Xinyu Huayuan Wollastonite Co., Ltd, Jiangxi Wollastonite
Silica is widely found in nature and forms rocks with other minerals. Natural silica is called silica, which accounts for about 12% of the mass of the earth's crust. It exists in two forms: crystalline and amorphous. Quartz crystals are crystalline silica with different crystal forms and colors. The colorless and transparent crystals in quartz are commonly known as crystals. The colored ring band or layer is called agate (containing impurities).

 

Silica minerals refer to minerals with the same chemical formula (SiO₂) but with different structures. These minerals are collectively called quasi-heteromorphs, and mainly include quartz, cristobalite, and scaly quartz. These minerals are mainly found in granite, sandstone and black silica on the earth, but they are scarce on the moon. The main reason is that: on the evolution of chemical composition, the moon forms a low-silicon, high-aluminum moon crust and high-silicon granite. Rocks are extremely scarce; the moon has evolved lacking a water system and hot water system that can crystallize silica minerals like the earth. Although silica minerals are extremely rare on lunar rocks, they have an important role in studying the classification and genesis of lunar rocks.

 

The quartz minerals on the moon were first discovered in several pieces of granite-like fragments, and many fine cristobalite minerals were also filled in the fine fine rock. Analysis of their fine structure and composition showed that these quartz actually Quartz changed. Later, quartz minerals were also found in the coarse-crystalline lunar granite fragments. According to the analysis results of their isotopes, these minerals were crystallized in a relatively deep environment around 4.1 billion years, indicating that these quartzs were not formed in magmatic rocks. During the crystallization.

 

The vast majority of silica minerals in Yuehai Wuyan are cristobalite, with a volume percentage of up to 5%, almost no quartz minerals, only a small amount of quartz minerals exist in fine-grained Yuehai basalt. These cristobalite have The typical double crystal structure shows that during the cooling of the melt, cristobalite is formed from high temperature to low temperature. In addition, cristobalite and scale quartz also exist in some coarse-crystalline moon-sea basalts, but from the structural characteristics, cristobalite is transformed from scale quartz, because scale quartz is generally embedded between irregular particles.

 

Physicochemical properties

 

Physical properties

 

[1] Silica, also known as silica, has the chemical formula SiO₂. There are two kinds of crystalline silica and amorphous silica in nature.

 

The main component of sand is silica

 

Due to their different crystal structures, crystalline silica is divided into three types: quartz, scale quartz and cristobalite. Pure quartz is a colorless crystal, and large and transparent prismatic quartz is called crystal. If the crystals containing trace impurities have different colors, such as amethyst, tea crystal, etc. Ordinary sand is fine quartz crystal, with yellow sand (more iron impurities) and white sand (less impurities, more pure). In silicon dioxide crystals, 4 valence electrons of silicon atoms form 4 covalent bonds with 4 oxygen atoms, the silicon atom is located in the center of the regular tetrahedron, 4 oxygen atoms are located at the 4 vertices of the regular tetrahedron, SiO₂ It is the simplest form to represent the composition, and only represents the ratio of the number of silicon and oxygen atoms in the silicon dioxide crystal. Silica is an atomic crystal. [2]

 

Diatomite existing in nature is amorphous silica, which is the remains of diatoms of lower aquatic plants. It is a white solid or powder, porous, light and soft solid, with strong adsorption.

 

not soluble in water.

 

Chemical properties

 

The chemical properties are relatively stable. Does not react with water. It is an acidic oxide and does not react with normal acids. Gaseous hydrogen fluoride reacts with silicon dioxide to form gaseous silicon tetrafluoride. Reacts with hot concentrated strong alkaline solution or molten alkali to form silicate and water. It reacts with various metal oxides to form silicate at high temperature. Used in the manufacture of quartz glass, optical instruments, chemical vessels, ordinary glass, refractory materials, optical fibers, ceramics, etc. Silica is inactive in nature. It does not interact with halogens, hydrogen halides, sulfuric acid, nitric acid, and perchloric acid (except hot concentrated phosphoric acid) other than fluorine and hydrogen fluoride.

 

Common concentrated phosphoric acid (or pyrophosphoric acid) can corrode silica at high temperature and generate heteropoly acid [3]. At high temperature, molten borate or boric anhydride can also corrode silica. In view of this property, boric acid Salt can be used as a flux in the firing of ceramics. In addition, hydrogen fluoride can also dissolve the acid of silicon dioxide to produce fluorosilicic acid that is easily soluble in water: 6HF + SiO2 = H2SiF6 + 2H2O

 

Silica generally does not react with water, that is, it does not generate silicic acid in contact with water, but it is artificially specified that silica is an anhydride of silicic acid.

 

In the temperature range of interest for most microelectronics processes, the crystallization rate of silica is so low that it can be ignored.

 

Silica structure

 

Although fused silica is not long-range ordered, it shows a short ordered structure, and its structure can be considered as four oxygen atoms located on the apex of the tetrahedron. At the center of the polyhedron is a silicon atom.

 

In this way, every 4 oxygen atoms are approximately covalently bonded to silicon atoms, which satisfies the valence shell of silicon. If each oxygen atom is part of two polyhedra, the valence of oxygen is also satisfied, and the result is a regular crystal structure called quartz.

 

In fused silica, certain oxygen atoms become oxygen bridge sites and bond with two silicon atoms. Some oxygen atoms do not have an oxygen bridge and only bond to one silicon atom.

 

It can be considered that the thermally grown silica is mainly composed of a polyhedral network in any direction. Compared with the anaerobic bridge site, the larger the aerobic bridge part, the greater the adhesion of the oxide layer, and the smaller the tendency to damage. The ratio of aerobic bridge to anaerobic bridge of dry oxygen oxide layer is much larger than that of wet oxygen oxide layer.

 

use

 

Silica is used to manufacture flat glass, glass products, foundry sand, fiberglass, ceramic glaze, sandblasting for rust prevention, sand for filtration, flux, refractory materials, and the manufacture of lightweight air-filled concrete. Silica is very versatile. The rare crystals in nature can be used to manufacture important parts, optical instruments and crafts in the electronics industry. Silica is an important raw material for manufacturing optical fibers. Generally pure quartz can be used to make quartz glass. The expansion coefficient of quartz glass is very small, which is equivalent to 1/18 of ordinary glass. It can withstand rapid temperature changes and has good acid resistance (except HF). Therefore, quartz glass is often used to make high-temperature resistant chemical instruments. Quartz sand is often used as glass raw material and building material.

 

Silica is the raw material for manufacturing glass, quartz glass, water glass, optical fiber, important parts of the electronics industry, optical instruments, handicrafts and refractory materials. It is also an important material for scientific research.

 

When the silica crystal is perfect, it is crystal; silica gel is agate after dehydration; silica gel containing water becomes solid when it becomes solid; when the silica crystal grains are less than a few microns, they form chalcedony, flint and secondary Quartzite. Mineral resources with very stable physical properties and chemical properties. The crystal belongs to the oxide mineral of the trigonal system, that is, low-temperature quartz (α-quartz), which is the most widely distributed mineral species in the quartz family of minerals. Quartz in a broad sense also includes high-temperature quartz (β-quartz). Quartz blocks, also known as silica, are mainly raw materials for producing quartz sand (also known as silica sand), as well as raw materials for quartz refractories and sintered ferrosilicon.

 

Used as anti-caking agent, anti-foaming agent, thickener, filter aid and clarifier in the food industry. China's "Sanitary Standards for the Use of Food Additives" (GB2760-2011) stipulates: it can be used for egg powder, sugar powder, milk powder, cocoa powder, cocoa butter, plant powder, instant coffee, soup powder, etc., the maximum usage is 15g / kg , Powder essence, the maximum usage is 80g / kg; solid beverages, the maximum usage is 0.2g / kg; food, 1.2g / kg