silicate

[guī suān yán]

The general term for compounds formed by combination of silicon, oxygen and other chemical elements

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

In chemical terms, the so-called silicate refers to the general name of compounds formed by the combination of silicon, oxygen and other chemical elements (mainly aluminum, iron, calcium, magnesium, potassium, sodium, etc.). It is widely distributed in the crust and forms most rocks (such as Granite ）And the main components of soil. Most of them have high melting points and stable chemical properties, and are the main raw materials for silicate industry. Silicate products And materials are widely used in various industries, scientific research and daily life.

Chinese name: silicate
Foreign name: Silicate
Distribution: It is widely distributed in the crust

Type: chemical compound
Main elements: Aluminum, iron, calcium, magnesium, potassium, sodium, etc
Structure: Oxygen tetrahedron
Features: Most of them have high melting points and stable chemical properties

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Basic structure

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Due to its structural characteristics, there are many kinds（ Silicate mineral The basic structure of silicon -- Oxygen tetrahedron; In this tetrahedron, silicon atoms occupy the center and four oxygen atoms occupy the four corners. These tetrahedrons form various silicates according to tetrahedrons and different coordination. Silicate structure There are many kinds: island olivine, layered quartz, ring montmorillonite, etc. Most of them have high melting points and stable chemical properties Silicate industry Main raw materials of. Silicate products And materials are widely used in various industries, scientific research and daily life.^[1]

Mineral classification

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Chain structure

Silicate minerals with a series of [ZO4] tetrahedrons connected by corner tops to form a one-dimensional infinite chain like silica backbone. The chains are connected by metal cations (mainly Ca, Na, Fe, Mg, Al, Mn, etc.). More than 20 types of chains have been found, the most important of which are pyroxene single chain [Si2O6] 4 - and amphibole double chain [Si4O11] 6 -.

In the chain structure silicate minerals, because the silica backbone is always extended and distributed in parallel, the anisotropy of its crystal structure is much more prominent than the island and ring ones. The minerals are always elongated in shape, often in columnar, needle and fibrous shapes. In physical properties, cleavage is more developed in the direction parallel to the chain, with higher refractive index in the direction parallel to or near the chain, lower in the direction perpendicular to the chain, and larger birefringence than island or ring minerals. Chemical composition includes Transitional element The polychrome and absorbability of the minerals of the pyroxene group and amphibole group rich in iron, titanium and other elements are very obvious.

Layered structure

Silicate minerals with a series of [ZO4] tetrahedrons connected by corner tops to form a two-dimensional infinite extension of layered silica backbone. The most common type of silica backbone is that each tetrahedron is connected with three surrounding tetrahedrons by three corner tops to form a hexagonal mesh like monolayer, and all its active oxygen points to the same side. It widely exists in mica, chlorite, talc, pyrophyllite, serpentine and clay minerals, and is commonly called tetrahedral slice. The tetrahedral sheet is combined with other metal cations (mainly Mg2+, Fe2+, Al3+, etc.) through active oxygen. These cations all have octahedral coordination, and each coordination octahedron is connected by a common edge to form a two-dimensional infinite extended octahedral sheet. The combination of tetrahedron sheet and octahedron sheet forms the structural unit layer.

If the structural unit layer consists of only one tetrahedral sheet and one octahedral sheet, it is a 1:1 structural unit layer, such as the layer in kaolinite and serpentine. If it is composed of two tetrahedral pieces with opposite active oxygen and one octahedral piece, it is a 2:1 type structural unit layer, such as the layer in mica, talc and montmorillonite. If the electricity price of the structure unit layer itself is not balanced, there can be low-cost large radius cations (such as K+, Na+, Ca2+, etc.) between the layers, such as mica, montmorillonite, etc. Water molecules also exist between the layers of the latter. In addition, three octahedrons with triangular centers are matched with a six element ring range of the tetrahedron sheet in the octahedron sheet. When the octahedron position is occupied by divalent cations, cations must exist in the three octahedrons to reach the electricity price balance. If it is a trivalent cation, only two cations are needed to reach equilibrium, and the other octahedron position is empty.^[2]

chemical property

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Silica tetrahedron

Chemically, it refers to the combination of silicon and oxygen Composed compounds（

）, sometimes including one or more metals or Hydrogen element 。 Conceptually, we can say that silicate is a general term for compounds composed of silicon, oxygen and metals. It is also used to indicate that silicon dioxide or Silicic acid Salt produced. It can react with acid to form silicic acid solid. In general, the most stable silicate is silicon dioxide (SiO two ）And other substances.^[4] Silicon dioxide often contains traces of silicic acid（

）At Equilibrium state 。 Chemists believe that silica is insoluble, but it can flow in a long time scale. In addition, under alkaline conditions

。 Most silicates are insoluble.

Silicate minerals are characterized by their Regular tetrahedron Structure, sometimes these regular tetrahedrons chain 、 Double chain , sheet, and three-dimensional shelf. According to the degree of tetrahedral polymerization, silicates can be subdivided into island silicates, cyclic silicates, etc.

The solubility of silicic acid in water is very low, about 0.002mol ·L-1 at 25 ℃. In the solubility range, silicic acid exists in the form of monosilicic acid. Silicic acid polymerizes when its concentration in solution is higher than its solubility. Polymerization is an important property of silicic acid, that is, monosilicic acid polymerizes into oligomeric silicic acid, and then polymerizes into homopolysilicic acid.^[2]

Analytical method

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principle

Microwave is electromagnetic wave An electromagnetic radiation between far-infrared and radio. Its frequency range is 300MHz ～ 3 × 105MHz. Microwave heating is different from the traditional heating method. Microwave heating is an internal heating method. The heated sample and acid mixture generate instant deep heating by absorbing microwave energy. At the same time, the alternating magnetic field generated by the microwave will make the dielectric molecules polarize, and the polar molecules can be alternately arranged with the high frequency magnetic field, resulting in the high-speed oscillation of molecules. For this kind of oscillation Molecular thermal motion The obstruction and interference of interaction with adjacent molecules produce a friction like effect that enables molecules to obtain high energy. Therefore, this high energy can break the chemically stable silicate in rocks and minerals, so that its molecules can be measured.

Test part

1 Instruments and reagents

Instrument: household microwave oven.

Reagent: Cement clinker Standard sample; Ordinary Portland Cement Standard sample; raw meal Standard sample; TEA( Triethanolamine ）(volume mix ratio 1:2); Hydrochloric acid; KOH solution; EDTA standard sample; Calcein - Methyl thymol blue -Phenolphthalein mixed indicator (CMP mixed indicator).

2 Experimental methods

(1) Calibration of EDTA standard solution

First, take a certain volume of CaCO3 solution and dilute it 8 times. For example, in the actual experiment process, take 10mL of the solution and dilute it to 80mL, then add an appropriate amount of CMP mixed indicator A magnetic stirrer Add 200g/L KOH solution dropwise under stirring until green fluorescence appears, and then titrate about 2mL excess. Titrate with EDTA standard solution until the green fluorescence disappears and turns red.

(2) Sample digestion

a、 First, digest the cement clinker standard sample or ordinary Portland cement standard sample: weigh 0.1000g of the cement clinker standard sample or ordinary Portland cement standard sample that has been baked at 105~110 ℃ for two hours, put it into a 400mL beaker, add 20~30mL of distilled water and 3~4mL of hydrochloric acid solution with a volume ratio of 1:1 to dissolve the sample, and cover the watch glass, Put it into the microwave oven for digestion, take it out, cool it to room temperature, fix the volume in a 100mL volumetric flask, and wait for use.

b、 Digestion of cement raw meal standard sample: weigh 0.2000g of cement raw meal standard sample that has been baked for two hours at 105~110 ℃, put it into a 400mL beaker, add proper amount of distilled water and HCl solution to dissolve the sample, cover the watch glass, and place it in the microwave oven for heating digestion. Then take it out and cool it to room temperature, and fix the volume in a 100mL volumetric flask for later use.

3 Results and discussion

The content of CaO in cement clinker and ordinary Portland cement was mainly determined.

It can be seen that the calcium oxide content has been within the tolerance range after digestion for more than 2min under the microwave low fire condition, meeting the determination requirements.^[2]