JV. L. Bowen — The Binary System. 561 



that this temperature is considerably higher than that at which 

 inversion will take place if the time is sufficiently long. 

 E~e 9o An 10 may be held at 1282° (34° above the inversion point 

 of JNTe) for four hours without change. Held at 1294° for one 

 hour a trace of the carnegieite form and at 1296° for one hour 

 considerable carnegieite separates. 



These results indicate that carnegieite first separates from 

 Ne 90 An 10 between 1282-1294° or at about 1290°. This rapid 

 rise in the temperatures of inversion of nephelite as CaA! 2 Si 2 8 

 is taken into solid solution was confirmed in Ne 95 An 5 (1265°) 

 and in N"e 80 AD 20 which shows only the nephelite form at 1340°. 



It is not possible to confirm these inversion temperatures by 

 starting with the carnegieite form and studying the reverse 

 change as was done with Ne and Cg because the behavior of 

 these solid solutions is much less simple than that of the pure 

 compound. 



It is well known that according to theory a solid solution 

 should, in general, melt, not at a definite temperature to form a 

 liquid of its own composition, but that melting should be dis- 

 tributed over a temperature interval, the liquid first formed 

 differing in composition (concentration) from the solid. 

 Similarly the inversion of a solid solution takes place over an 

 interval, the concentration in the first small quantity of the 

 new crystal form being either greater or less than that in the 

 old. It is mainly in these characteristics that a solid solution 

 differs from a definite compound. 



Both melting interval and inversion interval are exhibited 

 by the nephelite solid solutions. Any point on the curve E C 

 represents the temperature of the lower limit of the inversion 

 interval for the corresponding composition and was located, as 

 has been described by holding nephelite of that composition at 

 higher and higher temperatures until evidence of inversion to 

 carnegieite was obtained. Any point on the curve E D rep- 

 resents the temperature of the upper limit of the inversion in- 

 terval for the corresponding composition. Only the end points 

 E and D are accurately located and the middle portion qualita- 

 tively indicated (Table II (f) and (1)). 



The concentration of CaAl 2 Si 2 8 in the carnegieite solid 

 solution (E D) is much less than that in the nephelite solid solu- 

 tion (E C) in equilibrium with it at any temperature. Beckman 3 

 has developed an equation for the amount of change in the in- 

 version (or melting) point of a substance by the solution in it 



02T 2 

 of another substance. It reads A T = —y- (C 2 — O a ) where A T = 



change in inversion temperature, T= absolute temperature of 

 the inversion point in the pure substance, Z=its heat of trans- 

 formation, C 1 = concentration of the solution above the inver- 



