Gaylussite: thermal properties by simultaneous thermal analysis

Abstract

Simultaneous thermal analysis apparatus was used to determine the thermal properties of gaylussito over a temperature range from ambient to 1000'C at 10 per minute in several carefully controlled atmospheres. In a dry atmosphere, dehydration occurs in a single step, while in the presence of water, dehydration occurs in two steps. Dehydration converts gaylussite to the double carbonate, Na,Ca(CO), which generates the thermal effects above 250'C. From 250C to 500'C thermal analysis indicates one major crystal change and two crystal inversions in the double carbonate. Above 500'C the thermal properties of the double carbonate are controlled by the presence or absence of CO in the atmosphere surrounding the sample. In a COs atmosphere NaCa(COe)a first melts, then decomposes to form NarCO and CaO. In a CO,-free atmosphere the double carbonate decomposes to form Na'COs and CaO. Melting is again detected by thermal analysis; however, the material that melts is Na,CO, formed from decomposition of the double carbonate. fffroduction Gaylussite (Na2COs.CaCOs.5fLO) is found in Wyoming in Eocene Green River Formation oil shales (Fahey, 1962). Because minerals contained in oil shale are heated in the retorting process, a knowledge of the thermal properties of these minerals contributes to a basic understanding of oil shale's thermal behavior. Literature on thermal analysis of gaylussite is limited to Beck's (1950) interpretation of his difierential thermal analysis curve that shows water loss in two steps. He suggests that the remaining highly irregular curve is produced by slow fusing of the mineral. His large exotherm between 500' and 600C could not be duplicated in this study. Thermal properties of the double carbonate similar to the residue formed after gaylussite loses its water of hydration have also been discussed in the literature-Smith, Johnson, and Robb (D7A); Billhardr (1969); Wilburn, Metcalfe, and Warburton (1965); and Niggli (1916). Because little is known about gaylussite, this paper reviews briefly the mineral's history and presents new data on the thermal properties of natural and synthetic gaylussite. The importance of the eftect of different atmospheres on the thermal analysis of gaylussite is demonstrated.