Chert is a type of sedimentary rock made up of cryptocrystalline and microcrystalline quartz, which is the mineral form of silicon dioxide. This occurs as layered deposits, concretionary masses, and nodules. Chert can break with conchoidal fracture that usually produces extremely sharp edges.
People in the ancient times took advantage of the way chert breaks and they used it to fashion weapons and cutting tools. Flint and chert are names being used to refer to the same material. These two are both varieties of chalcedony.
How Does Chert Form?
Chert forms if silicon dioxide’s microcrystals grow inside soft sediments which are going to become chalk or limestone. In the sediments, large numbers of silicon dioxide’s microcrystals form into concretions or nodules with irregular shapes once dissolved silica gets transported to the site of formation through groundwater movement.
If there are numerous concretions or nodules, these can grow big enough to merge with each other to form an almost continuous layer of chert in the sediment mass. The chert that is formed through this process is considered a chemical sedimentary rock.
A certain amount of silicon dioxides present in chert is assumed to have biological origin. There are some parts in the shallow seas and oceans that have large numbers of radiolarians and diatoms living in the waters. The said organisms have glassy silica skeleton. There are also sponges that produce spicules made up of silica.
Upon the demise of these organisms, their silica skeletons descend to the ocean floor where these dissolve, re-crystallize then possibly become part of chert nodule. There are areas where the rate of sedimentation of the materials is sufficiently high to form laterally extensive and thick layers of rock. Chert formed through this method can be regarded as a biological sedimentary rock.
Composition of Chert
In most instances, chert is a biogenic rock made of the siliceous tests of siliceous sponge spicules, radiolarians, diatoms, and others. There are times when these sea creatures’ microscopic fossilized remains could get preserved in the rocks.
The siliceous tests are not composed of quartz at first but following their burial, diagenesis, and compaction, opaline siliceous sediments turn into quartz. Even though the material that this is made of came from the marine species’ siliceous tests, the rock itself doesn’t usually get deposited in situ. This might move as a liquid rich in silica and form some nodules in rocks through replacing the original material that is usually carbonate. This is why it is often considered as a rock of chemogenic origin. Its bedded variety is usually linked with turbidity currents.
Properties and Characteristics of Chert
The hardness of chert is similar to that of crystalline quartz that has a hardness rating of 7 in Mohs scale, probably somewhat softer at 6.5 when this still contains hydrated silica. Aside from its hardness, chert is also a pretty tough rock. This stands on top of landscapes in the outcrops that can resist erosion. This is dreaded by oil drillers since it is too difficult to penetrate chert.
Its curvy conchoidal fracture is also less splintery and smoother compared to pure quartz’s conchoidal fracture. This is a favorable thing for ancient toolmakers and the top quality rock became an item for trading among tribes.
Chert is not like quartz in the sense that it is not translucent all the time and is never transparent. It also has a resinous or waxy luster different from the glassy luster that quartz has.
Color of Chert
Chert can occur in a plethora of colors. The continuous gradients of color exist between black and white or brown and cream. It is also common to see red, orange, yellow, and green cherts. The darker colors are usually due to the inclusions of organic matter and mineral matter. Red color is produced when the chert is abundant in iron oxides. Jasper is the name usually used to refer to the reddish cherts. An abundance of organic material can produce black or gray chert. These darker chert colors are called flint.
Types and Classification of Chert
Chert comes in numerous varities classified according to their visible, physical, and microscopic characteristics.
- Flint is a type of high microcrystalline quartz. The name was originally used for chert found in marly or chalk limestone formations formed when silica replaces calcium carbonate.
- Common chertis a chert variety that forms in formations of limestone when silica replaces calcium carbonate. This is the most abundant type of chert.
- Radiolite is the variety of rock formed as the primary deposits and contained radiolarian microfossils.
- Jasper is the variety formed as the primary deposits in connection with or found in magnetic formations that owes the red color to iron inclusions. It is common to see jasper in green, yellow, or black color depending on the specific type of iron that it contains. This is often opaque to near opaque.
- Chalcedony is microfibrous quartz.
- Onyx is banded agate with parallel line layers in white or black.
- Agate is notably banded chalcedony that has successive layers that differ in value or color.
- Opal is hydrated silicon dioxide. This is usually of Neogenic origin. This is actually a mineraloid and not really a mineral and is not considered a chert variety in general. However, there are some opal varieties that are microcrystalline containing less to no water at all. People sometimes confuse chert with opal because of the same physical and visible characteristics.
- Magadi-type chert is the variety forming from sodium silicate precursor in the highly alkaline lakes like Kenya’s Lake Magadi.
- Siliceous sinter is light colored, low density, and porous siliceous rock that is deposited by hot spring waters and geysers.
- Porcelanite is the term used to refer to fine-grained siliceous rocks with a fracture and texture that resemble those of unglazed porcelain.
- Mozarkite is a distinct type due to its unique color variation and the ability of taking a high polish.
Some other not so common terms used to refer to chert, with many of them archaic, include silex, firestone, chat, flintstone, chat, and silica stone.