8 Morey & Bowen — Melting Potash Feldspar. 



leucite crystals and glass in this 360° temperature-inter- 

 val. When held at 1510° for 2 hours glass and small 

 crystals of leucite were obtained. When the heating was 

 continued at this temperature for 20 hours the leucite 

 crystals became larger and less numerous. They evi- 

 dently grew freely in equilibrium with the liquid. On 

 the other hand when the same material was held at 1535° 

 for only 2 hours the crystals completely disappeared. 

 There can, therefore, be no question that the persistence 

 of crystals at 1510° for 20 hours is the result of the fact 

 that they are in equilibrium with the liquid. Incidentally 

 it may be noted, too, that, for the attainment of equi- 

 librium, prolonged heating is unnecessary at the higher 

 temperatures (in the neighborhood of 1500°). 



Incongruent Melting. 



Definite crystalline compounds may melt in one of two 

 different ways. A compound of one class melts com- 

 pletely at a definite temperature, giving a liquid of the 

 same composition as the crystals, for which reason it is 

 said to melt congruently. The temperature at which this 

 occurs is a true melting point. A compound of the other 

 class forms, at a definite temperature, a liquid of different 

 composition from its own and at the same time one or 

 more new crystalline compounds. Such a compound is 

 said to melt incongruently or, sometimes, to melt with 

 decomposition. The temperature at which this occurs is 

 not a true melting point. Thus the compound CaAl 2 Si 2 O s 

 (anorthite) has a true or congruent melting point at 

 1550°. At that temperature crystals of the composition 

 CaAl 2 Si 2 8 are in equilibrium with a liquid of the same 

 composition. It constitutes, by itself, a one-component 

 system. 



The compound MgSi0 3 is a familiar example of a com- 

 pound of the other class. At 1557° the crystalline clino- 

 enstatite'(MgSi0 3 ) melts incongruently or breaks up into 

 liquid and another crystalline compound Mg 2 Si0 4 (for- 

 sterite). There is no temperature at which clinoenstatite 

 is in stable equilibrium with a liquid of its own composi- 

 tion. It, therefore, has no true melting point. The com- 

 pound MgSi0 3 does not constitute, by itself, a one-compo- 

 nent system but can be treated only as a part of a 



