THE PROBLEM OF THE ANORTHOSITES loi 



of temporary minerals unstable during the later stages of the crys- 

 tallization of such a magma. 



Let us now make several calculations which will show that even 

 if we lay aside all the foregoing considerations and take Bowen's 

 viewpoint, it is impossible to expect such effective differentiation 

 as is assumed by Bowen from his experiments. Bo wen studied 

 the crystallization of a gabbroic magma consisting of 50 per cent 

 labradorite (AbiAui) and 50 per cent diopside. We will examine 

 a magma of nearly this composition but containing also magnetite, 

 as it usually does. 



Let us take two mixtures : 

 L 6 parts diopside: molecular weight 216X6 = 1296, or 41.3 per 



cent (in molecular percentages) . 



2 parts labradorite: 804X2 = 1608, or 51.2 per cent. 



I part magnetite: 232, or 7.4 per cent. 



(The sum of the molecular weights is 3 1 56 ; the total of molecular 

 percentages 99.9). 

 II. A similar mixture but containing less magnetite, namely, 



Per cent 



Diopside 42.3 



Labradorite 52 .4 



Magnetite 5 



We begin with the second mixture and examine two cases: 



a) If two-thirds of the enstatitic silicate of our magma splits into 

 olivine and free silica there will remain a pyroxene 2 (MgO • 3 CaO • 

 4 SiOa), and we will have olivine 2(2 MgO-Si02) + 2 SiOz. The 

 free sihca will be combined with the Fe304, i.e., to 2 FeO, and conse- 

 quently there will be no formation of quartz. 



h) If we assume the improbable case that the whole mass of 

 the magnesian metasilicate will split and the rest will be 6 CaSiOj 

 there will be 3 (2 MgO-Si02)+3 SiOa, and after the neutralization 

 of 2 SiOz by the ferrous oxide the remaining free silica can give 

 1. 9 1 per cent of quartz. 



We take now the first mixture (I). In the same conditions as 

 already stated, even if the whole mass of the magnesian metasiHcate 

 splits, there will be no free silica. 



