The high-temperature transformation of andalusite (Al₂SiO₅) into 3/2-mullite (3Al₂O₃2SiO₂) and vitreous silica (SiO₂)

Summary The high-temperature transformation of andalusite (Al 2 SiO 5 ) into 3/2-mullite ( 3 A 2 O 3 2SiO 2 ) plus vitreous silica (SiO 2 ) has been studied within the temperature interval from 1300 to 1600 °C by means of X-ray powder and single-crystal techniques and by infrared spectroscopy. Results are interpreted in terms of a topotactic transformation in which (011) and (01 ) planes of andalusite transform into (20 ) and (201) planes of 3/2-mullite; in this way the a - and b -axes of the two phases are interchanged. From the structural and orientational relationship between the andalusite and 3/2-mullite latrices it is concluded that the aluminium-oxygen octahedral chains, running in both structures parallel to the crystallographic c -axis, are preserved during phase transformation.