252 N. L. Boiven — The Ternary System: 



crystallized. In such a case it is often plainly discernible that 

 the resultant pyroxene crystals have a different composition in 

 their inner and outer portions. 



It was desirable to resort to this method of growing the 

 pyroxene crystals in a liquid, only in the case of the more 

 magnesian pyroxenes, thereby to increase the width of the 

 twinned bands and to facilitate optic axial angle measure- 

 ments. The width of the bands is sufficiently great in the 

 more calcic members, when formed by crystallizing glass of 

 the composition of the desired pyroxene. The composition of 

 the pyroxene is, of course, known absolutely, when prepared 

 by the latter method. When prepared by growing the crys- 

 tal in liquid the composition is known only as closely as it can 

 be determined from extinction angle measurements (within 5 

 per cent MgSi0 3 ). Other things being equal, the material 

 whose composition is certain is preferable, but, for the pur- 

 pose of studying the change in the plane of the optic axes, the 

 crystals whose composition is known only approximately have 

 the advantage already pointed out. 



Pyroxenes prepared by either one of these methods could 

 hardly be termed excellent material for optic axial angle* 

 measurement, but fortunately the change of angle is very 

 great and the fact of continuous variation between the two end 

 members readily fixed. 



Table VIII. 



Pyroxene Prepared by Crystallization of Glass. 



Composition 

 Diopside Clino-enstatite Plane of Optic Axes 2 V 



//010 59°±3° 



//010 56°±3° 



//010 47° ±6° 



//010 40°±6° 



//010 very small 



_L010 20°±6° 



JL010 53°±3° 



The composition of the pyroxene, which for monochro- 

 matic light is uniaxial, was placed between diopside 37 per 

 cent and diopside 19 per cent by using pyroxenes crystallized 

 from glass (Table VIII). By using pyroxene grown in a 

 liquid (Table IX) it was placed within the narrow limits, diop- 

 side 35 per cent and diopside 30 per cent. 



In ligure 23 the refractive indices for the various composi- 

 tions, as shown in Table VII, are plotted. The values for fi 

 are calculated approximately from the above determinations of 



100 





75 



25 



65 



35 



56 



44 



37 



63 



19 



81 





100 



