gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/minmag.1978.042.322.05 The High-Temperature Transformation of Andalusite (A12SiO5) into 3/2-Mullite (3Al2O32SiO2) and vitreous silica (SiO2) Pannhorst Wolfgang Kristaliographisches Institut der Universität Schneider Hartmut * Mineralogisches Institut der Universitäit, Karlsruhe, Germany

Present address: Forschungsinstitut der Feuerfestindustrie, An der Elisabethkirche 27, 5300 Bonn, Germany.

 6 1978 42 322 195 198 25 5 1977 22 9 1977 Copyright © 1978, The Mineralogical Society 1978 Summary

The high-temperature transformation of andalusite (Al2SiO5) into 3/2-mullite (3A2O32SiO2) plus vitreous silica (SiO2) 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¯1) planes of andalusite transform into (20¯1) 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.