Argillaceous rock

Sedimentary rocks with more than 50% argillaceous particles

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This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

Argillaceous rock refers to argillaceous particles (mainly clay mineral ）Sedimentary rock with content more than 50% and particle size less than 0.0039 mm. also called Claystone 。

Argillaceous rocks include mudstone 、 Claystone 、 shale 、 slate Etc. Due to different formation conditions, there are white, gray white, gray, yellow, green, reddish brown, brown and other colors.

Chinese name: Argillaceous rock
Foreign name: argillaceous rocks
Rock: mudstone 、 Claystone , shale slate etc.

Color: White, grey, yellow, green, reddish brown, brown, etc
Composition: Kaolinite Illite Isoclay mineral
Structure: Argillaceous texture, oolitic texture

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Rock Introduction

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Argillaceous rock is mainly composed of clay mineral (Kaolinite, halloysite, montmorillonite, hydromica, sepiolite, etc.), detrital minerals (quartz, feldspar, mica, etc.) and some authigenic non clay minerals (oxides of iron and manganese, hydroxides, carbonate minerals, etc.). Colloidal, pisolitic, oolitic texture and scaly, felted lattice structure 。

According to the mixture, it can be divided into silty argillaceous rock, sandy argillaceous rock, etc; According to clay minerals, it can be divided into kaolinite argillaceous rock, montmorillonite argillaceous rock, hydromica argillaceous rock, etc.

Argillaceous rock is the most widely distributed rock in sedimentary rock, accounting for about 60% of the total area of sedimentary rock.

In black shale and carbonaceous shale, deposits containing nickel, molybdenum, vanadium, lead, platinum, palladium, cerium, yttrium and other elements have been found.

physical property

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Pure single mineral argillaceous rock has Plasticity 、 Fire resistance 、 Sinterability , drying shrinkage Adsorbability It is widely used as ceramic materials, refractory materials, bleaching agents, purifying agents and drilling mud raw materials by industrial departments due to its characteristics such as water absorption and expansion. However, some properties (such as plasticity, water absorption, etc.) have extremely adverse effects on engineering construction, which should be noted.

mineral composition

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The composition of argillaceous rock is very complex, mainly kaolinite 、 Illite 、 montmorillonite 、 Chlorite And mixed layer clay and other clay minerals, which are often difficult to identify in hand samples and thin slices because of their small particle size scanning electron microscope It can be distinguished according to the characteristic morphology of each clay mineral. Their crystal structure, chemical composition, physical properties are different, and their origins are also different. Kaolinite is formed in acid water with abundant rainfall and good drainage. It is a typical product of the tropics and subtropics. Montmorillonite is usually composed of Volcanic glass It is altered and common in alkaline soil. During burial diagenesis, montmorillonite will be transformed into illite. Illite is the most abundant clay mineral, most of which comes from pre formed shale and is also the main component of deep buried shale, in which illite is often associated with chlorite.

Sericite can also appear in argillaceous rocks after deep burial diagenesis. In addition, it often contains quartz, feldspar, mica clasts of silt or mud size and other authigenic components, such as calcite, authigenic quartz, iron aluminum oxide and its hydrate, pyrite, organic matter, etc.

The average mineral composition of argillaceous rock is: clay mineral accounts for 58%, quartz 28%, feldspar 6%, carbonate mineral 5%, iron oxide mineral 2%.^[1]

chemical composition

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The chemical composition of argillaceous rock mainly depends on its mineral composition and the composition and content of adsorbed ions. The chemical composition of argillaceous rock is characterized by SiO two 、Al two O three And H2O, including SiO two Is the most. Average chemical composition content of shale (%): SiO two 58.10，Al two O three 15.40，Fe two O three 4.02， FeO 2.45，MgO 2.44，CaO 3.11，Na two O 1.30，K two O 3.24，CO two 2.63，C 0.80，H two O 5.00。

SiO in argillaceous rock two The content mainly depends on the quartz content, and most CaO and a small amount of MgO depend on Carbonate mineral How many. Al two O three 、Fe two O three 、FeO、Na two O、K two O mainly occurs in clay minerals, and the content change of these oxides depends on different clay minerals.

depositional environment It also has certain influence on the chemical composition of argillaceous rocks. For example, Pennsylvanian marine clay contains more calcium than non marine clay.

Structure and construction

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Argillaceous texture

It is mainly composed of particles smaller than 4 μ m, so the rock is compact and uniform. When silt and sand are contained, various transitional structures are formed, such as silt argillaceous structure, sand argillaceous structure, etc.

Ooid and pisolite texture

Clay particles are agglomerated around the core, and those with a diameter of less than 2mm are called oolites, and those with a diameter of more than 2mm are called pisolites, which are commonly found in clay rocks of colloidal origin.

Syngenetic gravel structure

After the clay material is deposited, it has not been fully consolidated. It is scoured by water to form gravel chips and cemented by clay material. The structure of claystone can be divided into macro structure and micro structure. The most obvious in the macro structure is the exfoliation due to the directional arrangement of clay minerals, which is most common in shale. The common microstructures are: scale structures formed by the disorderly distribution of extremely small scale like clay minerals, which are mostly found in mudstones; Felt structure formed by interlacing fibrous clay minerals and directional structure formed by directional arrangement of lamellar clay minerals. It is often formed in a still water environment, and can also be the result of compaction.

Classification and naming

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Divided by whether the page is developed

1. Mudstone: no paging.

2. Shale: with page arrangement.

It is an international common practice to define shale with page theory, but some people at home and abroad also define shale as mudstone with horizontal texture.^[1]

Classified by clay mineral composition

1. Single component clay rock: refers to a certain clay mineral in the clay ore that contains more than 50%, and this mineral can directly participate in naming. Such as kaolinite clay rock, montmorillonite clay rock Illite clay rock 。 If there is any paging, "clay rock" can be changed to "shale".

2. Multi component clay rock: refers to that the content of any kind of clay mineral in clay ore is not more than 50%. If necessary, two clay minerals with high content can be named in the order of "less before more after". Such as kaolinite illite clay rock, illite montmorillonite clay rock. Most argillaceous rocks in nature are of composite composition, so the name "complex clay rock" is seldom used, and it can become mudstone or shale.^[1]

Named according to other components other than clay

In addition to certain content specifications for silt, this classification is relatively less strict for other components (authigenic components).

Rock type	Mudstone (shale)	Silty mudstone (shale)	Silty mudstone (shale)
Volume fraction of silt	0——5%	5%——25%	25%——50%

Main rock types

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The main types of argillaceous rocks are as follows:

kaolinitic claystone

Also known as kaolin, it was first discovered in Gaoling Village near Jingdezhen, Jiangxi Province, China. It is mainly composed of kaolinite, and the content of pure kaolinite can reach more than 90%, followed by halloysite and illite. The mixture includes quartz, feldspar, mica, pyrite, siderite and organic matter. Its chemical composition is mainly SiO2, Al2O3 and H2O. The rocks are mostly white and gray, and the color becomes dark when containing impurities. Dense block or loose soil, greasy and slippery, low plasticity, small cohesiveness, high fire resistance, up to 1770~1790 ℃, good insulation and chemical stability 。 Mainly used as paper coating, filler, rubber, plastic, paint filler and ceramic raw materials. The synthetic zeolite made from kaolin can be used for the adsorption, drying, separation and purification of gas and liquid, as well as the catalysis and cracking of petroleum. kaolinite clay (Rock) can be divided into weathered residual type and sedimentary type. The former is mainly developed in the weathering crust of acidic and intermediate acidic intrusive rocks. Under warm and humid climate conditions Aluminosilicate mineral It is decomposed. Famous places of origin include Jingdezhen, Jiangxi, Hengyang, Hunan, etc. The latter is formed by coagulation of SiO2 and Al2O3 colloids in acidic medium, or by mechanical deposition of erosion and destruction products of weathered residual kaolin (rock). The reserves of sedimentary kaolin (rock) in China's Late Paleozoic and some Mesozoic coal bearing rock series are far greater than those of weathered residual kaolin (rock). Famous places of origin include Tangshan, Hebei, Zibo, Shandong, etc. There is a kind of kaolinite mudstone found in many coalfields in China, which is produced as coal seam dirt band, almost composed of pure kaolinite. It is a high-quality kaolin deposit with small thickness, stable horizon and wide distribution range, and can be used as a marker for stratigraphic and coal seam correlation. In kaolinite mudstone, special heavy minerals such as high temperature quartz, sanidine, euhedral zircon, apatite and brown biotite are found. Combined with its geological occurrence, people believe that the formation mechanism of kaolinite mudstone is that volcanic ash falls and then is altered.

Montmorillonite clay rock

It is mainly composed of montmorillonite, often containing a small amount of muscovite, chlorite Carbonate mineral , gypsum, organic matter and undissolved volcanic tuff materials. The rocks are often white, pink, light green, light yellow, etc. The water absorption, plasticity and cohesiveness are strong, but the fire resistance is weak. According to its technological properties and uses, it is also divided into bentonite (also known as porphyry) and bleaching earth. Bentonite is a kind of clay with montmorillonite as the main component, which has strong water absorption. After water absorption, its volume can expand 10~30 times, and it also has strong cation exchange performance. According to chemical composition, it can be divided into Sodium bentonite and Calcium bentonite 。 Bentonite is used as decolorizing agent in petrochemical industry, sugar making and grease industry, binder in foundry industry and mud material in drilling. Bleached clay is a kind of colloid clay with similar composition to bentonite, but it has more calcium, less sodium and strong adsorption performance. It can be used as a decolorant or bleaching agent when refining petroleum products, mineral oil and vegetable oil. The main producing areas in China are Shulan in Jilin, Yuhang and Lin'an in Zhejiang, Jiangning in Jiangsu, Toksun in Xinjiang, etc. Montmorillonite clay (Rock) is tuff or glassy extrusive rock, which is the decomposition product under the action of seawater or groundwater in alkaline medium (pH=7~8.5), accumulated in situ or deposited in lakes, bays and deep sea.

Illite clay rock

It is the most widely distributed clay (rock) mainly composed of illite, but often contains other clay minerals, as well as quartz, feldspar, mica and other debris and organic matter. The rocks are often gray, yellow and brown, and horizontal bedding is developed. Because it often contains many impurities, it is generally used as raw material for coarse ceramic products and bricks and tiles. Illite clay (rock) can be generated in both continental and marine environments. The older the stratum age is, the greater its relative content is, which is related to diagenesis. ④ Mudstone and shale. Mudstone is massive argillaceous rock without texture or foliation, and shale is argillaceous rock with texture or foliation (see color picture). The two kinds of rocks are mainly composed of illite, in addition, they often contain other clay minerals, some clastic minerals and some Authigenic mineral 。 According to the chemical composition of the mixture, it can also be divided into: those containing CaCO3 (<25%) are called calcareous mudstone and calcareous shale. When iron ions are contained, the rocks are red, purple red, gray green and other colors, which are called ferruginous mudstone and ferruginous shale. Those rich in SiO2 (up to 85%) are called siliceous mudstone and siliceous shale. Carbonaceous mudstone and carbonaceous shale contain a large amount of carbonaceous organic matter. Black mudstone and black shale are black because they contain more organic matter and finely dispersed iron sulfide. Black shale often contains various metal elements, sometimes reaching industrial grade. The abnormal content of iridium and carbon in black shale and its widespread distribution in the world in some geological periods (such as Precambrian, Early Paleozoic, Carboniferous, etc.) have attracted great attention of geologists. It is believed that the black shale was formed under an unusual anoxic event. Shale containing a certain amount of kerogen (>10%) is called oil shale, from which several useful gases and tar can be extracted.

Diagenesis

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Argillaceous rock mainly occurs in diagenetic stage Compaction And the transformation of mineral composition. The soft mud is rich in water, which is up to 70~90% of the total volume. Most of the water is discharged under the compaction of the overlying strata. Therefore, at a depth of about 1km, argillaceous sediments only contain 30% water, and argillaceous rocks have been formed at this time. With the increase of burial depth, dehydration occurs to varying degrees, and finally only a few percent of water is left in the argillaceous rock. The transformation of mineral composition is mainly due to the increase of burial depth and temperature. Montmorillonite starts to become mixed layer clay (such as montmorillonite illite) when the ground temperature is 75-95 ℃, and gradually transforms into illite and chlorite when the ground temperature increases above 150 ℃. When kaolinite is above 100 ℃, when the pore solution (K + ）/（H + ）When the ratio increases, it turns into illite. When transition to initial metamorphism (temperature: 200-300 ℃, pressure: 1 × 10 eight -2×10 eight Pa), pyrophyllite and laumontite can appear. With the strengthening of initial metamorphism and subsequent low-grade metamorphism, illite was finally replaced by sericite and chlorite. Therefore, studying the transformation of clay minerals in diagenesis can provide important geothermal information, indicating the generation and preservation of oil and the change of coal rank.

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