Alnoitic Rocks at Isle Cadieux, Que. 33 



melilites so formed are analogous to natural melilites in 

 composition and optical properties. 



It is thus proved experimentally that nephelite reacts 

 with diopside to form melilite, a reaction analogous to 

 that which is considered to have taken place between 

 augite and alkalic liquid in the natural rocks. 



This reaction is of the nature of a desilication of the 

 diopside, and while the formation of monticellite in this 

 manner has not been demonstrated in the relatively sim- 

 ple experimental mixtures, the demonstration of desilica- 

 tion of diopside in these mixtures is believed to give 

 support to the idea that monticellite is so formed from 

 augite in the more complex natural mixture. 



An outstanding difference between the natural, replac- 

 ing (reacting) liquid and the artificial mixtures is the 

 presence of potash and water in the former. As a result 

 of this difference the reaction products instead of being 

 forsterite and melilite were, in the natural rock, biotite 

 and melilite with monticellite as an intermediate step. 

 In fact chrysolite, originally present, was itself a prin- 

 cipal source of biotite. The liquid, as modified, by the 

 reaction, passed on and possibly gave rise to analcite 

 dikes. 



It is probable that monticellite occurs fairly frequently 

 as an igneous rock mineral in rocks related to those 

 described. Re-examination of the original alnoite shows 

 its presence there with, apparently, a similar relationship. 

 Even the pure lime olivine itself (£Ca 2 Si0 4 ) has been 

 foundin a related rock from Tasmania, a fact to which 

 attention is here directed because it appears to have been 

 overlooked in general petrologic literature. 



Such lime-rich minerals, including also melilite and 

 the related minerals garnet, vesuvianite and others, may 

 be considered to have formed in alkalic magmas as the 

 result of desilication of the more normal cafemic 

 molecules (metasilicates). Their presence in alkalic 

 rocks may therefore be the result of normal equilibrium 

 processes in the magma and not of addition to the magma 

 of lime-rich rocks (limestone, etc.). 



Geophysical Laboratory, 



Carnegie Institution of Washington, 

 May, 1921. 



Am. Jour. Sol— Fifth Series, Vol. Ill, No. 13.— January, 1922. 

 3 



