Thermal decomposition of jarosites of potassium, sodium and lead

Jarosites are a group of minerals formed in evaporite deposits and form a component of efflorescence. As such the minerals can function as cation and heavy metal collectors. Thermogravimetry coupled to mass spectrometry has been used to study three Australian jarosites which are predominantly K, Na and Pb jarosites. Mass loss steps of K-jarosite occur over the 130 to 330 and 500 to 622 degrees C temperature range and are attributed to dehydroxylation and desulphation. In contrast the behaviour of the thermal decomposition of Na-jarosite shows three mass loss steps at 215 to 230, 316 to 352 and 555 to 595 degrees C. The first mass loss step for Na-jarosite is attributed to deprotonation. For Pb-jarosite two mass loss steps associated with dehydroxylation are observed at 390 and 418 degrees C and a third mass loss step at 531 degrees C is attributed to the loss of SO3. Thermal analysis is an excellent technique for the study of jarosites. The analysis depends heavily on the actual composition of the jarosite.