168 A. L, DAY MINERAL RELATIONS FROM LABORATORY VIEWPOINT 



homogeneous mix-crystals of that composition. This was the case with 

 all the lime-soda feldspars crystallizing in the laboratory. Crystalliza- 

 tion did not begin until the molten mass had passed entirely through the 

 region in which the changes in concentration occur, and all the crystals 

 and the glass residue, if any was left, were of one concentration, corre- 

 sponding to that originally taken. We made no effort in this investiga- 

 tion to produce the banded structure in imitation of nature, on account of 

 the time required to do it and the fact that it would add but little to our 

 knowledge of the subject. All that is necessary is a furnace in which the 

 temperature can be held constant within the temperature region where 

 the changes of composition occur and sufficient patience to wait for the 

 results (with the intermediate feldspars, probably some weeks). 



Simple Mixtures — Summary 



These simple cases, very briefly stated, serve to illustrate the behavior 

 of cooling mixtures of two components. In the first case (the eutectic 

 series) one of the components first appears separately, followed by a mix- 

 ture of both in fixed proportions, called the eutectic; in the second (the 

 isomorphous series), the two components always appear together, the first 

 crystals to form being usually considerably richer in the higher melting 

 component, followed in gradually changing proportions by the lower melt- 

 ing mixtures. Both these cases may occur in the same series of mixtures. 

 For example, if the metasilicates of lime and magnesia be mixed together 

 in a number of proportions and studied after the manner outlined above, 

 it will be found that a compound (diopside) forms in the middle of the 

 series. On one side, this compound forms a simple eutectic series with 

 pseudo-wollastonite ; on the other, a partial series of mix-crystals with 

 the magnesian metasilicate. It is not uncommon to find two or three 

 compounds forming between two components, with an appropriate num- 

 ber of eutectics between. Lime and silica, for example, yield two com- 

 pounds, a metasilicate and an orthosilicate, and eutectics occur between 

 pure silica and the metasilicate, between the metasilicate and the orthosili- 

 cate, and finally between the orthosilicate and pure lime. In this last 

 series (orthosilicate-pure lime), not only both components, but the eutec- 

 tic itself, melt at temperatures above 2000 degrees. 



Molecular Composition at the Moment of Crystallization 



There is opportunity, in a series like this, to study a situation to which 

 Harker has called attention especially, namely, that the molecules of the 

 liquid magma are not the simple oxides (SiOg and CaO), but are silicates 

 of a composition appropriate to the percentages present in the solution 



