deep red chert is called jasper, while high quality, dark-colored chert is known as flint.

Rocks & Minerals