to Calcium and Magnesium Metasilicates. 33 



a\h:G~ 1*0931: : 1 : 0*5891. Both the angular values and the 

 calculated erystallographic constants prove the close resem- 

 blance of the artificial diopside crystals to the natural mineral, 

 the differences observed being within the limits of error possible 

 with the quality of reflection signals _, 11 



obtained from the artificial crystals. 

 Except for the form X, which was observed 

 only once, the crystal habit is simple 

 and requires no comment. Twinning after 

 100 is common and one of the three 

 crystals measured was thus twinned, the 

 twinning plane dividing the crystals into 

 two nearly equal halves. Postsynthetic 

 twinning after 100, however, occurred only 

 rarely and is not characteristic. 



Prismatic cleavage after 110 is good ; in diopside crystallites 

 from the melt indications of a parting after a dome or basal 

 pinacoid face at an angle of about 66° with the prismatic 

 cleavage cracks were also recorded. 



The refractive indices were measured on polished plates of 

 the crystalline material on the total refractometer in sodium 

 light : 



y Na = 1-694 ± -002 ; /? Xa = 1-671 ± '002 ; a Na = 1*664 ± -002 

 y — a — -030 ; y — /3 = "023 ; f3-a = *007 



Direct determinations of the birefringence were also made in 

 the thin sections with the result : y — a — "030 ± "002 (average 

 of three measurements on good sections). The optic axial 

 angle was measured on a number of different sections both 

 with the universal stage and with the two-screw micrometer 

 ocular. The average of four good determinations is : 

 2Y = 59°*3 =t 1° ; 2E = 114°. The optical axial dispersion 

 is weak, 2V/o> 2Y V . The plane of the optic axes is the plane 

 of symmetry (010). 



Extinction angles were measured both on the clinopinacoid 

 and the prism face 110. On 010 c : c = - 38°'5 =b 1°. On 110 

 c : c = — 32°'9 ± 1°. The position of total extinction was 

 ascertained in each case by use of the new bi-quartz wedge 

 plate* and the values should be correct within 1°. Extinction 

 angles were also measured for different faces in the prism 

 zone. Suitable crystals, held in a specially constructed device, f 

 were immersed in a liquid of the refractive index /3 and the 

 extinction angle read for different angles of revolution of the 

 crystal from its position of zero extinction when the ortho- 

 pinacoid is normal to the axis of the microscope., 



* This Journal, xxvi, 377-379, 1908. f This Journal, xxvi, 388, 1908. 

 Am. Jour. Sci.— Fourth Series, Vol. XXVII, No. 157.— January, 1909. 



