Andersen— System Anorthite-Forsterite-Silica. 419 



The crystals are monoclinic, tabular after (100) or prismatic 

 after (110), always elongated after the oaxis. Poly synthetic 

 twinning after (100) is very characteristic. Good prismatic 

 cleavage after (110). 



Optical properties : y = 1*660 ; a = 1*651 ; 7 — a = 0*009 ; 

 2V = 53*5° : Extinction angle c : 7 = 22°. Plane of optic 

 axes normal to (010). 



Near the MgSiO, point of the anorthite-forsterite-silica sys- 

 tem, the crystals of clinoenstatite were comparatively large, 

 often with distinct faces. The general type was tabular after 

 (100), elongated after the c-axis, with (100) and (110) and some 

 " pyramid "* as prevailing forms. Prismatic crystals with 

 about equal development of (100), (010) and (110) were also 

 seen. Some rough measurements of the prism angle 110 : 110 

 could be made under the microscope on crystals of approxi- 

 mately vertical orientation. Average of 8 measurements (vary- 

 ing from 86*5° to 89°) gave 87*5° with the orientation of the 

 prism as usual in monoclinic pyroxenes (i. e., acute angle 

 bisected by the plane of symmetry). Cleavage cracks after 

 (110) were often seen. Sometimes some rough cracks after 

 (010) and (100) were also noticed. Very characteristic of the 

 clinoenstatite in most of the mixtures were some curved cracks 

 roughly perpendicular to the c-axis. These cracks were prob- 

 ably due to contraction during the cooling of the crystals. 



Some of the optical properties were determined on crystals 

 from different mixtures containing from 10 to 55 per cent 

 anorthite. Fairly good determinations of refractive indices 

 could be made only occasionally when crystals free from the 

 including glass were obtained. On mixtures containing 30, 40 

 and 55 per cent anorthite, the following determinations were 

 made, respectively : 



y = 1*663 ± 0*003, a = 1*655 ± 0*003 ; 

 7 = 1*660 ± 0*003, a = 1*655 ± 0*003 ; 

 y = 1*662 ± 0-003, a = 1*652 ± 0*003. 



Numerous rough determinations gave the approximate values 

 of the average refractive index from 1*65 to 1*66. The 

 extinction angle c : 7 was measured on carefully selected crys- 

 tals. Values from 21° to 23*5° were obtained on crystals from 

 various mixtures. Average of 8 good determinations gave 

 c: 7 = 22-1°. 



The crystals showed the usual polysynthetic twinning after 



( 10 °)- 



In the viscous mixtures near the ternary eutectic, anorthite- 

 clinoenstatite-silica the crystals became very small, and their 



* Actually some combination of monoclinic hemipyramids, which, owing 

 to the polysynthetic twinning, acquires the appearance of an orthohombic 

 pyramid. 



