sosman: metasilicate minerals and rocks 57 



* 



This is the analysis marked p. Its two constituents, marked b 

 (bronzite )and d (diopside) should lie at opposite ends of a straight 

 line passing through p. The line passing through these three 

 points is almost exactly straight, the small deviation being simply 

 the error of analysis. 



Under conditions of complete equilibrium, the analyses of 

 minerals from igneous rocks of these compositions should all lie 

 along the border line of the "eutectic field." If equilibrium were 

 incomplete during solidification, the composition of the constit- 

 uent minerals might lie some distance away from this boundary, 

 as some of those plotted actually do. The diagram also includes 

 many secondary minerals, which may have almost any composi- 

 tion within the solid solution portion of the diagram. 



3. The mineral and rock relations just discussed agree excel- 

 lently with the experimental data of this laboratory on the system 

 MgSi0 3 -CaSi0 3 . Beginning with MgSi0 3 , Allen and White 

 find a solid solution of diopside in clino-enstatite extending to 

 about 2 or 3 molecular per cent CaSi0 3 . Mixtures with somewhat 

 more CaSiOs than this separate into two solid solutions, one of the 

 composition just given, and one having 30.5 molecular per cent 

 CaSi0 3 . The eutectic proportion of these two falls at the point 

 E (29 molecular per cent CaSi0 3 ). 



Corresponding to this 2-phase region we find the 2-phase pyrox- 

 enites and websterites. According to Williams, the bronzite has 

 crystallized first in the websterites which he investigated. This 

 fact would place the eutectic line close to the diopside side of the 

 field, agreeing excellently with the determination of its position 

 in the system MgSi0 3 -CaSi0 3 , as determined by Allen and White. 



Next comes the compound diopside, MgSi0 3 .CaSi0 3 , near 

 which we find grouped a large number of natural diopsides con- 

 taining varying amounts of FeSi0 3 , very few of them corre- 

 sponding exactly to the formula because of the range of solid 

 solution possible. On the CaSi0 3 side of diopside the solid solu- 

 tion is found to end at about 52 or 53 molecular per cent CaSi0 3 . 

 The mineral analyses end here also. 



No 2-phase rocks corresponding to the 2-phase region between 

 the two solid solutions of wollastonite in diopside, and diopside in 



