Figure 7-2. Differential-thermal curves of various minerals. 



Curve F, kaolinite (H4AI0S12O9), shows a strong endothermic reaction at 600C, 

 representing destruction of the crystal lattice. At about 990C there is a sharp 

 exothermic reaction due to recrystallization of amorphous alumina produced in 

 the lattice breakdown. Curve G, montmorillonite (Wyoming type) — (Mg,Ca)0- 

 Alo0 3 5Si0 2 ' nH 2 ? ) , exhibits several well-defined peaks. The lower endo- 

 thermic peak at about 160C represents the loss of water of hydration (not to be 

 confused with moisture, which may be absorbed on the surface of any mineral 

 and which would appear as an endothermic bulge at about 100C. The large endo- 

 thermic peak at about 700C and the smaller one at about 900C are due to the 

 stepwise destruction of the lattice structure. After the final breakdown, crystal- 

 lization of spinel (MgAloC^) results in the exothermic peak which follows im- 

 mediately (about 950C). 



SEMIQUANTITATIVE When a mineral is present in less than 100 



ANALYSIS percent concentration, DTA peaks become 



correspondingly smaller and peak temperature 

 is generally reduced. This fact is illustrated in Figure 7-3. The three curves at 

 the left portray 25, 50, and 100 percent calcite, with inert alumina as the diluent. 

 Although decomposition starts at the same temperature, the peak is reached 

 sooner (at a lower temperature) with lower concentrations. This is brought 



124 



