Chert

Lore & Background

Chert is found in settings as diverse as hot spring deposits (siliceous sinter), banded iron formation (jaspilite), or alkaline lakes. However, most chert is found either as bedded chert or as nodular chert. Bedded chert is more common in Precambrian beds, but nodular chert became more common in the Phanerozoic as the total volume of chert in the rock record diminished. Bedded chert is rare after the early Mesozoic. Chert became moderately abundant during the Devonian and Carboniferous and again became moderately abundant from the Jurassic to the present. Bedded chert, also known as ribbon chert, takes the form of thinly bedded layers (a few centimeters to a meter in thickness) of nearly pure chert separated by very thin layers of silica-rich shale. It is usually black to green in color, and the full sequence of beds may be several hundred meters thick. The shale is typically black shale, sometimes with pyrite, indicating deposition in an anoxic environment. Bedded chert is most often found in association with turbidites, deep water limestone, submarine volcanic rock, ophiolites, and mélanges on active margins of tectonic plates. Nodular chert is most common in limestone but may also be found in shales and sandstones. It is less common in dolomite. Where chert occurs in chalk or marl, it is usually called flint.

Reader's Guide

Chert is significant as a major component of the rock record, particularly in Precambrian and Phanerozoic sedimentary sequences. Its formation is closely tied to the biological extraction of silica from seawater by organisms such as diatoms, radiolarians, and glass sponges, which produce siliceous ooze that is later transformed into chert through diagenesis. The transition from opal-A to opal-CT to microquartz occurs at specific temperatures, with the presence of magnesium hydroxide hastening recrystallization. Chert's purity and lack of detrital sediments indicate deposition in areas with little influx of river-borne silt and clay. The Folk classification divides chert into three textural categories: granular microquartz, chalcedony, and megaquartz. Subvarieties such as diatomaceous chert, radiolarite, and spicularite reflect the dominant organisms responsible for silica extraction. Precambrian bedded cherts may have formed nonbiologically in oceans more saturated in silica. Chert's color, ranging from white to black, is an expression of trace elements, with red and green often related to traces of iron in its oxidized and reduced forms. The study of chert provides insights into ancient ocean chemistry, biological productivity, and tectonic settings.

Did You Know?

• Chert is typically composed of the petrified remains of siliceous ooze, which contains the silicon skeletal remains of diatoms, silicoflagellates, and radiolarians.
• Precambrian cherts are notable for the presence of fossil cyanobacteria.
• Where chert occurs in chalk or marl, it is usually called flint.
• The Folk classification divides chert into three textural categories: granular microquartz, chalcedony, and megaquartz.

Mineral Composition & Textural Classification

In petrology, chert encompasses all chemically precipitated sedimentary rocks built primarily from microcrystalline, cryptocrystalline, and microfibrous silica. Most specimens are nearly pure silicon dioxide, carrying fewer than 5% of other minerals such as calcite, dolomite, clay minerals, hematite, and organic matter. Yet the purity spectrum is remarkably wide: some cherts exceed 99% silica, while impure nodular varieties may dip below 65%. Aluminium leads the minor-element roster, followed by iron, manganese, potassium, sodium, and calcium. Extracrystalline water—tiny inclusions lodged within and around quartz grains—contributes less than 1% of most cherts. The Folk classification sorts chert into three textural categories: granular microquartz, composed of roughly equidimensional grains typically 8 to 10 microns across; chalcedony, a microfibrous variety of radiating crystal bundles around 100 microns long; and megaquartz, with equidimensional grains exceeding 20 microns. Most chert is dominated by microcrystalline quartz with minor chalcedony and occasional opal, though the full compositional range stretches from nearly pure opal to nearly pure quartz. Notably, little opal persists beyond 60 million years.

Biogenic Origins & the Fossil Record

Chert is characteristically of biological origin, though it can also form inorganically as a chemical precipitate or through diagenetic replacement, as in petrified wood. Typical chert is built from the petrified remains of siliceous ooze—the biogenic sediment blanketing vast stretches of the deep ocean floor. This ooze preserves the silicon skeletal remains of diatoms, silicoflagellates, and radiolarians. In Precambrian cherts, fossil cyanobacteria are a notable inclusion. Beyond microfossils, chert occasionally harbors macrofossils, though some specimens are entirely devoid of any fossil content. Living organisms such as diatoms, radiolarians, and glass sponges extract silica from seawater even at concentrations as low as 0.1 ppm—far below what inorganic processes could precipitate. Diatoms can double their numbers eight times daily under ideal conditions. Together, these organisms produce an estimated 12 cubic kilometers of opal annually in the world's oceans. Once dead, their skeletons dissolve rapidly unless buried on the ocean floor, where they accumulate into siliceous ooze containing 30% to 60% silica.

Color Palette & Trace-Element Chemistry

Chert displays an impressive chromatic range, spanning from white to black, with the most common hues being gray, brown, grayish brown, and light green to rusty red, occasionally dark green. This color diversity is a direct expression of trace elements embedded in the rock. Both red and green coloration are most frequently linked to traces of iron, appearing in its oxidized and reduced forms respectively. The presence of aluminium as the most abundant minor element, followed by iron, manganese, potassium, sodium, and calcium, contributes to the subtle variations seen across specimens. In bedded chert specifically, the rock is usually black to green, and impurities such as authigenic pyrite and hematite form in the sediments after deposition. The typically high purity of bedded chert, comparable to other chemically precipitated rocks, points to deposition in environments with minimal influx of detrital sediments like silt- and clay-laden river water. The color of a given chert thus serves as a chemical fingerprint, recording the specific trace-element conditions under which the silica was deposited and subsequently altered.

Geological Settings & Diagenetic Transformation

Chert occurs in remarkably diverse settings, from hot spring siliceous sinter deposits and banded iron formations known as jaspilite to alkaline lakes, though most is found either as bedded or nodular chert. Bedded chert, also called ribbon chert, consists of thin layers ranging from a few centimeters to a meter thick of nearly pure chert separated by very thin silica-rich shale. These sequences can extend several hundred meters in total thickness and are most commonly associated with turbidites, deep-water limestone, submarine volcanic rock, ophiolites, and mélanges on active tectonic plate margins. The black shale interbeds, sometimes bearing pyrite, indicate deposition in anoxic environments. Nodular chert became more prevalent in the Phanerozoic as total chert volume in the rock record diminished, while bedded chert is rare after the early Mesozoic. The diagenetic pathway transforms opal-A, an amorphous silica, into opal-CT, a microcrystalline form of bladed cristobalite and tridymite crystals, at roughly 45°C in deep ocean water, and further into microquartz at about 80°C. Magnesium hydroxide hastens these transitions by providing nucleation sites for recrystallization.