Yang Xiaoyong et al: Research Progress and Development Trend of High Purity Quartz

of

High Purity Quartz Yang Xiaoyong ¹ , Sun Chao ¹ , Cao Jingya ¹ , Shi Jianbin ²¹

. School of

Earth and Space Sciences, University of Science and Technology of China 2.

Introduction:

High-purity quartz is widely used in new industries, and China needs to import it. According to statistics, the proportion of high-purity quartz imported from China with SiO2 content ≥ 99.99% is 91%. The content of SiO2 used in chip manufacturing is ≥ 99. The research of high purity quartz is highly concerned in China.

High-purity quartz is produced by purifying natural crystal, vein quartz, granite pegmatite and other mineral raw materials , which are widely distributed in China. Granite pegmatite vein groups in East Qinling and Altay can be used as high-quality high-purity quartz mineral raw materials. High-quality hydrothermal vein quartz from Qichun in Hubei, Donghai in Jiangsu, Jingde in Anhui and Taihu Lake may have the potential of high-purity quartz raw materials. With the deepening of geological survey and research work, more high-purity quartz raw materials will be discovered.

For a long time, the mining industry has not fully realized the great potential industrial value of this precious and non-renewable high-purity quartz raw material, which is often used as a common building material, and the precious resources are wasted.

During the formation and evolution of quartz crystal, impurity elements will exist in quartz crystal more or less due to the influence of environmental conditions, fluid properties and modification after crystallization. Quartz of pure SiO2 composition does not exist in nature. The types, contents and occurrence forms of impurity elements in quartz, especially the characteristics of inclusions, will directly affect the quality and industrial use of quartz resources.

At present, the basic research on quartz is insufficient and the purification process is relatively backward in China, which makes a large number of high-purity quartz rely on imports. However, the United States and Europe have regarded high-purity quartz as a strategic resource and restricted its export to China, resulting in the price of high-purity quartz almost doubling in recent years, which has caused great economic losses to Chinese enterprises. Therefore, strengthening the investigation and research of high-purity quartz resources is an inevitable requirement to meet the demand for high-purity quartz materials in China and to get rid of the shackles of imports. According to different types of quartz resources, Professor

Yang Xiaoyong et al. Used geochemistry, mineralogy, mineral geochemistry, inclusions and other methods . The chemical composition, microstructure characteristics, inclusion characteristics and impurity element characteristics of different types of quartz ores are described, and the formation mechanism, impurity occurrence and purification potential of quartz ores are judged, and the feasible purification process scheme is explored and put forward; It lays a theoretical foundation for optimizing the quartz mineral producing areas as high-purity quartz raw materials and building a large-scale high-purity quartz raw material resource base. The

research results of this paper provide guidance for the exploration of high-purity quartz raw material deposits and the basic research of high-purity quartz purification, which has attracted wide attention of domestic scholars and mining industry. --

-- Content Outline --

Abstract

0 Introduction

1 Mineralogical Characteristics

of Quartz 2 Impurity Elements and Occurrence States

in Quartz Crystals 2.1 Lattice impurity elements

2.2 Submicron and nanoscale inclusions

2.3 Minerals, Melt and Fluid Inclusions

3 Cathodoluminescence Characteristics

of Quartz 4 Quality Classification and Industrial Standards

of High Purity Quartz 5 High Purity Quartz Raw Materials

5.1 Comprehensive Evaluation

of High Purity Quartz Raw Materials 5.2 Ideal source rock

of high-purity quartz material 5. 3 Geological origin

of high-purity quartz material 5. When quartz crystal is formed and evolved , Impurity elements such as P, Ti, Ge, Al, B, Be, Ca, Na, K, H, Li, etc. Will exist in quartz crystal more or less due to the influence of environmental conditions, fluid properties during crystallization and transformation (such as metamorphism, structural deformation and thermal disturbance) after crystallization. Therefore, quartz of pure SiO2 component does not exist in nature. At the same time, based on the trace element composition of quartz, the source area and formation and evolution process of quartz can be inverted, and it can be used to study the petrogenesis, deposit genesis and other scientific issues. The impurity elements in quartz crystal exist in three forms: isomorphous substitution, gas-liquid inclusion and mineral inclusion (which can not be divided according to the size, but should be divided according to the state) . The types, contents and occurrence forms of impurity elements in quartz, especially the characteristics of inclusions, will directly affect the quality and industrial use of quartz resources. In this paper, the luminescent properties and technological properties of quartz, the quality classification and standards of high-purity quartz, and the raw material sources of high-purity quartz are also systematically reviewed. In this study, the distribution of fluid inclusions and the types of mineral inclusions in quartz from Zhangjinzhuang quartz sand in northern Jiangsu and Guposhan granite in Guangxi have been preliminarily identified by optical microscopy and laser Raman testing . This is the basis for further research. High-purity quartz is a high-quality quartz raw material obtained by purifying and processing high-quality quartz resources, which is used in high-tech industries such as quartz glass, quartz crucible , semiconductor, high-temperature lamp, optical fiber, precision optics, microelectronics, solar energy, etc. The study on the evaluation system, ideal source rock and formation mechanism of high-purity quartz raw materials is beneficial to the sustainable supply of high-purity quartz raw materials and greatly improves the industrial added value of quartz raw materials.

Key words: quartz; high-purity quartz; high-purity quartz raw material; Zhangjinzhuang quartz sand in northern Jiangsu;

Quartz is a stable mineral in the surface environment. Because of its stable physical and chemical properties, wide distribution on the surface and easy exploitation, it has become a necessary raw material for traditional industrial production such as metallurgy, glass, construction, chemical industry and optics. In recent years, with the development of in-situ analysis technology, scholars have a new understanding of the trace element composition of quartz and its indicative significance, and the occurrence of trace elements, which has an important reference value for the subsequent purification and processing of quartz and its industrial use. High-purity quartz is a kind of high-quality quartz formed naturally (such as crystal) or purified from relatively pure quartz raw materials, which is an indispensable raw material for some high-tech industries (such as semiconductor, high-temperature lamp, communication, precision optics, microelectronics, solar energy, etc.) To produce high value-added quartz products. It is an important green strategic resource. For a long time, the mining industry has not fully realized the huge potential industrial value of this precious and non-renewable resource. Quartz is often used as a common building material, and many high-purity quartz raw materials are wasted. This phenomenon must be changed. High-purity quartz is mainly obtained by purifying natural quartz raw materials. Establishing the evaluation system of high-purity quartz raw materials, determining the ideal source rock of high-purity quartz raw materials and studying the formation mechanism of high-purity quartz raw materials are conducive to the sustainable supply of high-purity quartz raw materials and the purification of high-purity quartz.

This paper introduces the research progress of quartz and high-purity quartz, including the trace element composition of quartz and its indication significance, the occurrence state of impurity elements in quartz, the quality definition of high-purity quartz, as well as the evaluation of high-purity quartz raw materials, ideal source rocks and genesis. In order to arouse the attention of domestic scholars and mining industry to the research of quartz and high-purity quartz raw materials.

1 Mineralogical characteristics

of quartz The chemical formula of quartz is SiO2 . It is an oxide mineral formed by ordered arrangement of silicon-oxygen tetrahedra [SiO4 ] composed of silicon atoms (Si) and oxygen atoms (O) in a three-dimensional space, wherein the oxygen atoms (O) are connected with two silicon-oxygen tetrahedra [SiO4 ]. Generally speaking, quartz refers to low-temperature alpha-quartz (trigonal system) which can exist stably in the surface environment. It is one of the important polymorphs of silicon dioxide (SiO2 ); In addition, other crystalline and amorphous polymorphs of silicon dioxide (SiO2 ) exist . At atmospheric pressure (1. The higher the pressure, the higher the temperature at which α-quartz can exist stably; The maximum temperature and pressure at which α-quartz can exist stably are 1 380 ℃ and 3.

Quartz widely exists in magmatic rocks, metamorphic rocks, sedimentary rocks and hydrothermal veins in nature , and is an important rock-forming mineral and an important component of the lithosphere. According to statistics, the total amount of quartz in magmatic rocks, sedimentary rocks and metamorphic rocks accounts for 93.6%, 3.2% and 3% of the total amount of quartz in the lithosphere, respectively. In the mineral composition of the exposed upper crust, quartz accounts for about 20%, second only to feldspar (about 35%); Quartz accounts for about 23.2% of the mineral composition of the upper crust, second only to feldspar (about 39%).

Quartz is an important mineral resource with stable physical and chemical properties. It is widely used in industry, traditionally in metallurgy, glass, construction, chemical, optical and other industries. The development of science and technology and industry has put forward higher requirements for quartz raw materials. Traditional natural quartz raw materials can not meet the production needs of high-tech industries such as semiconductors, high-temperature lamps, communications, precision optics, microelectronics, solar energy and so on. The operation of these industries needs the support of high-purity quartz raw materials and their purified and processed products. High-purity quartz is a kind of quartz composed almost entirely of SiO2 , which is rare in nature (such as crystal), and is usually obtained by purifying and processing naturally formed pure quartz. The physical and chemical properties of high-purity quartz are better than those of general quartz raw materials. It is a raw material for the production of quartz products with high added value and widely used in high-tech industries. It is also a green strategic resource with broad application prospects.

2. Impurity elements and occurrence state in quartz crystal The theoretical chemical composition of

quartz is SiO2 , but there is no pure SiO2 quartz in nature. Quartz more or less contains some impurity elements (such as Al, Ti, K, Na, Ge, etc.), whose types and contents are related to the melt/fluid when quartz is crystallized, the external environment and the transformation after crystallization. Inside the quartz crystal , the occurrence forms of impurity elements include (from small to large scale) : lattice impurity elements (lattice scale), submicron (100 nm to 1 μm) and nanometer (<100 nm) inclusions, and microscopic inclusions (> 1 μm).

2. The point defects of quartz crystal include vacancies, substitutional atoms and interstitial atoms, among which the latter two are the main ones that can introduce impurity elements. Extraneous ions such as P5 ⁺ , Ti4 ⁺ , Ge4 ⁺ , Al3 ⁺ , Fe3 ⁺ , B3 +, etc.) By displacing Si4 ⁺ , Occupying the site of Si4 + to form a substitutional impurity element; At the same time, when some ions (such as Al3 ⁺ , Fe3 ⁺ , etc.) replace Si4 +, In order to keep the electrovalence balance, electrovalence compensation ions such as Na ⁺ and K ⁺ are introduced between atoms, which are interstitial impurity elements.

According to whether the electrovalence of the substitutional ions is the same as that of Si4 ⁺ , The replacement of Si ^{4 +} by foreign ions can be divided into two categories:

(1) isovalent isomorphous replacement, The electrovalence of the substitutional ions is the same as that of Si4 ⁺ . For example, Ti4 ⁺ and Ge4 ⁺ can directly react with Si4 + in [SiO4 ]. Isomorphous substitution into the quartz lattice (fig. 1);

(2) Non-equivalent isomorphous substitution, the electrovalence of the substitutional ions is different from that of Si ⁴ +, It can be:

① Si4 ⁺ is replaced by trivalent ions, and a monovalent charge compensation ion is needed. For example, when Si4 ⁺ is replaced by Al3 ⁺ , Fe3 ⁺ and B3 ⁺ , Na ⁺ , K ⁺ , Li ⁺ , and H ⁺ are introduced between Si atoms (fig. 1);

② Two pairs of heterovalent ions are replaced at the same time. Uch as while Al3 ⁺ replaces Si4 +, The substitution of P5 ⁺ for Si4 ^{+ occurs in the center of the adjacent silicon-oxygen tetrahedron} , that is, in the form of pair-wise substitution, to maintain the balance of electricity price (Fig. 1);

(3) when the external ion is a divalent ion (such as Be2 ⁺ ), the divalent ion can enter the vacancy of the quartz crystal, At the same time, two trivalent ions (usually Al3 ⁺ ) are needed as electrovalence compensation elements to replace Si4 ⁺ . In the process of replacement, the crystal structure is locally adjusted in order to keep the lattice type unchanged (Fig. 1).

Along with that advance of analytical technique, there is a new understanding of the non-equivalent isomorphous substitution in quartz crystal. M Müller and Koch-M Müller, Namely the substitution ions Al3 ⁺ , The total number of moles of Fe3 ⁺ and B3 ⁺ should be equal to the number of atomic interstitial P5 +, H ⁺ , Total moles of Li ⁺ , Na ⁺ , and K ⁺ . At the same time, the author points out that the electronic defects (such as vacancies) have little or no effect on the above relationship, although they can also maintain a certain balance of electricity price. Therefore, the correction scheme proposed by M Müller and Koch-M Müller is of great significance in determining the chemical composition and purity of quartz. For example, in quartz, if the content of Al, Fe and B elements is high, the content of H, Li, Na and K elements must not be very low , because the introduction of monovalent electrovalence compensation ions is necessary to maintain the overall electrovalence balance.

2. At present, there are few studies on this kind of submicron (100 nm ~ 1 μm) and nanometer (<100 nm) inclusions, mainly focusing on colored quartz, especially blue magmatic quartz. Therefore, the content and distribution of such inclusions in general quartz crystals are uncertain. To date, Submicron-sized inclusions observed include rutile, anatase, mica, tourmaline, and Al-Si phase minerals (possibly representing Al2SiO5 polymorphs, AlOOH, or corundum). M Müller et al. Studied the deformed granite of the Lachlan fold belt ( Lachlan Fold Belt ) in Australia. Localized Al-K-rich sites or bands were found by electron probe line scanning of quartz in the granite. The author believes that the analysis results may be affected by the submicron inclusions, and according to the Al/K ratio obtained by the analysis is similar to that of muscovite, it is considered that there are submicron inclusions with chemical composition similar to muscovite in quartz. Combined with the deformation history of the region, M Müller et al speculated that after the formation of the granite, Al and K originally uniformly distributed in the quartz lattice were redistributed due to the disturbance of multiple deformation events in the later period, and Al and K accumulated locally in the quartz, eventually forming submicron inclusions of muscovite composition.

Seifert et al. systematically studied submicron (100 nm to 1 μm) and nanoscale (<100 nm) inclusions in blue quartz. It is generally believed that the blue color of quartz is caused by Rayleigh scattering of incident light by tiny mineral inclusions in quartz, and has nothing to do with the composition of trace elements in quartz. In the study, the authors collected typical blue quartz from all over the world and analyzed their chemical composition and microscopic morphology in situ. Backscatter images show numerous submicron (100 nm to 1 μm) and nanoscale (<100 nm) inclusions in this type of quartz , including rutile, anatase, and mica (Figure 2). Their grain size is smaller than that of inclusions seen in optical microscopy (often larger than 1 μm). The density of these submicron and nanoscale inclusions in quartz is estimated to be 5,000 to 41,500/mm2 based on the backscatter images.

et al. It is inferred that the genesis of submicron and nanoscale inclusions in blue quartz may include : (1) exsolution of solid solution; (2) simultaneous crystallization with host quartz; (3) capture of previously crystallized minerals. The test results show that there are more nano-sized needle-like rutile in the blue quartz core than in the mantle , suggesting that the Ti content in the core is higher than that in the mantle . It seems that solid solution exsolution may well explain the appearance of nanoscale needle-like rutile; or such needle-like rutile may be formed by co-growth with quartz, not necessarily by exsolution. However, the large amount of submicron mica and anatase is difficult to explain by solid solution exsolution, because it requires a large amount of K, Na, Al, Fe, Mn and other elements in the quartz lattice. On the contrary, mechanism ② can better explain the origin of submicron inclusions, that is, these submicron inclusions grow at the interface between quartz crystal and melt, and the growth rate of host quartz is higher than that of these submicron inclusions. Mechanism ③ is more common in micron-sized and larger inclusions. Therefore, the authors believe that the nanoscale and submicroscale inclusions in quartz have different formation mechanisms. The former is usually formed by the exsolution of solid solution after the cooling of quartz, while the latter is usually crystallized at the same time as the quartz crystal.According to M Müller et al., submicron inclusions may be formed in a specific crystallization environment. When they appear, they will reduce the purity of quartz and become one of the sources of quartz pollution because they contain many impurity elements (depending on the type of submicron minerals). Further systematic study of submicron and nanoscale inclusions in quartz is needed to clarify their genesis, distribution and influence on quartz purity.

2. The type and abundance of inclusions depend on the crystallization environment, alteration and deformation after crystallization. If there are a lot of inclusions in the quartz crystal, they will have a great influence on the chemical purity and quality of the quartz raw material.

Fluid inclusions are the most common and abundant inclusions in quartz. They can be captured by quartz during the growth of quartz crystal and form primary fluid inclusions, and can also be formed when the fluid permeates along the micro-cracks of quartz and the quartz crystal heals in the later period, which is called secondary inclusions . For example, when we studied Guposhan granite in Guangxi and Zhangjinzhuang quartz sand in northern Jiangsu, we found that secondary fluid inclusions developed along quartz microfractures were found in both quartz (Fig. 3A, B, e); At the same time, isolated or grouped fluid inclusions were found in the quartz of both of them (Fig. 3C, d, f, G). Water is most commonly present in fluid inclusions, but in some cases, CO2 , CH4 , heavy hydrocarbons, N2 , etc. Can also be seen. Gerler found that the contents of Cl, Br, Na, Ca, Sr and Mn in quartz are positively correlated with the amount of fluid inclusions in quartz, suggesting that almost 100% of Cl, Br and I in quartz may be concentrated in fluid inclusions. The research results of Monecke et al also show that the fluid inclusions also contain a large number of Rb, Sr, REE and other components. To sum up, when the fluid carries a large amount of dissolved substances, after the quartz crystal is cooled, these substances are supersaturated and crystallized to form daughter minerals. Common daughter minerals include chlorides (such as halite), silicates, carbonates (such as calcite, dolomite, etc.), sulfates (gypsum, barite, etc.), etc.

Silicate melt inclusions often occur in magmatic rocks as small glassy or crystalline "bubbles" (about 1 to 300 μm). They are relatively rare in quartz crystals compared to fluid inclusions. The composition of the melt inclusions corresponds to that of the silicate melt trapped by quartz, which is mainly composed of Si, Al, Fe, Ca, Na and K. Melt inclusions in quartz from pegmatites formed in the late stage of magmatic evolution often contain a large number of alkali elements (such as Li, Na, K, Rb, Cs), volatile elements (such as B, P, F, Cl) and some rare elements. The impurity elements in these melt inclusions can affect the chemical purity of quartz and are a major source of pollution.

In theory, the mineral phases occurring in the host rock can occur in the mineral inclusions of quartz. Mineral inclusions of quartz in magmatic rocks mainly include feldspar, mica, rutile, apatite, etc . Most of these minerals are rock-forming minerals or important accessory minerals of magmatic rocks. For example, when we studied Guposhan granite in Guangxi, we found apatite mineral inclusions in quartz (Fig. 4A), which is one of the common accessory minerals in granite. In metamorphic rocks, the mineral inclusions in quartz are related to the metamorphic grade of the host rock, and the different metamorphic grade of the host rock corresponds to the different mineral assemblages in the mineral inclusions in quartz. In low-grade metamorphic rocks , the mineral inclusions of quartz may include chlorite, muscovite, or hornblende; in high-grade metamorphic rocks , the mineral inclusions of quartz may include kyanite, staurolite, or garnet. In sedimentary rocks , in addition to mineral inclusions in quartz of clastic magmatic rocks and metamorphic rocks, Quartz formed in the sedimentary environment (such as quartz secondary enlargement edge) will also contain anhydrite, gypsum, salt minerals, organic matter and other minerals formed in the sedimentary environment. For example, when we studied Zhangjinzhuang quartz sand in northern Jiangsu, we found calcite, microcline, rutile and other inclusions in quartz sand (Fig. 4 b-d). They are related to the source rocks of detrital quartz, and their formation and source rocks need to be further studied.

the above lattice impurity element (lattice scale), Submicron (100 nm ~ 1 μm) and nanometer (<100 nm) inclusions, as well as microscopic inclusions (> 1 μm) , are the basic occurrence forms of impurity elements (such as P, Ge, Ti, Al, Fe, B, K, Na, Li, Be, etc.) in quartz crystals. At the same time, on the surface of quartz crystal, other mineral crystallites can be attached; in crystalline rock, quartz will be disseminated with other minerals.

At the present stage, through mineral purification processes such as grinding, separation, magnetic separation, gravity separation, electric separation, flotation, pickling, acid leaching, heating and roasting, Most of the microcrystals attached to the surface of the quartz crystal, the minerals embedded with the quartz and most of the microinclusions in the quartz crystal can be removed . However, for the impurities in quartz, such as lattice impurity elements, possible submicron (<100 nm) and nanoscale (100 nm ~ 1 μm) inclusions, and microscopic inclusions (> 1 μm), It is difficult to remove them completely by existing methods, and their existence will greatly affect the chemical purity and quality of quartz. Therefore, the number and distribution of lattice impurity elements, submicron and nanometer inclusions, and microscopic inclusions in quartz crystal are important restrictive factors to determine whether quartz crystal can become high-purity quartz.It is very important to find out the occurrence, quantity and distribution characteristics of impurity elements in quartz crystal in detail for the subsequent mineral purification and processing and the discussion of its industrial use. Cathodoluminescence (CL)

of quartz is of great significance for the study of the composition and microstructure of quartz.

Scanning electron microscopy-cathode luminescence (Scanning Electron Microscopy, SEM-CL) technology makes it possible to obtain fine CL images of quartz. Because of its higher magnification and resolution, finer CL structures can be observed. The luminescence intensity of SEM-CL mainly depends on the chemical composition and structural changes inside the quartz crystal, which makes this technique an ideal technique for detecting the composition and microstructure of quartz. Observing quartz

under SEM-CL, we can observe some SEM-CL microstructures of quartz that can not be observed in ordinary optical microscope , secondary electron images and backscattered images. The SEM-CL microstructure of quartz is of great significance for the inversion of quartz source rocks and the exploration of the formation and evolution of quartz. Seyedolali et al. And Zhang Dexian et al. Have shown that the microstructure of quartz under SEM-CL can be divided into two types , namely, the primary microstructure formed during crystallization and/or cooling after crystallization. Including annuli, randomly oriented micro-cracks or healing crack lines, uniform CL, non-uniform patch-like or mottled CL; Secondary microstructures formed during deformation and/or recrystallization, such as grain crushing, oriented microcracks, deformed lamellae, homogeneous CL, heterogeneous patchy or mottled CL. Different SEM-CL microstructures of quartz usually occur in specific rock types and have different formation conditions, which can be used to invert the source of quartz and constrain its genesis. In addition, the combination of traditional OM-CL and SEM-CL, which complement each other, will better constrain the source rock and origin of quartz. The SEM-CL of

quartz can also be used to constrain the formation and evolution of quartz. For example, Rusk and Reed and Rusk identified quartz with different SEM-CL microstructures when studying the Butte porphyry copper deposit in Montana, USA. Their different luminescence intensities reflect the complex evolution history of quartz. Larsen and Young studied quartz from granites in the Oslo Rift, Norway, and identified four stages of quartz with different SEM-CL microstructures and luminescence intensities, and inferred that the primary magmatic quartz would be gradually infiltrated, dissolved or recrystallized by fluids. The original quartz is changed. In addition, the SEM-CL microstructure of quartz in magmatic rocks indicates its complex growth history, which is also of certain significance for the inversion of magmatic evolution.

4 High-purity quartz quality classification and industrial standard

High-purity quartz sand uses high-grade quartz stone (SiO2 content is greater than 99. High-purity quartz sand products range from low-end to high-end The general application paths are light source industry (99.5% -99.99%), high-end optical devices, laser devices (more than 99.99%), optical fiber communication, semiconductor, photovoltaic, microelectronics and other fields (99.995% -99. With the rapid development of China's electronic information industry, electric light source industry and the explosive growth of photovoltaic industry, the total industrial output value of China's quartz products industry has maintained a relatively rapid growth . Quartz is an indispensable raw material in the semiconductor field. Quartz is used to a greater or lesser extent in almost all processes, from the production of auxiliary components to the tools used in the actual processing of silicon wafers. The quartz crucible is used for manufacturing monocrystalline silicon, the quartz glass bell jar is used for photolithography engineering, and the quartz boat and the quartz support made of the quartz tube can be used for IC epitaxy, diffusion, photolithography engineering and the like . The rapid development of semiconductor industry will drive the demand growth of upstream high-purity quartz materials.

In the field of optical fiber communication, high-purity silica glass products are important materials in the process of optical fiber production. It is widely used in optical fiber preform manufacturing and optical fiber drawing process, in which the purity of the core rod is required to be the highest (greater than 5 N). The optical fiber semiconductor market has high requirements for the purity, specification accuracy and quality stability of quartz materials. Most domestic quartz products manufacturers do not have the ability to produce high-purity quartz sand and electronic grade quartz products. Therefore, at present, domestic optical fiber semiconductor manufacturers still mainly import quartz products from foreign enterprises. Internationally renowned quartz enterprises, such as Heraeus, Maitu and Shin-Etsu Quartz, occupy most of the optical fiber semiconductor application market in China. The raw materials for the manufacture of optical fiber core rods and sleeves are mainly monopolized by Unimin. At present, optical fiber preform quartz casing products mainly rely on imports, the cost of imported optical fiber quartz casing is high, and the demand for domestic substitution is strong .

At present, the state has formulated 25 current national standards, industry standards and local standards for quartz industry. For example, for photovoltaic high-purity quartz sand, it is required to have white particles with certain transparency and no different colors; the particle size of quartz sand should be in the range of 70 ~ 350 μm, and the cumulative mass fraction in this particle size range should be greater than or equal to 90%. The cumulative mass fraction of particles with particle size less than 100 μm or more than 300 μm shall be less than 1%. The content of silicon dioxide shall be greater than or equal to 99.99%, and the loss on ignition shall be less than or equal to 0.01%; The content of impurity elements should be less than or equal to 25 μg/G, in which the total content of potassium, lithium and sodium is less than 2.

According to the purity of commercial quartz products, Harben classifies the processed quartz according to the total amount of impurities. In this classification scheme, quartz having a total amount of impurity elements less than 50.m u.g/G is defined as a high-grade quartz material , including high purity quartz (HPS). Total impurity elements: 8 to 50 μg/G), very high-purity quartz (Ultra High Purity Quartz, total impurity elements: 1 to 8 μg/G), and ultra-pure quartz (Hyper Purity Quartz, total impurity elements <1 μg/G) (Fig. 5). Among them, the latter two are scarce or not in nature, and need to be processed and synthesized with natural quartz raw materials with high purity.

the content of Al and Ti in quartz is the main factor to restrict the purity of quartz. Accordingly, when the content of Al and Ti in quartz is less than 25 μg/G and 10 μg/G respectively, the natural quartz can be attributed to the category of high-purity quartz (Fig. 6).

so it is necessary to consider the upper limit of the content of the main impurity elements, rather than simply setting the upper limit of the total amount. Although the standard proposed by M Müller et al can be applied to natural quartz and mainly considers the influence of Al and Ti content, it fails to fully consider other impurity elements .

Based on the above reasons and the progress of quartz research, M Müller et al revised the evaluation criteria. In the revised scheme , the contents of nine harmful elements in quartz, including Na, K, Li, Al, Ca, Fe, Ti, B and P, were investigated. The allowable upper limits of each impurity element are Al <30 μg/G, Ti < 10 μg/G, Na < 8 μg/G, K < 8 μg/G, Li < 5 μg/G, Ca < 5 μg/G, Fe < 3 μg/G, P < 2 μg/G and B < 1 μg/G, respectively. At the same time, the total amount of these elements should not exceed 50 μg/G , so that the quartz can be used as high-purity quartz (Fig. 7). The evaluation scheme of M Müller et al. Is not only applicable to natural quartz, but also to processed quartz products, which has wider applicability . In addition, M Müller et al pointed out that the trace elements of quartz can be obtained by in-situ analysis. Uch as electron probe microanalysis (EPMA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) or secondary ion mass spectrometry (SIMS) , This is to avoid the influence of foreign minerals and inclusions. In theory, the result of in-situ analysis is the superposition of the content of lattice impurity elements and the content of submicroscopic inclusion elements. If the content of impurity elements in quartz is too high, the potential of purifying natural quartz into high-purity quartz is relatively small. At present, the highest purity of large-scale production can reach 99. The price of its products is closely related to the purity. The content of silicon dioxide in low-end products is 99.5% ~ 99. The purity of middle-end products is 99.995 ~ 9. The price of products is about 30,000 yuan/ton. The price of high-end high-purity quartz sand above 5 N is 60,000 yuan/ton, and the market is in short supply .

naturally occurring high-purity quartz is scarce or extremely limited (such as crystal) . In order to obtain high-purity quartz, Quartz raw materials with high purity in nature are often purified into high-purity quartz. Therefore, the evaluation of high-purity quartz raw materials and the study of source rocks and formation mechanism of high-purity quartz raw materials will be beneficial to the sustainable supply of high-purity quartz raw materials and the processing and purification of high-purity quartz .

5. As far as the quartz mineral itself is concerned, the five factors of chemical composition, disseminated grain size, paragenetic gangue minerals, inclusions and lattice impurities of quartz should be considered. See Table 1 for specific evaluation scheme.

Table 1 evaluation index

of high purity quartz raw material the five indexes of a certain quartz raw material, the quartz is more pure and is an ideal high-purity quartz raw material ; on the contrary, the quartz raw material is not ideal and needs to be purified through a complex process. Of course, the external impurities can be removed relatively easily by physical, chemical treatment, etc. Internal impurities include isomorphous substitution, gas-liquid inclusions and mineral inclusions. It is difficult to remove isomorphous substitution impurities and mineral inclusion impurities in quartz. The gas-liquid inclusion impurity in that quartz can be removed by heat, bursting and pure water washing, and of course, the bigg the gas-liquid inclusion is, the easier the gas-liquid inclusion is to burst, and the easier the gas-liquid inclusion is to be washed and removed.

In the actual production process, we need to integrate various factors, weigh the pros and cons, and make a comprehensive evaluation.

5. Studying and identifying which rocks can be used as raw materials for high-purity quartz is not only beneficial to prospecting, but also helpful to the subsequent mineral purification and processing.

Intermediate-acidic magmatic rocks such as granite, diorite and rhyolite are the main quartz-rich rocks , containing a large amount of quartz. However, other rock-forming minerals such as feldspar, hornblende and mica are often embedded with quartz, which is not a good source rock for quartz. In actual production, alaskite, pegmatite and hydrothermal veins are often selected as the source rocks for extracting quartz raw materials, because they are mainly composed of quartz and have relatively few impurity minerals, which are convenient for separation and purification. For example, the IOTA-type high-purity quartz produced by Unimin (a well-known supplier of high-purity quartz) is processed from alaskite in Spruce Pine, western North Carolina, USA (mainly composed of light-colored minerals, with an average grain size of about 1.Zhang Ye and Chen Peirong compared the pegmatite exposed in Spruce Pine area of the United States and Altai area of Xinjiang. The Results show that the geochemical characteristics of some pegmatite from Altay can be correlated with those from Spruce Pine, such as high contents of large ion lithophile elements Sr and Ba, and low contents of high field strength elements and rare earth elements. It is inferred that these pegmatites may be derived from the fluid produced by the highly fractionated melt-vapor of granitic magma, so the content of impurity elements in quartz is relatively low, which indicates that some pegmatites in Altay area have the potential to produce high-purity quartz.

Metamorphic rocks contain many quartz-bearing rocks , such as mylonite, amphibolite, gneiss, granulite and so on . However, most of the quartz crystals in these rocks are fine-grained and closely embedded with other rock-forming minerals, which is not an ideal raw material for quartz. For metamorphic rocks, rocks with high whole rock SiO2 content and almost composed of quartz are often selected. Such as metamorphosed quartzite, some siliceous slates and quartz veins differentiated by metamorphism . These rocks are either transformed by metamorphism, resulting in the migration of some original impurity elements, thus improving the purity of quartz (such as metamorphic quartzite and siliceous slate), or formed by the crystallization of metamorphic fluids with few impurity elements (such as metamorphic veins). For example, M Müller et al. studied kyanite quartzite in northern Norway, and found that the content of impurity elements in quartz is low, fluid inclusions are scarce, and the grain boundaries of quartz and other minerals are mostly flat and straight. Based on the regional geological background, the authors speculate that retrograde metamorphism affected the original trace element composition of quartz. During retrograde metamorphism, the crystal lattice of quartz is recovered, accompanied by the shrinkage of grain boundary zone and the migration of grain boundary , which is conducive to the healing of crystal lattice defects of quartz. Impurity elements (such as Al, Ti, etc.) in the crystal lattice are expelled to grain boundaries or/and condensed into inclusions .

Sedimentary rock is the main supply rock of silica raw materials in industry. Among sedimentary rocks, relatively pure quartz-bearing rocks include quartz sand and quartzite of sedimentary origin. Sedimentary quartz sand is usually formed in the environment with strong external dynamic geological action, such as strong weathering area and strong beach washing area; the chemical purity of sedimentary quartzite is high , and the content of cryptocrystalline and/or amorphous silica cement is increased.

In addition to the above-mentioned high-purity quartz raw material rocks with quartz as the main mineral composition, other quartz-bearing rocks and their quartz still need to be studied to comprehensively evaluate whether they can become raw materials for producing high-purity quartz. This will be beneficial to the sustainable supply of high-purity quartz raw materials. Relatively pure quartz formed in

different geological environments has different chemical purity and impurity elements, so their preferred application fields are different. We have summarized the properties and preferred application areas for the formation of higher purity quartz in different geological environments, as detailed in Table 2.

Table 2 Properties and preferred utilization fields

of quartz raw materials of different origins 5. Therefore, relatively pure quartz can be directly crystallized in melts/fluids with suitable external environment and few impurities; If the purity and particle size of quartz formed by melting/fluid are not good at the beginning, it can also be "purified" by removing impurities through lattice recovery and grain boundary migration in the later transformation process (such as tectonic deformation, metamorphism, hydrothermal metasomatism, etc.); of course, it can also be formed by the superposition of the above two ways. The Nedre Øyvollen pegmatite in northern

Norway is a high-quality high-purity quartz raw material , which produces quartz grains with large crystals, high purity and uniform chemical composition. The chemical composition of Nedre Øyvollen pegmatite shows that the pegmatite itself contains low impurity elements. M Müller et al. concluded that the well-behaved quartz in the Nedre Øyvollen pegmatite was crystallized directly from a silicate melt with relatively few impurity elements . Similarly, the raw rock produced by Unimin Company in the United States for processing and producing high-purity quartz is white granite in Spruce Pine, western North Carolina, USA. Coarse-grained quartz crystals may also originate from highly differentiated magmas with relatively few impurity elements. In addition, the Nesodden and Kvalvik quartz veins in Norway are potential sources of quartz with high chemical purity and few impurity elements, and it is speculated that the hydrothermal fluid itself from which these quartz veins are crystallized has few impurity elements. Of course, these two quartz veins are disseminated with other minerals and contain inclusions and submicroscopic inclusions, which will increase the difficulty of purification. The modification of

quartz in the later stage may also improve its purity. For example, M Müller et al. And Van den Kerkhof and Hein studied granulite (Bamble sector) and kyanite quartzite (extensive locality) in Norway. It is found that the chemical purity of quartz crystals formed during retrograde metamorphism is relatively high and impurities are relatively low. This is because the primary quartz contains many point defects (substitutional atoms and interstitial atoms), which will increase the internal energy of the quartz crystal and make it in a thermodynamically unstable state . In the retrograde process , the crystal lattice of quartz will be restored by the way of grain boundary migration (recrystallization process) , and the defects will be gradually eliminated and the impurity elements in quartz will be expelled. Impurity elements will migrate to the grain boundary or aggregate to form inclusions . In addition, the later dynamic disturbance and thermal disturbance will also remove some impurity elements in quartz.For example, when studying the Sveconorwegian pegmatite in Finland, M Müller et al. Found that compared with quartz in undeformed pegmatite which has not been subjected to thermal contact metamorphism, The contents of Li and Al in quartz from mylonitization and thermal contact metamorphism pegmatites are relatively lower , indicating that the later dynamic and thermal disturbance can remove the two impurity elements Li and Al in quartz. However, quartz in pegmatites with mylonitization and thermal contact metamorphism has a relatively high Ti and Ge content (similar to the conversion of one impurity element into another), which also increases the difficulty of quartz purification.

In addition to the two main quartz "purification" mechanisms mentioned above (crystallization from low-impurity melts or fluids, and later modification of the original quartz), other endogenous and exogenous geological processes that are conducive to quartz purification need to be studied . This is not only beneficial to the search for high-purity quartz raw materials , but also enlightens and benefits the purification and processing of quartz raw materials.

5. Quartz contains more or less impurity elements (such as Al, Ti, K, Na, Ge, etc.) . Its type and content are related to the melt/fluid at the time of quartz crystallization, the external environment and the modification after crystallization. The content and occurrence state of impurities in quartz crystal are the important restrictive factors to determine whether quartz crystal can become high-purity quartz. When the metallogenic potential of high-purity quartz is comprehensively evaluated in combination with technological indicators and commercial value, the dissemination characteristics of quartz minerals, coexisting gangue minerals and their types should be comprehensively examined. It is very important to find out the occurrence, quantity and distribution characteristics of impurity elements in quartz crystal in detail for the subsequent mineral purification and processing and the discussion of its industrial use. In the aspect of purification technology, in the whole process flow, through the procedures of roasting, water quenching, magnetic separation and acid leaching, Impurity elements such as Fe, Cr, Ni, Na, K, Ca, Mg and Cu in quartz can be greatly reduced . However, after a series of purification processes, the removal effect of Al is limited . This is mainly because Al3 ⁺ enters the lattice to replace Si4 ⁺ , and the ionic radius is relatively close, so it is not easy to purify. Similarly, there are Ti4 ⁺ , B3 ⁺ , P3 + and other impurity elements . It can be seen that the impurities in natural quartz, especially those existing in the isomorphous state, directly restrict the production of high-purity quartz products. When the content of impurity elements such as Al, Ti, Li, B and P in the raw ore is high, it is not easy to obtain high-purity quartz .

Conclusion and prospect

(1) High-purity quartz is a kind of high-quality quartz produced in nature (such as crystal) or processed from relatively pure quartz raw materials. Because of its low impurity content and unique physical properties, it has become a necessary raw material for the production of high value-added quartz products in some high-tech industries (such as semiconductors, high-temperature lamps, communications, precision optics, microelectronics, solar energy, etc.). From quartz to high purity quartz, from general quality to high quality, from traditional industrial raw materials to high-tech industrial raw materials , from low value-added quartz products to high value-added quartz products. The improvement of quartz quality, the extension of its use and the improvement of its value are the qualitative changes of quartz.

(2) Crystal lattice impurities, submicron inclusions and microscopic inclusions are the main occurrence States of impurity elements in quartz, which have an important impact on the purity of quartz. When evaluating the quartz mineral raw material for producing high-purity quartz, not only the chemical purity and internal impurities of the quartz raw material should be considered, but also the dissemination characteristics of the quartz mineral, the coexistence of gangue minerals and types should be examined, and the comprehensive evaluation should be carried out according to the process index and commercial value. The pegmatite and hydrothermal veins in magmatic rocks, quartzite and metamorphic veins in metamorphic rocks, and quartzite and quartz sand of sedimentary origin are relatively good choices for quartz raw materials used to purify high-purity quartz. The relatively pure quartz can be directly crystallized from the melt or fluid with less impurities, can be formed by purifying the primary quartz in the later transformation, and can also be the superposition of the above two effects.

(3) High-purity quartz is an important resource and raw material for industrial production. Because there are relatively few studies on the basic mineralogy and geochemistry of quartz and high-purity quartz raw materials in China, the purification and processing technology of quartz raw materials needs to be improved urgently. A large amount of relatively pure and high-purity quartz is dependent on import. In Order to make better use of and develop the quartz resources, on one hand, from the angle of science, we should strengthen the basic research on quartz mineralogy, find out the characteristics of quartz in different quartz-bearing rocks and the potential of purifying and processing quartz into high-purity quartz. Perfecting the comprehensive evaluation mechanism of high-purity quartz raw materials and discussing what kind of internal and external dynamic geological processes are beneficial to the "purity" of quartz will become the cornerstone of subsequent processing and purification of high-purity quartz. On the other hand, from the engineering point of view, improving the process flow of purifying high-purity quartz from quartz raw materials, optimizing the purification scheme and studying the new technology route of quartz purification will be conducive to the sustainable supply of high-purity quartz and meet the growing demand in the high-tech field. Only by combining theoretical research with practical processing and production can quartz raw materials glow with new vitality.