On the thermal stability of manganese(II) sulfate and its reaction with zeolite A to form the sodalite Na6Mn2[Al6Si6O24](SO4)2

Abstract

Thermal gravimetric analysis and in situ high-temperature powder X-ray diffraction (HT-PXRD) revealed that MnSO4 begins to thermally decompose under air at ~625 °C, which is significantly lower than the values reported in the literature. The reaction is kinetically sluggish and yields bixbyite, Mn2O3, ≳625 °C and hausmannite, Mn3O4, ≳850 °C. Na6Mn2[Al6Si6O24](SO4)2 was prepared as a novel sodalite-type phase by reacting zeolite A, Na6[Al6Si6O24], with MnSO4 as a compacted mixed-powder monolith at 650 °C under air. In situ HT-PXRD indicated that this reaction commenced at about 620 °C. Rietveld refinement of the PXRD data recorded at 20 °C showed that Na6Mn2[Al6Si6O24](SO4)2 crystallizes with a cubic cell, a = 0.89780(2) nm, and is isostructural (I4¯3m) with the mineral haüyne, Na6Ca2[Al6Si6O24](SO4)2. The product material contained traces of Mn2O3 which imparts a purplish-brown coloration; highlighting the problem of preparing Na6Mn2[Al6Si6O24](SO4)2 as a monophasic material within the narrow temperature ‘window’ available for this reaction.