Siliceous rocks are sedimentary rocks that consist of SiO2, either in the form of amorphous silica (opal) or microcrystalline, cryptocrystalline quartz or chalcedony. These rocks are generally referred to as cherts. Chert is synonym of jasper and flint, although it is mostly used by geologists: flint tends to be more popular among archaeologists, and jasper has a gemological connotation. Most cherts originate from the accumulation of siliceous ooze, consisting of fragments of some organisms (e.g. sponges) or shells of single-celled organisms (diatoms and radiolarians). Precambrian cherts contain microfossils of cyanobacteria. Cherts may also occur as nodules in limestones, where they form as a result of diagenetic processes. SiO2 – rich rocks may also form due to inorganic chemical precipitation, for example in the presence of silica-rich waters (i.e. geysers), but they are quite uncommon in the sedimentary record. Cherts contain other particles than silica, like clay and oxides, that originate from pelagic deposition of siliciclastic sediments together with siliceous ooze, commonly due to the settling of aeolian dust in oceans. Trace elements may color these rocks, which can show white, black, green, red, or grey hues.
Several organisms produce shells or skeletal fragments that consist of amorphous silica. The most important silica producers are diatoms and radiolarians, unicellular planktonic organisms that live in the upper 200 meters of the water column. Diatoms are single-celled algae (protists) that rely on photosynthesis and produce shells made of opaline silica between 2-200 micrometers (0.002 – 0.2 mm) in diameter. Radiolarians are protozoa that are part of the zooplankton and build 0.1 – 0.2 mm large shells of amorphous silica.
These silica shells slowly settle and accumulate on the seafloor after the death of these organisms: the descent of the shells through the water columns may take up to several weeks or months and, during this time, the remains may be consumed by other organisms or partially dissolve in water. The amorphous silica produced by diatoms and radiolarians is, indeed, very unstable and tends to dissolve in the silica-undersaturated marine waters producing silicic acid (H4SiO4). Dissolution processes are stronger in the shallower part of the water column, while deep marine waters, generally more acidic, are less aggressive to silica. The deposition of siliceous ooze on the seafloor is also an indirect effect of the dissolution of carbonates, which increases with depth in present-day oceans. In more acidic, deep waters rich in CO2 the carbonatic shells of other organisms (like foraminifera) dissolve, enriching the resulting sediment in siliceous ooze. Indeed, in present-day oceans, siliceous oozes accumulate on abyssal plains at more than 4000-5000 meters depth.
Cherts that originate from the lithification of siliceous ooze are classified according to the prevailing microfossils that constitute them as radiolarite (or radiolarian chert), if they are radiolarians, or diatomite, if they are diatoms. In present-day oceans, radiolarian cherts form close to the tropical regions, while diatomites are more common in the arctic and antarctic oceans. There are other organisms that may produce important deposits of siliceous oozes, such as sponges and even cyanobacteria in the Precambrian Earth. Sponges are benthic organisms (living anchored to the seafloor) that produce spicules, which are needle-like skeletal fragments that sustain the organism and offer defense against predators. Spicules can be microscopic to larger than a few millimeters. Cherts consisting of spicules are called spiculite.
After the deposition of siliceous ooze, a series of mineralogical transformations starts to take place. Amorphous, hydrous silica, which is an unstable compound, is substituted first by metastable cristobalite and trydimite, then by microcrystalline or cryptocrystalline quartz. These phase transformations reduce the porosity and cement the original sediment into relatively strong cherts.
Nodules of cherts
Many carbonatic rocks contain non negligible content of silica-rich particles, produced by remains of silica-secreting organisms that deposit together with calcareous oozes. Since the amorphous silica produced by organisms is really unstable, it tends to dissolve and recrystallize after its deposition, during lithification and diagenesis. As a result, silica starts to migrate in the calcareous sediments, and redeposit as nodules in specific horizons where the pH conditions allow the precipitation of silica. Cherty nodules grow as the surrounding carbonates are progressively replaced by silica, producing cherty limestones with nodules of chert. Nodules of cherts are also known as flint, especially when they occur in chalk or fine-grained limestone.
Waters in some hot springs contain high amounts of dissolved silica, which is chemically deposited close to springs and geysers, due to fluctuations in pH and temperature decrease. These waters commonly originate from the interaction of water with underlying, degassing magma, from which silica is dissolved. They are very rare rocks in the sedimentary record.
Identification of siliceous rocks
Siliceous rocks occur in sedimentary sequences, usually interbedded with shales, or carbonatic rocks. They consist of microcrystalline quartz and, therefore, have hardness around 7, which implies that they are not scratched by metal. The fracture of cherts is typically conchoidal, as this rock does not contain specific planes of weakness. Siliceous rocks do not react to HCl, while the surrounding limestones do: this is useful to identify nodules of chert within limestone. In sedimentary sequences, cherts are very resistant to erosion and tend to be in relief compared to the surrounding carbonates and shales. The identification of the microfossils that constitute cherts requires a microscope, since most of these fossils are smaller than a few hundreds of micrometers. However, some large radiolarian or spicule may be visible with the help of a hand lens.
Geyserites are different from other siliceous rocks, since they form due to chemical precipitation: they usually show structures that indicate cyclical deposition over a substratum, like concentric layers, botroidal textures or globular aggregates. They may contain encrusted rocks or fossils, like remains of plants that were present in the depositional environment.
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