A thermogravimetric and infrared emission spectroscopic study of alunite

Thermogravimetric and differential thermogravimetric analysis has been used to characterize alunite of formula [K-2(Al-6(3+)(SO4)(4)(OH)(12)]. Thermal decomposition occurs in a series of steps (a) dehydration up to 225C, (b) well defined dehydroxylation at 520C and desulphation which takes place as a series of steps at 649, 685 and 744C. The alunite minerals were further characterized by infrared emission spectroscopy (IES). Well defined hydroxyl stretching bands at around 3463 and 3449 cm(-1)are observed. At 550C all intensity in these bands is lost in harmony with the thermal analysis results. OH stretching bands give calculated hydrogen bond distances of 2.90 and 2.84-7 angstrom. These hydrogen bond distances increase with increasing temperature. Characteristic (SO4)2-stretching modes are observed at 1029.5, 1086 and 1170 cm(-1). These bands shift to lower wavenumbers on thermal treatment. The intensity in these bands is lost by 550C.