l82 



N. L. BOWEN 



whatever the conditions, the eutectic is the goal of all liquids; all 

 attain it; none pass it; it is a "fen of stagnant waters." 



OF THE REACTION PAIR AND DISCONTINUOUS REACTION SERIES 



The kind of reaction relation introduced by the existence of 

 a soKd solution series is not the only kind that is of importance 

 in crystallizing magmas. Another type of crystalKzation phenom- 

 enon, ordinarily to be thought of as quite distinct from solid 



solution, has, nevertheless, 

 consequences of a similar 

 nature. This t)rpe is illus- 

 trated in several systems 

 that have been investi- 

 gated experimentally, but 

 for our present purpose 

 we shall discuss only cases 

 showing phases closely 

 related to rock minerals. 

 The equilibrium 

 diagram of the system 

 MgO-SiOa is shown in 

 Figure 3. A liquid of 

 composition 42 per cent MgO, 58 per cent Si02 begins to crystallize 

 with separation of olivine Mg2Si04 and this continues until 1557° 

 when the olivine reacts with the hquid to form the pyroxene clino- 

 enstatite. If the opportunity for reaction^ is perfect the liquid is 

 completely used up by the reaction and the mass consists entirely 

 of oHvine and pyroxene. If, on the contrary, liquid and oHvine are 

 not free to continue in intimate contact for any reason, then upon 

 completion of all reaction possible under the circumstances, some 

 Hquid will be left over. This will proceed to crystalKze in the 



' The phrase, opportunity for reaction, is used throughout this paper without 

 explanation. It is considered that the various factors involved have been sufficiently- 

 discussed elsewhere. The removal of crystals from a part of the liquid by their 

 sinking is a factor limiting the opportunity for reaction as far as that part of the liquid 

 is concerned. And so it is with the formation of a reaction armor about crystals or 

 the squeezing out of liquid from a crystal mesh. Usually the rate of cooling is the 

 fundamental control over these factors and therefore over the opportunity for reaction. 



WT. PER CENT 



Fig. 3. — Equilibrium diagram of the system, 

 forsterite-silica. 



