MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 235 



Thin at 1400°. — Silicate melts with 



SiO^ CaO MgO AI3O3 FeO, Mno 



50 25-30 15-20 2-5 1-3 



Further, melted diopside 



SiO^ CaO MgO 



55 26 19 



But when SiOa amounts to 60-65 P^^^ cent, as 



SiO. CaO MgO ALO3 



60-65 20-25 about 10 about 5 



the molten masses are rather viscous. And with still more SiOa 

 and decreasing CaO + MgO, the degree of viscosity is considerably 

 increased so that with about 75-80 per cent SiOz, even at tempera- 

 tures as high as 1500°-! 600°, a very considerable viscosity is 

 involved. As to pure SiOa, see below. 



Al^O^'-bearing silicate melts. — ^A few per cent of AI2O3 seem to 

 have no traceable influence on the viscosity of basic silicate melts 

 with predominant CaO, MgO, FeO, etc. Basic slags with somewhat 

 higher percentages of AI2O3, as 



SiO^ ALO3 CaO MgO 



30-35 15-25 30-40 5-10 



wherein meHlite-gehlenite (with spinell, etc.) crystallizes to a 

 considerable extent, seem throughout to be rather thin at a tempera- 

 ture about 1400°. (As to anorthite see below.) 



If we pass to somewhat more acid slags, we find that AI2O3 

 induces considerable viscosity. Thus slags, as e.g., the Mansf elder 

 slags with 



SiOa AW3 CaO FeO K^O 



46-50 16-18 16-22 4-6 4 



at a temperature as high as i5oo°-i55o°, are fairly thin but become 

 very viscous even at about 1300°, some 100° above the beginning 

 of crystallization. 



