152 A. L. DAY MINERAL RELATIONS FROM LABORATORY VIEWPOINT 



mination becomes a straightforward question which can be treated along 

 recognized lines. 



Other Minerals lower the Melting Temperature 



Most of us are familiar with the typical entectic diagram as it appears 

 in the text-books of physical chemistry and in the newer treatises upon 

 rock formation ( Vogt,® Iddings/^ Harker^^) . This diagram tells us plainly 

 that if to the component mineral (A) of simple and definite composition 

 with a definite melting point (a) we add a small quantity of another 

 mineral (B) (which forms no. appreciable solid solution^^ with it), this 



melting temperature will be meas- 

 urably lowered (to &), and a suffi- 

 ciently large addition will bring it 

 down to the lowest point of the curve 

 which, following the physical chem- 

 ists, we have come to call the entectic 

 (c). If our first mineral (A) is 

 pure wollastonite, and we add 5 per 

 cent of pure diopside (B), the melt- 

 ing point is lowered from 1510 de- 

 grees (pure wollastonite) to 1502 

 degrees. If more is added until the 

 mixture contains 60 per cent of 

 diopside and 40 per cent of wol- 

 lastonite, the mixture will crys- 

 tallize as a entectic (c) at 1348 

 degrees (see figure 7, page 172). 

 This is perhaps not the time nor the place for a long physico-chemical 

 discussion of the conditions of vapor tension in a solution which result in 

 the lowering of the melting point when one substance is mixed with an- 

 other and melted. It can be found, with numerous illustrations, in any 

 text-book of physical chemistry. The illustrations are drawn for the 

 most part from the study of aqueous solutions at low temperatures, but 



Concentration 



Figure 1. — Diagram shoiving Tempera- 

 tures of Melting and Solidification in a 

 Entectic Series 



»J. n. L. Vogt: Die Silikatschmelzosungen (2 vols., Christiania, 1903-'4) ; Physika- 

 lisch-chemische Gesetze der KrystaUisationsfolge in Eruptivgesteinen, Tscherm. Min. u. 

 Pet. Mitth., 24, 1905, pp. 437-542 : 25, 1906, pp. 361-412. 



^" J. P. Iddings : Igneous rocks, 1909. 



11 Alfred Harker : The natural history of igneous rocks, 1909. 



'^ Some minerals are ahle to take up a very limited quantity of another ingredient in 

 solution so intimately that the second invariably crystallizes out with the first like an 

 isomorphous pair (page 167), in which every crystal unit contains both minerals. Such 

 crystals are called "mix-crystals"' and such a solution after crystallization a "solid solu- 

 tion." When saturated in this way, its further behavior is independent and normal. 



