vertical base line in an endothermic direction. An exothermic reaction will ap- 

 pear as a deviation to the left of the base line. 



Thus, Figure 7- IB shows the heating curve of sulfur but in the form of a 

 differential-thermal curve. The peak BCD represents melting ; EFG, evaporation ; 

 and GHI, oxidation. The temperature of a peak is not the temperature at which 

 the reaction commences, it is the temperature at which the greatest differential 

 exists between the sample and the alumina standard. Also, the reaction is 

 complete before the curve has come back fully to the base line, i.e., points 

 C, F, or H. The area within each peak is a function of the type and con- 

 centration of element or compound causing the reaction. Observance of the 

 temperature, shape, and size of the peaks provides a means of qualitative and 

 semiquantitative analysis. More complete theory and mathematical relation- 

 ships of differential-thermal analysis may be found in existing authoritative 

 publications (Spiel, Berkelhamer, Pask, and Davies, 1945; Kerr, Kulp, and Ham- 

 ilton, 1949). 



It is not within the scope of this discussion to explain the effects of varying 

 the kind of gas surrounding the sample or of varying the pressure of this gas. 

 Research is progressing continuously on these more complicated phenomena and 

 results appear periodically in the literature (Rowland and Lewis, 1951; Stone, 

 1954; Stone and Rowland, 1955). However, inasmuch as these specialized re- 

 finements of thermal-analysis technique are still largely of a research nature, it 

 will suffice for this presentation to consider only the methods and results ob- 

 tained under ordinary conditions of atmospheric-pressure and air environment. 



QUALITATIVE ANALYSIS Typical DTA curves of several common min- 

 erals are shown in Figure 7-2. A brief ex- 

 planation of each curve will aid in illustrating the type of results that may be ob- 

 tained from thermal analysis. Curve A, representing alumina (A1 2 3 ), is a 

 straight vertical line, indicating that no thermal reactions occur within the tem- 

 perature range covered (room temperature to 1000C). Curve B, quartz (Si0 2 ), 

 shows one small sharp endothermic reaction at 573C, representing the crystal 

 inversion from a to /? quartz. Calcite (CaC0 3 ), curve C, is thermally inert up 

 about 800C but the large endothermic reaction at about 925C represents the 

 decomposition of the carbonate and the expulsion of carbon dioxide. Curve D, 

 dolomite (MgC0 3 CaC0 3 ), shows two endothermic peaks at about 780 and 925C 

 respectively, which represent the successive decompositions of magnesium car- 

 bonate and then calcium carbonate with the consequent loss of carbon dioxide. 

 Curve E, siderite (FeC0 3 ), shows the decomposition at 550C with loss of 

 carbon dioxide. The remaining iron oxide (FeO) immediately oxidizes to Fe 3 4 , 

 as indicated by the sharp exothermic peak at 600C. The small exothermic bulge 

 centered at about 750C represents the further oxidation to hematite (Fe 2 3 ). 



123 



