THE PHYSICAL CHEMISTRY OF THE CRYSTALLIZATION 



AND MAGMATIC DIFFERENTIATION OF 



IGNEOUS ROCKS 



J. H. L. VOGT 



Trondhjem, Norway 



VIII 



THE TEMPERATURE-INTERVAL OF THE CRYSTALLIZATION 

 OF THE IGNEOUS ROCKS 



We will take as the starting point for the calculations in this 

 chapter the determinations made at i atmosphere pressure (in 

 water-free melts) of the minerals, the eutectics, the mix-crystals, 

 the liquidus- and solidus-curves, etc. 



We wiU then have to make two corrections: (a) on account of 

 the effect of pressure, and (b) on account of the effect of light 

 volatile compounds dissolved in the magma (H2O, CO2, etc.). 



As explained in an earlier chapter^ the melting points of the 

 siHcate minerals increases with the pressure, but only in a relatively 

 insignificant degree. In flows, for instance, we have to deal with 

 only 2.5° or at most 10° C, and even in deep-seated rocks crystallized 

 at very great depths, the rise of the temperature may be only about 

 25 or 50° C. Hence the effect of pressure is practically out of the 

 question. But not so with the effect of H2O, CO2, etc., to which 

 we wiU return below. 



As typical examples of anchi-monomineralic rocks' we will choose 

 the anorthosites and the peridotites. The solidification of Abm Aun 

 (without any admixture of foreign materials), according to Bowen's 

 investigations, in water-free melts at i atmosphere pressure, takes 

 place at the following temperature-intervals, when we assume 



^This/owrwaZ/or 1922, p. 611-614. 



^ In a later paper I shall try to prove that these rocks must have crystallized 

 from ordinary, thoroughly melted magmas. I cannot agree with Bowen's exposition 

 of the anorthosites as formed in a kind of magma-gray with large quantities of plagio- 

 clase-crystals and in addition a magma in subordinate quantity. 



407 



