gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/minmag.1968.283.036.13 On the Infra-Red Spectrum and Thermal Decomposition Products of Thaumasite,1 Ca3H2(CO3/SO4)SiO4.13H2O Kirov G. N. Chair of Mineralogy and Crystallography, University of Sofia, Sofia, Bulgaria Poulieff C. N. X-ray Laboratory, Geological Survey of Bulgaria2

This paper was presented in part at the Seventh I.U.Cr. Congress in Moscow, July 1966.

Present address: Institute of Physical Chemistry, Bulgarian Academy of Sciences, Sofia 13, Bulgaria.

 9 1968 36 283 1003 1011 16 2 1968 Copyright © 1968, The Mineralogical Society 1968 Summary

The infra-red spectrum of untreated thaumasite Ca3H2(CO3/SO4)SiO4.13H2O, as well as the spectra and X-ray diffraction patterns of thaumasite heated in air at 200, 360, 550, 725, 800, 900, and 1150° C were investigated. The release of CO2 and H2O upon heating was followed. It was established that thaumasite changes between 200 and 550° C to a glass-like, amorphous state. Ca2SiO4-polymorphs and anhydrite are formed upon heating at temperatures up to 800° C. From 900° C calcium silicosulphate and anhydrite are the thermal decomposition produets. These results are used in the assignment of the IR-absorption bands under 1400 cm−1 of untreated thaumasite, which are shown to be of a composite nature, i.e. attributable to overlapping vibrations of both sulphate and silicate tetrahedra.