MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 629 



for the nepheline syenite-pegmatite dikes, further in contact zones, 

 etc.), give rise to the foimation of helvine (and danalite). At a 

 XQoderately high pressure also some double-compounds of silicate 

 and NaCl or Na2S04 (sodalite, nosean and hauyn) crystallize. 

 But these minerals — ^and according to my experience this is also 

 true of scapolite — may not be reproduced by melting the respective 

 silicate with admixture of NaCl or Na2S04 at common low pressure. 



In this connection it may be further mentioned that the mag- 

 matic crystallization of minerals as FeSz and CaCOj imply a high 

 pressure. 



This review proves that the formation of many minerals, or 

 combinations of minerals, may take place at low pressure as well as 

 at moderately high or very high pressure, and further, that the 

 determinations undertaken at one atmosphere pressure in regard to the 

 eutectics and mix-crystal systems, so far as these minerals are concerned, 

 may also he made valid with only slight corrections at high pressures, 

 even at the high pressure prevailing during the crystallization of 

 the deap-seated magmas. 



But this may not be applied to all minerals, or combinations of 

 minerals. The formation of some minerals, as melilite and leucite, 

 is favored by low pressure; the formation of other minerals, as 

 garnet, is favored by high pressure. For some minerals, as 

 especially muscovite, further HaO-bearing biotite and hornblende, 

 a more or less high pressure may be an indispensable condition for 

 crystallization from the magma; indeed indirectly depending on 

 the fact that the presence of sufficient H2O in the magma implies a 

 high pressure. For pyrite and calcspar, which at one atmosphere 

 are dissociated by moderate heating, high pressure is necessary for 

 their crystallization from the magma. 



It is to be noted that garnet has a much higher density (Mg-Al2- 

 garnet 3.7-3.8, Ca-Al2-garnet 3.9-4.0, Ca-Fca-garnet 3.8-4.1, etc.) 

 than the combination of those minerals which result from re-melting 

 it at one atmosphere. On the other hand, leucite has a much lower 

 density (2.45-2.50) than the combination of those minerals, espe- 

 cially orthoclase (2.55) and the various KzO-rich micas (2.76-3.0) 

 which generally replace leucite in the deep-seated magmas. And 

 melilite (2.90-2.93) has somewhat lower density than the combina- 



