Andersen — System Anorthite- For sterile- Silica. 449 



tion at N. As in this case no crystals were removed, the final 

 solid would consist of anorthite, clinoenstatite, forsterite and 

 tridymite. It is obvious that this mixture does not represent 

 a stable system, as the solid formed during a normal cooling, 

 when equilibrium obtained and all reactions were completed, 

 would consist of anorthite, clinoenstatite and forsterite (and no 

 silica). Theoretically, therefore, a reaction should take place 

 in this mixture in such a way that all the silica disappeared 

 and combined with part of the forsterite to form clinoenstatite. 

 The rate of this reaction, however, would in the solid state be 

 infinitely low, and we may, therefore, say that the four com- 

 pounds mentioned practically represent a stable mixture under 

 the conditions considered. 



We may consider next some mixtures whose normal crystalli- 

 zation curves pass the quintuple point M, but not the bound- 

 ary curve F M, viz. the mixtures within the section A K D 

 M. In a mixture on the forsterite side of the conjugation 

 line the normal course of crystallization would be : First, 

 separation of one of the phases anorthite or forsterite, then 

 simultaneous crystallization of both along the boundary 

 curve K M, and finally, reaction at M with partial dissolution 

 of forsterite and crystallization of clinoenstatite and anorthite. 

 The part of this crystallization which takes place before M is 

 reached would not be influenced by any removal or protection 

 of the solid phases. If, however, all forsterite were removed 

 before M was reached, or the first reaction at JVI resulted in 

 the formation of protecting coatings round the forsterite crys- 

 tals, or if the latter crystals had been imbedded in anorthite 

 during the crystallization along K M, then the crystallization 

 would not end at M as in the normal course, but would pro- 

 ceed along M N and end at the ternary eutectic N. In this 

 case we would again have a mixture of anorthite, clinoensta- 

 tite, forsterite and tridymite, which could remain unchanged 

 for an indefinite time, although it did not represent equilibrium. 



VI. Some Applications of the Results to Petrologic 



Problems. 



General remarks. 



Earlier writers have already intimated how these physico- 

 chemical principles may afford simple explanations of certain 

 textural features of igneous rocks. The phenomena of mag- 

 matic resorption and recurrent crystallization have received 

 particular attention. 



J. H. L. Vogt* has explained magmatic resorption as an 

 effect of undercooling, whereas the recurrent and also the so- 



*Tsch. Min. petr. Mitt., xxiv, 453, 1905. 



