Alnoitic Rocks at Isle Cadienx, Que. 19 



Since both components melt congruently there is, at 

 either end, a portion which behaves at higher tempera- 

 tures as an ordinary two-component mix but towards the 

 middle the relations are much more complicated. In 

 particular it is to be noted that the early crystals are 

 olivine and melilite, that is, nephelite reacts with diop- 

 side in such a way as to produce these very basic 

 molecules which are then precipitated. The separation 

 of these necessarily renders the liquid with which 

 they are in equilibrium correspondingly more siliceous. 

 Another important general feature is the very low 

 temperature at which there is still some liquid in the 

 intermediate mixtures. Thus at temperatures in the 

 neighborhood of 1000- some liquid remains and in many 

 mixtures it is impossible to tell at what temperature com- 

 plete solidification occurs, on account of the slowness 

 with which crystallization proceeds at such temperatures. 



The behavior of any particular mixture is given by the 

 diagram, but will be put in words for certain of these 

 that are of special significance. A mixture of 30 per cent 

 nephelite, 70 per cent diopside begins to crystallize at 

 1270° with separation of forsterite. This continues to 

 separate until a temperature of 12-10° is reached, when 

 diopside begins to separate and forsterite to redissolve. 

 The re-solution of forsterite continues until the tem- 

 perature has fallen to about 1180° and at about the same 

 temperature melilite begins to separate. At tem- 

 peratures below this and as far as 1100° the mass 

 consists of diopside, melilite and liquid. Below 1100 : 

 the crystals formed are so small and the attainment of 

 equilibrium so slow that further changes can not be 

 definitely described, the temperature of final consolidation 

 being also unknown. Xo nephelite is formed at any tem- 

 perature where the identification of phases is certain; 

 indeed the reaction which forms melilite presumably 

 destroys the nephelite and part of the diopside. 



A mixture of 50 per cent nephelite and 50 per cent 

 diopside begins to crystallize at 1220 2 with separation 

 of forsterite which continues down to about 1180°. At 

 this temperature melilite begins to crystallize, and below 

 this temperature the mass consists of melilite, forsterite 

 and liquid down to temperatures where the crystals 

 formed are no longer identifiable. Here again the tern- 



