Alnoitic Rocks at Isle Cadieux, Que. 21 



also minute stringers of glass. As more nephelite is 

 added the forsterite and glass increase somewhat and the 

 birefringence of the melilite further decreases. When 25 

 per cent nephelite is added the melilite is sensibly iso- 

 tropic and its index about 1.630, and further addition of 

 nephelite produces negative melilite. The series of nieli- 

 lites is evidently strictly analogous to the akermanite: 

 sarcolite : soda-sarcolite series of Buddington (p. 56). 

 An equation can be written showing the formation of these 

 molecules by reaction of akermanite and nephelite (the 

 akermanite being in excess) with the formation of some 

 forsterite and a more siliceous alkalic liquid that remains 

 as glass. The equation is as follows : 



nephelite akermanite sarcolite 



15(NaAlSi0 4 ) + 6(2CaO.Mg0.2SiO a ) = 4(3CaO.Al 2 3 .3SiO a ) -f 

 soda-sarcolite olivine 



2(3Na 2 O.Al 2 3 .3Si0 2 ) + 3Mg 2 Si0 4 + 3XaAlSi 2 O t 



The melilites formed in the nephelite-diopside mixtures 

 are exactly like those formed in the nephelite-akermanite 

 mixtures except that they do not show as great range of 

 composition. As noted, they may be either positive or 

 negative but are never far removed from the isotropic 

 member; in other words they never approach the pure 

 akermanite end member but appear to center around the 

 low melting mixture which Buddington has decided is 

 the case in natural melilite (p. 87). The reactions taking 

 place when nephelite is added to diopside may be pictured 

 by imagining that the first step is the production of for- 

 sterite and akermanite from diopside by desilication as 

 follows : 



4CaMgSi 2 6 + 3NaAlSi0 4 = 



2(2CaO.Mg0.2Si0 2 ) + Mg 2 Si0 4 + 3NaAlSi 2 6 



The akermanite then further reacts with nephelite after 

 the manner indicated in the first equation to produce the 

 other molecules of the typical melilites. The forsterite 

 and the melilites resulting are the first crystals precip- 

 itated from the intermediate mixtures. 



The correspondence with the melilites of the rocks 

 described is quite marked for it will be recalled that in the 

 portion of the rock where reaction with the alkalic liquid 

 has not proceded far the melilite is positive (akermanite- 

 rich) whereas in the biotite-melilite seams where reaction 



