On the classification of siliceous rocks, some are classified according to the field occurrence, some according to the microstructure of siliceous mineral components, and some according to the genesis (chemical, biological and biochemical). Siliceous rocks can be divided into two types according to their genesis:
(1) Biological or biochemical sources: diatomite, radiolarian rock, sponge rock, tabular diatomite and protein soil.
(2) Abiotic genesis: jasper rock, flint rock and silica. They may be chemical, secondary or related to volcanism.
(1) diatomite
Diatomite (or diatom rock) is mainly composed of diatom remains (diatom shells, whose composition is opal). There are many kinds of diatoms, including1100,000 species in known geological history and more than 5,000 species in existence. The shape of diatom shells is also varied, such as rectangle, square, circle, triangle and so on. And the individual is very small (0.002~0.05mm), mostly less than 0.05mm, and its content in rocks is uncertain, sometimes as high as 70% ~ 80%, and some in modern diatom mud can be as high as 90%. In addition, diatomite can also be mixed with a small number of radiolarians, sponge bone needles and other biological relics. Sometimes there are clay minerals, carbonate minerals, glauconite, detritus and mica. When the clay content exceeds 50%, it will be converted into diatom mud.
Diatomite is white, but it is often dyed light yellow or dark gray or even black by iron or organic matter. Rock is soft and light, and its density is only 0.4 ~ 0.9g/cm3. High porosity, up to 90% ~ 92%. The appearance of the rock is earthy, with loose structure, strong adsorption and sticky tongue. It has a typical biological structure under the microscope. Generally, the bedding is not obvious, and sometimes thin horizontal bedding can be seen. Diatomite looks like chalk, but it is not as delicate as chalk. Hand-ground into powder, it has a rough feeling, and it does not foam when added with acid.
Diatom mud is widely distributed in modern oceans in polar and mid-latitude regions. This deposit is yellowish to gray when wet and turns white when dry. In pure diatom mud, the content of diatom shells can reach 70% ~ 90%, and it can also contain 2% ~ 40% of calcium carbonate and 3% ~ 25% of other minerals. In geological history, most diatomaceous earth is produced in Paleogene and Quaternary sediments, and only a few diatomaceous earth is found in Cretaceous strata. In the older age, diatomaceous earth has undergone secondary transformation, becoming tabular diatomaceous earth or protein soil, and some diatomaceous earth may further become siliceous slate and jasper.
Diatomite produced in Shanwang, Linqu County, Shandong Province, is a Cenozoic Miocene continental freshwater lake deposit with gray-black and gray-white horizontal banded bedding, and the joints along the bedding direction are well developed. The diatomite layer as thin as a piece of paper, after weathering and evaporation of the water contained in it, tilts upward and becomes a beautiful page, so it is called "thousands of books".
(2) Sponge rock
Sponge rock is mainly composed of the remains of siliceous sponge, in which the main organism is sponge spicule, and the composition is mostly opal and sometimes chalcedony. In ancient rock varieties, besides spicules, sometimes a small amount of radiolarians and calcareous shells, as well as clay minerals, glauconite, silt and mineral particles can be seen.
Sponge rock is fine in appearance, light grayish green or black. There are two kinds: hard and loose. ① The hard sponge spicule is cemented by opal, Shi Ying, chalcedony and Yingshi in different proportions, and its appearance is earthy and impermeable. ② Pure and loose spongy rocks are rare and only found in Paleogene-Neogene sedimentary rocks in some areas.
In modern sediments, spongy ooze is rare, only found in the North Ocean, and the content of spongy spicules is 20% ~ 40%. Sponge rocks are common in Cenozoic sedimentary rocks in geological history.
(3) Radiolarian rock
Radiolarian rocks are mainly composed of radiolarians, which can also be divided into loose and hard.
Loose radiolarian rocks look like diatomite, soft, gray or yellow-gray. In addition to radiolarians, there are diatoms, spongy spicules, gray algae, foraminifera and other biological remains. It is usually mixed with clay, and sometimes calcite, glauconite and debris can be seen. This rock was found in Cretaceous and Paleogene sediments in some areas.
Hard radiolarian rock in which radiolarian shells are cemented by silica. There are two kinds: ① The shells and cements of opal radiolarian rocks are opal, and some opals are often transformed into chalcedony and autogenous time, which is very similar to protein rocks in appearance. This kind of rock is usually found in Cretaceous and Paleogene sediments. ② The chalcedony-chronoradiolarian rock shell and cement are chalcedony and spontaneous chronotropic. Radiolarian shells are sometimes replaced by calcite, and the rocks are hard, dense and impermeable. It looks like siliceous slate or jasper rock. This kind of rock is found in Mesozoic and Paleozoic sediments in geosyncline area. Hard radiolarian rocks are actually rare, and the common ones are radiolarian protein rocks or radiolarian siliceous slate and jasper rocks.
In modern marine sediments, the latitude of radiolarian ooze is lower than that of diatom, and the highest content of radiolaria can reach 60% ~ 70%, often mixed with clay and carbonate.
(4) protein soil (protein rock) and tabular diatomite (powder protein rock)
Both components are mainly opal. It is often an aggregate of tiny angular or spherical particles (the size is 0.0 1 ~ 0.005438+0 mm). They are different from diatomaceous earth or opal radiolarian rocks because they contain no or little siliceous biological remains. In addition to opal, rocks may also contain clay minerals, carbonate, pyrite, glauconite, zeolite, chalcedony, Shi Ying, detrital, organic matter and other inclusions. Sometimes there are a small number of diatoms, radiolarians, sponges, foraminifera and so on.
Both kinds of rocks have micropore structure, so they are attached to the tongue. The bedding is not obvious. Both are often produced in the form of lenses.
Protein soil is often harder than tabular diatomite, and the main differences between them are listed in Table 2-7- 1.
Table 2-7- 1 Main differences between tabular diatomite and protein soil
Some people think that the origin of these two rocks is primary chemical deposition, but it is also likely to be secondary changes from other biogenic siliceous rocks (mainly diatomaceous earth, some spongy rocks and radiolarian rocks). Protein soil and tabular diatomite can be further transformed into siliceous slate or jasper.
These two types of rocks are mainly distributed in Cretaceous and Paleogene-Neogene strata.
(5) Siliceous slate and jasper rock
This kind of rock is mainly composed of authigenic time, followed by chalcedony, and often mixed with iron oxide (more than 5%), as well as clay minerals, calcite, rhodochrosite, pyrite, chlorite, mica, organic matter and so on. Sometimes it can also contain a small amount of radiolarians, spongy spicules, cephalopods and brachiopods.
The difference between siliceous slate and jasper lies in its thin bedding.
This kind of rock has various colors, often red, green, grayish yellow or black. Sometimes it is mottled. Usually has a cryptocrystalline or colloidal structure. The particle size is about 0.0 1 mm, and the edge of the particle is serrated. The rock is dense and hard with shell-like cracks. It is mainly distributed in geosyncline area, found in sediments from Precambrian to Mesozoic, and often forms a very thick layer with volcanic rocks, which can reach hundreds of meters thick, and is called Jasper Formation.
There is also a layered siliceous rock, which looks like unglazed porcelain and is called white mud. Can be gradually transformed into fine tuff. Volcanic glass is mostly transformed into chalcedony, but a large number of feldspar fragments with unchanged edges and corners can be seen under the microscope. Fine-grained (siliceous) tuff rarely contains debris, but if there are a lot of terrigenous debris, it will be transformed into tuffaceous sandstone, and then white clay will become quartzite sandstone or siltstone. When the content of clay minerals and mica increases, white clay can be transformed into siliceous clay rock.
(6) Flint (Flint Rock)
Flint is the most common and important type of siliceous rocks, which is characterized by localized production, often nodular, lenticular or banded interlayer, and does not form a stable siliceous rock layer. The main mineral components are opal, chalcedony and spontaneous reaction time. The older you get, the more opals you have. The older you get, the more spontaneous you will be.
In addition to siliceous minerals, the composition of flint often contains clay minerals, carbonate minerals, organic matter and some biological remains, such as sponge spicules, radiolarians, foraminifera and so on.
Flint is a dense and hard cryptocrystalline or microcrystalline rock with conchoidal fracture. It comes in various colors, such as gray, black and dark, yellow, red and white. Flint can be divided into two types according to its occurrence:
(1) Layered flint: an unstable thick layer or a large lenticular body in the form of regular strips and thin layers. It usually occurs in carbonate rocks containing phosphorus or manganese, followed by clay rocks and sandstone. The thickness of single-layer flint is generally small, ranging from a few centimeters to 1m, but it is quite large with the original stone, which can reach tens to hundreds of meters. This kind of flint usually has aphanitic or granular structure, massive structure and occasionally oolitic structure. Oolitic grains are composed of aphanitic, microcrystalline chalcedony and isochron, with isochron as the core, clay and iron oxide as the edge and isochron as the matrix. Sometimes, fibrous chalcedony is arranged radially around oolitic grains, indicating that chalcedony is formed by recrystallization. Horizontal bedding or cross bedding can be seen in this kind of flint with oolitic structure, which is generally formed by silicification of carbonate rocks containing oolitic particles.
Layered flint associated with carbonate rocks is often mixed with different contents of carbonate minerals, and siliceous minerals are also common in carbonate rocks associated with carbonate rocks. There are a series of transitional types, from pure flint to carbonate flint and siliceous carbonate. Similar to clay rock, it is common in Precambrian strata.
(2) Nodular flint: This kind of flint is more common and studied. Commonly called flint nodules, they become regular or irregular nodules (Figure 2-7- 1) or irregular strips (Figure 2-7-2). Usually sandwiched in carbonate rocks, followed by clay rocks, often distributed along a certain horizon. Tuberculosis has various shapes, such as spherical, elliptical, rod-shaped, flat, gourd-shaped, annular, tubular, nodal, irregular and extremely irregular. Nodules can be distributed along the bedding, forming beaded or nodular layers, and sometimes beaded flint nodules or flint tubes are vertically or obliquely distributed between the nodules, forming a three-dimensional grid distribution. The relationship between nodules and bedding may be that bedding bypasses nodules or nodules cut off bedding. It can be the bottom nodules distributed between layers or sections, or the intralayer nodules distributed in rock strata.
Fig. 2-7- 1 Irregular nodular flint in limestone (Permian in Emei, Sichuan)
Figure 2-7-2 Irregular banded Flint (Permian in Emei, Sichuan)
The contact boundary between flint nodules and surrounding rocks is generally clear and abrupt, and the gradual transition relationship is rarely seen. Sometimes there is a light or loose capsule (as thick as 1 ~ 2cm) at the edge of the nodule, and there may be more than one layer, which is a concentric annular band structure. This light envelope has a gradual relationship with the flint itself.
Sometimes biological remains can be preserved in flint nodules, usually silicified. Some of these biological remains are the same as the species in the surrounding rock, and sometimes undiscovered fossils can be found in the surrounding rock.
(7) Silicon dioxide
This is a typical chemical siliceous rock, which is often formed in the place where hot springs gushed out of the surface in the late volcanism. Silica is porous and light in color, in which the content of SiO2 _ 2 is not fixed, and it often contains various inclusions, besides more Al _ 2O _ 3, it may also contain various other elements.
Second, the geological distribution and practical use of siliceous rocks
Flint and jasper are the most widely distributed in nature, and biogenic flint has been found in young strata after Cretaceous. This may be because older biogenic siliceous rocks disappeared due to secondary changes.
Siliceous rocks are widely distributed in geological history. In terms of quantity, the Precambrian is the most, and then it has a decreasing trend. On the one hand, during the Precambrian period, the outcrops of surface crystalline rocks were widely distributed, and CO2 in the atmosphere and surface water was sufficient, which was beneficial to the chemical decomposition of silicate and aluminosilicate, resulting in a large number of SiO2 _ 2, Fe and Mn enriched in the Precambrian basin. On the other hand, the submarine volcanism in the geosyncline was strong at that time, bringing a lot of silicon dioxide, iron and manganese into the seawater.
Judging from the mineral composition of siliceous rocks, the older the age, the more chalcedony and autogenous time there are in siliceous rocks. In young sediments, opal accounts for the majority.
The evaporite series contains less silica. Carbonate rocks do not contain flint, and sandstone is cemented by clay, carbonate or gypsum. Generally speaking, the environment of evaporation basin is favorable for carbonate precipitation with small Ca/Mg ratio, but unfavorable for siliceous precipitation. However, when the material sources are abundant, especially in the desalination period, siliceous precipitation can also occur.
There are a large number of siliceous formations in the carbonate rock series of the stable platform. Such as flint nodules, lenses, veinlets or thin interlayers and strips. Dolomitization often occurs in this kind of rock.
In the rock series of coal-bearing basins, carbonate deposits rarely appear in clay rocks and sandstone, with only a small amount of secondary SiO2 _ 2. It usually forms the secondary enlarged edge of debris. Even in the coal-bearing basin rock series with more limestone, there are generally no flint nodules, lenticular or other siliceous formations. But under special circumstances, siliceous rocks or siliceous rocks can also be deposited, such as in some Permian coal measures in China.
There are many siliceous rocks in the sedimentary rock series of geosyncline, and there may be a large number of flint nodules adjacent to the platform at the edge of geosyncline. In clay rock with thin limestone interlayer, carbonate substance is replaced by flint, or even completely.
Siliceous rocks are widely distributed in the sedimentary rock series of the geosyncline, mainly siliceous slate and jasper. The amount of siliceous rocks is so large that it is even difficult to explain it by normal marine deposition. It is generally believed that a large number of siliceous deposits are related to volcanism. In most cases, the precipitation of siliceous matter is rapidly carried out as the reaction product of sediment and pore water after sediment deposition.
Siliceous rocks are also widely distributed in China. For example, the pre-Cambrian Biyu iron mine in Anshan area is the most famous, while the Sinian siliceous limestone and dolomite, as well as flint nodules and bands in them, are widely distributed in North China and South China (there are many siliceous shales in Kunyang Group and Huili Group of Sichuan-Yunnan Proterozoic), and there are also flint nodules in Cambrian and Ordovician limestone in South China and parts of North China; However, flint nodules are more common in Carboniferous-Permian limestone in southern China.
Siliceous rocks have many uses in industry and actually become important mineral resources. For example, flint is used as the main grinding raw material because of its high hardness; Some pure flints with a large number can be used as silica refractories. Jasper is also regarded as an important boutique because of its hardness, compactness and beautiful color. Diatomite is an important siliceous sedimentary deposit, which is used as raw material for filtration and bleaching because of its strong adsorption, and is widely used in sugar industry, oil refining industry and water purification industry. White diatomite has fine particles, less impurities and good chemical stability, and can be used as filler in rubber, coating, paper making and other industries. Diatomite and tabular diatomite are also used as heat insulation, heat preservation and sound insulation materials in buildings.