JV. L. Boiven— Crystallization of Magmas. 161 



Aet. XVI. — The Crystallization of Haplobasaltic, Haplodio- 

 ritic and Related Magmas ; by 1ST. L. Bowen. 



Introduction. 



Basalt may consist essentially of labradorite and monoclinic 

 pyroxene and, if the nature of the plagioclase is regarded as 

 the determining factor, a mixture of andesine and monoclinic 

 pyroxene may be called diorite, though commonly diorites 

 favor hornblende rather than pyroxene. In their essentials, 

 then, the mixtures of diopside with the plagioclases, whose 

 thermal behavior is treated in the present paper, may be 

 regarded as basaltic, dioritic and so forth, according to the 

 nature of the plagioclase. The mixtures are, however, basalt, 

 diorite and so forth reduced to their simplest form, for the 

 pyroxene is pure, theoretical diopside, and the plagioclase pure, 

 lime-soda feldspar free from potash. I have referred to the 

 mixtures, therefore, as haplobasaltic and haplodioritic,* pre- 

 ferring to emphasize in the title the petrologic aspect of the 

 investigation rather than its physico-chemical aspect. The arti- 

 ficial mixtures are, moreover, believed to be sufficiently close 

 to basaltic and other magmas to throw considerable light on 

 the crystallization of these natural mixtures. 



Though five oxides, SiO a , A1 2 3 , MgO, CaO and Na 2 0, enter 

 into the composition of the mixtures studied, yet all the phases 

 appearing can be expressed quantitatively in terms of the three 

 components albite, anorthite and diopside. From the phase- 

 rule point of view, then, the mixtures studied constitute the 

 ternary system : diopside-anorthite-albite. 



Method of Working. 



In studying equilibrium in the various mixtures the quench- 

 ing method was used exclusively. By this method of sudden 

 chilling the phases present at the measured furnace-tempera- 

 ture are fixed and examined under the microscope at leisure. 

 Liquid becomes a glass, crystals remain as such and exhibit 

 their distinctive properties, with the result that the phases 

 present at all desired temperatures can be readily determined. 

 Equilibrium is assured by making the time of holding at the 

 constant furnace-temperature sufficiently long. 



The quenching system was calibrated against the following 

 fixed points: gold-1062-5, Li 2 SiO -1201°, diopside-1391-5 6 ', 

 anorthite-1550°. 



In making up the mixtures the following ingredients were 

 used : Na 2 C0 3 dried at 300°, A1 2 3 purified by boiling with 



* From the Greek airlooq — simple. 



Am. Jour. Sci. — Fourth Series, Vol. XL, No. 236. — August, 1915. 

 11 



