GEOPHYSICAL LABORATORY. 135 



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(416) Genetic features of alnoitic rocks frora Isle Cadieux, Quebec. N. L. Bowen. Am. 

 J. Sci., 3, 1-34. 1922. 



An occurrence of alnoitic rocks at Isle Cadieux near Montreal, Canada, is 

 found to consist principally of monticellite alnoite, a newly recognized (not a 

 new) rock type. Besides this type there is a variety, consisting almost exclu- 

 sively of melilite and biotite, which is apparently new. 



A study of the relations of the minerals indicates that the rock originally 

 consisted of augite and chrysolite and was nearly completely consolidated as 

 such. These minerals were then attacked, with lowering temperature, prob- 

 ably by their own interstitial liquid as it changed in composition, and they 

 were replaced by monticellite, melilite, and biotite, with marialite, perovskite, 

 and titaniferous magnetite as minor products of the reaction. The monticel- 

 lite is itself replaced by melilite and biotite, and the melilite-biotite rock is the 

 end-product of the replacement. This replacement was accomplished by an 

 alkalic liquid (magma) which formed monticellite from augite by desilicating 

 it, and later gave rise to melilite and the more definitely alkalic mineral 

 biotite. 



The melilites are both positive and negative and are sometimes conspicu- 

 ously zoned, with a positive core, an isotropic intermediate zone, and a nega- 

 tive rim. 



In an experimental part of the paper, equilibrium in mixtures of nephelite 

 and diopside is determined. It is found that, from intermediate mixtures, 

 forsterite and melilite are the first products to crystallize, and the melilites so 

 formed are analogous to natural melilites in composition and optical proper- 

 ties. 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 desihcation of the diopside; and while the formation of 

 monticellite in this manner has not been demonstrated in the relatively simple 

 experimental mixtures, the demonstration of desilication 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, replacing (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 

 principal 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. Reexamination of the original 

 alnoite shows its presence there with apparently a similar relationship. Even 

 the pure lime olivine itself (iSCa2Si04) has been found in 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 (limestones, etc.). 



