54 sosman: metasilicate minerals and rocks 



it is too small to be of practical importance, being of the same 

 order of magnitude as the effect of small changes in frequency. 

 The null methods now in use for the accurate determination 

 of ratio and phase angle all give theoretically currect results, well 

 within the experimental error, so that the accuracy attainable 

 is decidedly greater than is actually required in practice. 



GEOCHEMISTRY. — Minerals and rocks of the composition 

 MgSi0 3 -CaSi0 3 -FeSi03. Robert B. Sosman, Geophys- 

 ical Laboratory. Communicated by A. L. Day. 



This paper is a partial report on a study, from the statistical 

 standpoint, of certain rocks and minerals having a composition 

 corresponding to a mixture of the metasilicates of magnesia, 

 lime, and ferrous iron. The method of treatment is already 

 familiar to mineralogists and petrologists through the publica- 

 tions of Vogt. 



Although the studies of Vogt have indicated relationships of 

 great importance in the understanding of rocks, they are open to 

 the criticism that the systems treated are too complicated to 

 yield anything more than indications. It should be remembered 

 that the existence of a 2-component or a 3-component system 

 depends upon the properties of the chemical substances entering 

 into it, and upon the range of temperatures and pressures under 

 consideration, and not simply upon the fact that it is made up of 

 two or of three distinct minerals. To treat a mixture of diopside 

 and olivine, for example, as a 2-component system may serve to 

 conceal as many relationships as it brings out. This mixture 

 can be completely discussed only as a part of the 3-component 

 system Si0 2 -MgO-CaO. 



There are on record analyses of a few rocks and of a large 

 number of minerals that are practically completely made up of 

 the three metasilicates, MgSi0 3 , CaSi0 3 , and FeSi0 3 , with almost 

 no alumina, ferric iron, or other oxides. These analyses, in com- 

 bination with the experimental data of this laboratory on the sys- 

 tem MgSi0 3 -CaSi0 3 form the material for the accompanying 

 3-component diagram, fig. 1. The evidence is not complete that 

 these three can be treated as a 3-component system. But the 



