Pyrophyllite has a chemical formula of Al 2[Si 4O 10] (OH) 2 and is a 2:1 type of aluminum silicate with a layer structure (see chapter “Pyrophyllite,” Fig. 1); its crystal structure consists of two layers of [SiO 4] tetrahedral layers sandwiching a layer of [AlO 6] octahedra (Bailey 1966; Rayner and Brown 1966; Wardle and Brindley 1972). In nature, there are primarily two polytypes of pyrophyllite: two-layered monoclinic (2 M) and one-layered triclinic (1 Tc). Mixtures of the two forms (2 M + 1 Tc, or 1 Tc + 2 M) are also found in nature (Brindley and Warlde 1970; Wardle and Brindley 1972; Eberl 1979; Lee and Guggenhein 1981; Wang 1994; Yang and Zhang 1994). Pyrophyllite has been widely used in industry, especially in the manufacture of ceramics, glass, refractory materials, and pressure-transfer media. A deeper understanding of dehydroxylation and transformation of pyrophyllite would be useful for these industrial applications. The dehydroxylation of pyrophyllite involves the reaction of two OH groups (adjacent to each other on the [AlO 6] octahedron) with the liberation of water (Fitzgerald et al. 1996). Powder X-ray diffraction (XRD) measurements of 1 Tc pyrophyllite and its dehydroxylate have shown that pyrophyllite dehydroxylate consists of five-coordination, distorted, trigonal bipyramidal [AlO 5] structural units, sandwiched between two layers of twisted but intact [SiO 4] tetrahedral layers (Wardle and Brindley 1971, 1972). 29Si MAS-NMR data has confirmed that pyrophyllite dehydroxylate maintains the 2:1 layer structure (Frost and Barron 1984). Fitzgerald et al. (1989, 1996) used 27Al MAS-NMR to study the thermal-induced dehydroxylation of pyrophyllite. The results indicate that the six-coordinated Al in pyrophyllite becomes five-coordinated after dehydroxylation. Additionally, Heller (1962) studied the thermal transformation of pyrophyllite to mullite in the temperature range of 970–1200 °C; Fitzgerald et al. (1996) used the magic-angle spinning (MAS) nuclear magnetic resonance method to study the thermal transformation of pyrophyllite in the temperature range of 150–1350 °C; Wang and Zhang (1997a) used power XRD analysis to study the high-temperature phases of decomposed pyrophyllite and their evolutionary characteristics. Nevertheless, the process and behavior of thermal-induced dehydroxylation and transformation of pyrophyllite remain only partially understood.

Dehydroxylation and Phase Characteristics of Pyrophyllite During Annealing

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2025年

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Ling Wang

摘要

Pyrophyllite has a chemical formula of Al 2[Si 4O 10] (OH) 2 and is a 2:1 type of aluminum silicate with a layer structure (see chapter “Pyrophyllite,” Fig. 1); its crystal structure consists of two layers of [SiO 4] tetrahedral layers sandwiching a layer of [AlO 6] octahedra (Bailey 1966; Rayner and Brown 1966; Wardle and Brindley 1972). In nature, there are primarily two polytypes of pyrophyllite: two-layered monoclinic (2 M) and one-layered triclinic (1 Tc). Mixtures of the two forms (2 M + 1 Tc, or 1 Tc + 2 M) are also found in nature (Brindley and Warlde 1970; Wardle and Brindley 1972; Eberl 1979; Lee and Guggenhein 1981; Wang 1994; Yang and Zhang 1994). Pyrophyllite has been widely used in industry, especially in the manufacture of ceramics, glass, refractory materials, and pressure-transfer media. A deeper understanding of dehydroxylation and transformation of pyrophyllite would be useful for these industrial applications. The dehydroxylation of pyrophyllite involves the reaction of two OH groups (adjacent to each other on the [AlO 6] octahedron) with the liberation of water (Fitzgerald et al. 1996). Powder X-ray diffraction (XRD) measurements of 1 Tc pyrophyllite and its dehydroxylate have shown that pyrophyllite dehydroxylate consists of five-coordination, distorted, trigonal bipyramidal [AlO 5] structural units, sandwiched between two layers of twisted but intact [SiO 4] tetrahedral layers (Wardle and Brindley 1971, 1972). 29Si MAS-NMR data has confirmed that pyrophyllite dehydroxylate maintains the 2:1 layer structure (Frost and Barron 1984). Fitzgerald et al. (1989, 1996) used 27Al MAS-NMR to study the thermal-induced dehydroxylation of pyrophyllite. The results indicate that the six-coordinated Al in pyrophyllite becomes five-coordinated after dehydroxylation. Additionally, Heller (1962) studied the thermal transformation of pyrophyllite to mullite in the temperature range of 970–1200 °C; Fitzgerald et al. (1996) used the magic-angle spinning (MAS) nuclear magnetic resonance method to study the thermal transformation of pyrophyllite in the temperature range of 150–1350 °C; Wang and Zhang (1997a) used power XRD analysis to study the high-temperature phases of decomposed pyrophyllite and their evolutionary characteristics. Nevertheless, the process and behavior of thermal-induced dehydroxylation and transformation of pyrophyllite remain only partially understood.