tailieunhanh - handbook of extractive metallurgy IV Part 4

Tham khảo tài liệu 'handbook of extractive metallurgy iv part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 1902 Handbook of Extractive Metallurgy Silica Silica Modifications and Products Geochemistry and Crystal Structure Silica silicon dioxide SiO2 is the major constituent of rock-forming minerals in magmatic and metamorphic rocks it accounts for ca. 75 of the Earth s crust 391 . It is also an important component of sediments and soils 392-394 In rocks free silica predominantly occurs as quartz which owes its important role to the action of water in rock-forming processes on Earth. Quartz makes up 12-14 of the Earth s crust. In silicate bodies that crystallized under dry conditions moon rocks and silicate slags for example quartz is rare instead cristobalite and tridymite crystallize. Micro- and noncrystalline silica minerals are described in 395 . The structural framework of crystalline SiO2 phases can incorporate trace element impurities in two ways Figure 396 . The first is the substitution-addition mechanism Figure substitution of Si4 by Al3 or Fe3 and addition of M and M2 at interstices M Li Na M2 Mg2 Ca2 . The second is the formation of hydrolytic or alkaline nonbridging oxygens Trennstellen in the framework. Incorporation of alkali metal and aluminum ions in stoichiometric proportions leads to stuffed derivatives 396 . Structure type highquaitz high high tridymite cristobalite Stiffed eucryptite carnegieite kalsilite derivative Li A SiO Na AlSiO K AlSiOJ Quartz has a very high geochemical purity the dissolved trace elements rarely exceeding 100 ppm 395 397 398 . Water however can be incorporated in concentrations from hundreds to several thousands ppm Figure . The transitions from structural incorporation to microstructural inclusion are fluent 399 . Sil ica has a Sttong affinity for water. At elevated temperatures and pressures water migrates rapidly through the structural framework splitting Si-O-Si bonds and forming silanol groups. This mechanism of hydrolytic weakening activates reconstructive transformations .

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