MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 625 



As an argument for the primary crystallization of olivine with 

 following resorbtion of olivine under new-formation of orthopy- 

 roxene, several investigators have referred to the well-known kely- 

 phitic or coronation zones around olivine, with hypersthene in the 

 inner zone facing the olivine. As stated above (Vol. XXIX, pp. 645- 

 49) we are dealing here, however, with a reaction in the solid face be- 

 tween olivine and plagioclase at a maximum temperature of about 

 1 250°-! 200° and generally somewhat lower — thus at least 300° lower 

 than the temperature (1557°) at which, under a pressure of one 

 atmosphere, olivine may crystallize from a melt of MgSiOs. The 

 kelyphitic hypersthene zone next to the olivine may thus have 

 nothing to do with the primary segregation of olivine from a melt of 

 MgSiOs and the later transformation from olivine to orthopyroxene. 



As maintained by Andersen {loc. cit. [1915], p. 453) the olivine, 

 as a consequence of the splitting of MgSiOj, in the orthopyroxene- 

 bearing igneous rocks, should in every case have crystallized earlier 

 than the orthopyroxene, and the crystallization of the olivine should 

 have been finished even before the beginning of the solidification 

 of the orthopyroxene. But petrographic investigation of rocks 

 rich in orthopyroxene but poor in olivine, shows that this is not so. 

 We refer to the facts mentioned above (Fig. 25). 



The conclusion from this is that bronzite and hypersthene, under 

 high pressure, in the common deep-seated magmas (norites, etc.), 

 crystallized directly from the magma in the same way as the other 

 common silicate minerals. How it may be with enstatite in an 

 almost pure MgSiOj magma, remains, however, an open question. 

 In the igneous rocks, indeed, we never find orthopyroxene with less 

 than ca. 7 per cent FeSiOj (or more than 93 per cent MgSi03). 



C 



1. Many minerals, such as olivine, monoclinic pyroxene, the 

 feldspars, spinel, magnetite, etc., may be formed in melts at a 

 pressure of one atmosphere, as well as in effusives and in deep- 

 seated rocks — consequently at low, as well as at middle high, and 

 very high pressure. 



2. Some minerals, as melilite for instance, which crystallize 

 out of melts at a pressure of one atmosphere, occur, moreover, in 

 effusive rocks (melilite basalts, etc.) and in certain dike rocks (as 



