420 Andersen — System Anorthite-Forsterite-Silica. 



properties could not be determined exactly. Twinning could 

 only be detected occasionally. Nevertheless the identification 

 was certain on the basis of the high refractive index and the 

 low double refraction. 



In general it appears that the clinoenstatite crystals of the 

 present system are identical with those formed from the pure 

 MgSi0 3 glass. There is evidently no change in the properties 

 following changes in the composition of the mixtures. The 

 principal optical properties, refractive indices and extinction 

 angle, for instance, seem to be the same through all the sys- 

 tem, within the limits of error of the measurements. 



Spinel. 



Artificial spinel* (MgAl 2 4 ) forms regular octahedral crystals 

 without distinct 5 cleavage. The melting point is about 2135 °,f 

 n = 1-723 ± 0-002. 



In the mixtures of the anorthite-forsterite-silica system, the 

 spinel crystals were generally very small though they often 

 formed sharp octahedrons. Sometimes they also occurred in 

 more irregular, elongated grains with crystal faces only indi- 

 cated. Cleavage cracks were never seen. The refractive index 

 was determined in one preparation, n — 1*715 d= 0*003. 



It was essential for the general consideration of the whole 

 system to decide conclusively whether the present phase was 

 spinel. Although the properties mentioned above in general 

 characterize spinel, some doubt might arise as to their adequacy 

 for microscopic discrimination between spinel and periclase 

 (MgO), so much the more as periclase, a priori, should be 

 considered a more probable phase than spinel. The habit 

 of the crystals is exactly the same. Both phases are isotropic, 

 and the difference in refractive index (spinel 1*723 ; periclase 

 1*734) is not much larger than the limits of error of rough 

 determinations under unfavorable conditions. 



I therefore found it desirable to get some information on the 

 microscopic appearance of periclase in silicate melts as like 

 those of the anorthite-forsterite-silica system as possible in 

 physical qualities (e. g., melting interval and viscosity), but of 

 such composition that the formation of spinel was excluded. 



Three mixtures were made up : 



60 CaSi0 3 + 40 MgO 



50 CaMgSi 2 6 4- 50 MgO 



35 Si0 2 65 MgO (i. e., Mg 2 Si0 4 + MgO). 



* E. S. Shepherd, G. A. Rankin, and F. E. Wright, this Journal (4), xxviii 

 293, 1909. 



fG. A. Rankin. Personal communication. 



