to Calcium and Magnesium Metasilicates. 31 



on the goniometer. The reflexion signals were not of the best 

 and the angles of the table can be considered only approxi- 

 mately correct, an error of ±15' being easily possible. 









Table VI* 







No. 



Letter 



Miller 



Symbol 







p 



1 



I 



010 



Ooo 



0°03 / 



90°00' 



2 



m 



110 



00 



40 08 



90 00 



3 



k 



Oil 



01 



01 



25 09 



From the angles the crystallographic constants can be cal- 

 culated : j? = 0*40 q = 0*47 

 or a : h : c = 1-19 : 1 : 0'47. 



The crystals are often tabular and prismatic in shape 

 after 010 as indicated in fig. 10. In other cases the prism 

 zone is less prominent and the crystals are of 

 eqnant development. The forms b, m, and Jc Fl &- 10. 

 were observed on all three crystals. Cleavage after 

 100, good. 



The plane of the optic axes is the cleavage plane 

 100 and the acute bisectrix is c. The optical orien- 

 tation is, therefore, a = ft ; b = c ; and optical char- 

 acter + . The refractive indices were determined 

 by the immersion method in refractive liquids ; a 

 = 1-641 ± -003 ; £ = 1-648 ±'003 ; 7 = l-663db'003 ; 

 ry_ a =-022; 7— £ = '015; 0-a = -OO7. The birefringence 

 7— /3 = "016 was furthermore measured directly on a tabular 

 crystal 0*182 mm thick by use of the Babinet compensator. 

 The optic axial angle is large and was measured with the 

 two-screw micrometer ocular on two sections showing an 

 optic axis in the field of vision. The average of the two 

 values thus obtained (2Y = 59*5° and 61*0°) is about 2Y 

 = 60-3° and 2E = 111°. The axial dispersion is fairly 

 strong, 2 Vp > 2Yv. 



* The angles in the table are the averages of the different values obtained. 

 These measurements were made on January 17, 1906, and since that time no 

 suitable crystals for goniometric measurement have again been observed. 

 Although the original notes stated definitely that these "crystal angles do 

 not coincide with those of enstatite " while "the optical relations do not 

 correspond with those of olivine," and this same form was observed at that 

 time in at least seven different preparations, its importance was not realized 

 and its presence was ascribed to impurity. In later experiments practically 

 no chilling was done and not until the thermal data indicated to Dr. A. L. 

 Day the presence of a high-temperature phase, enantiotropic to the first, 

 were quenching experiments again resumed and with them the true signi- 

 ficance of the a-MgSi0 3 became apparent. 



