56 sosman: metasilicate minerals and rocks 



ages of the constituent oxides, Si0 2 , MgO, CaO, FeO, A1 2 3 , etc. 

 Only those analyses were used in which the sum of Si0 2 , MgO, 

 CaO, and FeO exceeded 94 molecular per cent, and in the major- 

 ity of cases this sum exceeds 97 per cent. The analyses were 

 then recalculated to a basis of 100 per cent metasilicates, and plotted 

 in the diagram. The black dots represent minerals having a 

 normal metasilicate composition. Those having a slight excess 

 of Si0 2 (2 to 4 molecular per cent) are represented by triangles, 

 and those with a slight deficiency of Si0 2 , by squares. The open 

 circles with crosses represent rock analyses. The rocks in ques- 

 tion all consist of two minerals. 



The following facts are brought out by the diagram: 



1 . The rock analyses and the mineral analyses fall into two 

 separate parts of the diagram and can be separated by a line. 

 This line probably represents the boundary between magma 

 compositions that will form homogeneous solid solutions, and mag- 

 mas that will separate into two different solid solutions between 

 which there is a eutectic. The rock analyses lie in the "eutectic 

 field," the minerals outside of it. 



To be consistent, such a diagram should represent conditions 

 of equilibrium throughout. This diagram is inconsistent in that 

 the mineral enstatite appears in the MgSi0 3 corner. Enstatite 

 has been shown to be an unstable form, next in order of stability 

 after clino-enstatite or /3-MgSi0 3 , which has rarely been found in 

 nature. It is to be expected, however, that the relations of solid 

 solution and eutectic proportion will not be greatly different for 

 this false equilibrium, which may indeed have been a true equi- 

 librium under the conditions of formation of these rocks. The 

 same may be true of the neighboring orthorhombic pyroxenes, 

 bronzite and hypersthehe. 



2. The two types of minerals of which these rocks are made 

 up are in each case the two solid solutions which appear on oppo- 

 site sides of the "eutectic field." On the upper or MgSi0 3 side 

 they are given the names enstatite, bronzite, and hypersthene; 

 on the CaSi0 3 side, diopside and diallage. 



In the case of one of these rocks, described and analyzed by 

 Williams, the composition of the constituent minerals is given. 



