Question

descripe the crystal structure kaolinite? with drawong, and where is hhe final crystalline phase in firred Kaolinite?

Answer 1

Kaolinite is a clay mineral, part of the group of industrial minerals, with the chemical composition Al₂Si₂O₅(OH)₄. It is a layered silicate mineral, with one tetrahedral sheet of silica (SiO₄) linked through oxygen atoms to one octahedral sheet of alumina(AlO₆) octahedra. Rocks that are rich in kaolinite are known as kaolin or china clay.oct The chemical formula for kaolinite as used in mineralogy is Al₂Si₂O₅(OH)₄, however, in ceramics applications the formula is typically written in terms of oxides, thus the formula for kaolinite is Al₂O₃·2SiO₂·2H₂O, octahedral sheet of alumina(AlO₆) octahedra.

Metakaolin

Endothermic dehydration of kaolinite begins at 550–600 °C producing disordered metakaolin, but continuous hydroxyl loss is observed up to 900 °C (1,650 °F).^[11] Although historically there was much disagreement concerning the nature of the metakaolin phase, extensive research has led to a general consensus that metakaolin is not a simple mixture of amorphous silica (SiO₂) and alumina (Al₂O₃), but rather a complex amorphous structure that retains some longer-range order (but not strictly crystalline) due to stacking of its hexagonal layers.^[11]

Al₂Si₂O₅(OH)₄ → Al₂Si₂O₇ + 2 H₂O.

Spinel

Further heating to 925–950 °C converts metakaolin to an aluminium-silicon spinel which is sometimes also referred to as a gamma-alumina type structure:

2 Al₂Si₂O₇ → Si₃Al₄O₁₂ + SiO₂.

Platelet mullite

Upon calcination above 1050 °C, the spinel phase nucleates and transforms to platelet mullite and highly crystalline cristobalite:

3 Si₃Al₄O₁₂ → 2 (3 Al₂O₃ + 2 SiO₂) + 5 SiO₂.

Needle mullite

Finally, at 1400 °C the "needle" form of mullite appears, offering substantial increases in structural strength and heat resistance. This is a structural but not chemical transformation. See stoneware for more information on this form.