Antao, S. M.; Hassan, Ishmael
Author Affiliation, Ana.
Department of Chemistry
Thermal analyses of sodalite, tugtupite, danalite, and helvite
Date of Publication
Differential thermal analyses (DTA) and thermogravimetric analyses (TG) were carried out on sodalite, tugtupite, danalite, and helvite using a Netzsch STA 409 EP/3/D simultaneous TG–DTA apparatus. In the DTA and TG experimental run, a weighed amount of finely powdered sample was heated from 20 to 1450°C at a constant rate of 5°C/min. Sodalite, Na8[Al6Si6O24]Cl2, melts at 1079°C. The NaCl component in the sodalite-derived melt is lost in two stages; about 4.5 wt.% of NaCl is lost slowly at about 1150°C, and about 7.0 wt.% of NaCl is lost at a faster rate at about 1284°C. The interpretation that the NaCl component behaves this way in sodalite is confirmed by DTA and TG analyses of halite. Tugtupite, Na8[Al2Be2Si8O24]Cl2, melts at 1029°C. The NaCl component in tugtupite is lost in two main stages; about 1.8 wt.% of NaCl is first lost at about 1007°C, and about 8.2 wt.% of NaCl is lost in several steps between about 1018 and about 1442°C. Danalite, ideally Fe8[Be6Si6O24]S2, and helvite, ideally Mn8[Be6Si6O24]S2, undergo an oxidation of (Mn, Fe)2+ to (Mn, Fe)3+ cations. This is followed by a loss of S2(g), and then melting at about 1060°C. Finally, there is another oxidation of Mn3+ to Mn4+. These oxidations occur because the samples were heated in a static air environment. The first oxidation in danalite and helvite begins at about 771 and 705°C, respectively. Danalite gains 4.0 wt.%, whereas helvite gains 5.0 wt.% in the first weight-gain stage. In the weight-loss stage, danalite loses 5.7 wt.% of S2(g) from about 1029°C, whereas helvite loses 4.7 wt.% of S2(g) from about 883°C. The second stage of oxidation begins at about 1300°C in both the danalite- and helvite-derived melts. At this stage, the weight gain in the danalite-derived melt is about 0.7%, whereas that in the helvite-derived melt is about 0.2%.....