CD 

 O CD 



o 



Composition 

 of pyroxene 

 from III, 

 % diopside 



2 V 



Plane 



of 

 optic 

 axes 



33-5° 



54 



40°±3° 



//010 



31-0° 



41 



20°±3° 



//010 



29-7° 



35 



10°±3° 



//010 



28-5° 



30 



15°±3° 



J_010 



25-5° 



17 



25° ±3° 



J_010 



Diopside — Forsterite — Silica. 253 



Table IX. 

 Pyroxene Prepared by Growing in Liquid.* 

 I II III IV V VI 



Total Composition of Charge afore 1 



(Di 56 En 44) 97< SiO s 3# 1375° 

 " " " <■ 1385° 



u a u u 1392 



" " " " 1400° 



(Di 37 En 63) 95f c Si0 2 bfo 1410° 



the optic axial angle. The curve for (3 is given because it 

 presents graphically the reason for the observed behavior of 

 the optic axial angle, the reason being, of course, that the 

 values of /3 and a converge, become equal and then again 

 diverge. A change from b = /3 to b=a necessarily takes place 

 at the composition of the uniaxial pyroxene. 



Summary of Optical Properties of Pyroxene Series. — The 

 optical properties of the series of pyroxenes, diopside-clino- 

 enstatite, may now be summarized briefly. With increasing 

 MgSiOgthe extinction angle decreases continuously from 38*5° 

 to 22°, the mean refractive index falls from 1*676 to 1*654, 

 the birefraction decreases from *030 to *009 and the optic 

 axial angle, in the plane //010, grows smaller, passes through 

 zero and opens again in the plane J^OIO. 



The decreasing optic axial angle, refractive indices, bire- 

 fringence and extinction angle were all observed by Wright 

 and Larsen on those pyroxenes which were prepared formerly 

 at this laboratory, namely those extending from diopside to 

 about diopside 65 per cent, clino-enstatite 35 per cent.f In 

 the present work these changes have been followed beyond 

 this limit, in fact, to clino-enstatite itself. 



G-. Zinke,J in an investigation of the system, diopside- 

 MgSi0 3 , noted pyroxenes of small optic axial angle in some 

 of his products. He also found, for one of these, an extinc- 

 tion angle of 32°. The pyroxenes observed by Zinke were, 

 no doubt, some of the intermediate pyroxenes of the series, for 

 the optical properties noted are those which the present 



* In each case the initial material was glass and the time of holding at the 

 given temperature 2 hours. 



+ Allen, White, Wright and Larsen, this Journal (4), xxvii, 38, 1909. 

 tNeues Jahrb. fur Min. 1911, Bd. II, p. 132. 



Am. Jour. Sci.— Fourth Series, Vol. XXXVIII, No. 225.— Sept., 1914. 



18 



