100 



200 



TEMPERATURE 



300 



CENTIGRADE 



400 



Figure 10-6. Eflect of fractional annealing on thermolumines- 

 cence. Glow curves of Niagrara limestone. Samples heated 

 at 175° C for T hours before being run. 



the inclusion of impurities which may affect thermoluminescence. For example, 

 in a solution from which calcium carbonate is being precipitated by the addition 

 of sodium carbonate, any iron or manganese impurities will be precipitated with 

 the first calcium carbonate to come down at about a pH of 6.5. As more carbon- 

 ate is added and the pH is increased to about 8, strontium and barium will pre- 

 cipitate; and above a pH of 9, magnesium will precipitate. 



The presence of strontium under properly controlled conditions of tempera- 

 ture and time leads to the precipitation of aragonite which changes over to cal- 



190 



