104: Wright — Optical Study of 



a rule the crystals formed out of a melt are larger than those 

 produced by heating the silicate glass. 



Wollastonite. — Under the microscope the artificial wollas- 

 tonite resembles closely natural wollastonite and often pre- 

 sents its characteristic arrangement of divergent fibers. The 

 colorless laths are usually transparent and show perfect cleav- 

 age cracks parallel to their long direction. Twinning after a 

 face in the orthodiagonal zone was recognized on several of 

 the crystals. Two of the crystals formed from the melt uf 

 calcium metasilicate and calcium vanadate were of sufficient 

 size ('1 x "2 x -5 mm ) to permit goniometric measurement of 

 their faces.* Both crystals were elongated in the direction of 

 the axis of symmetry (b) and showed evidences of cleavage after 

 u (001) and c (100). On the first crystal, the forms u (001),f v 

 (101), t (101), a (102) and e (110) were observed ; on the second, 

 which was less perfect, the forms c (100), u (001), t (101) and a 

 (102) occurred, with two uncertain forms (302)? and (203)?. The 

 forms (001) and (101) were the best developed and gave sharp 

 reflexion signals. The faces of the other forms were smaller 

 and less satisfactory in their measurement. The accordance 

 of the observed interfacial angles with those of natural wollas- 

 tonite, however, was sufficiently close to prove their identity. 

 The following comparison of their polar angles shows differences 

 which are not greater than had been anticipated from the 

 inferior reflexion signals obtained. 



Artificial Wollastonite. Natural Wollastonite. 



stte] 



: Symbol. 



Miller. 



<P 



P 



<P 



p 



u 







001 



90°00' 



5°30'±5' 



90°00 / 



5°30' 



c 



ooO 



100 



u 



90 27 



a 



90 00 



e 



GO 



110 



43 10 



89 58 



43 39 



90 00 



V 



+ 10 



101 



90 00 



45 06 



90 00 



45 33 



a 



-|0 



102 



a 



19 54 



IC 



20 03 



t 



— 10 



101 



a 



39 29 



u 



39 35 



The greatest and least refractive indices were determined by 

 Schrceder van der Kolk's;}; method of refractive solutions to be 

 about 1'621 and 1*636. Birefringence, about - 015. Optic 

 axial plane, perpendicular to the cleavage lines. Optical char- 

 acter of principal zone, both positive and negative. Optical 

 character of mineral negative with optic axial angle in air, 

 2 E = 69°30' — 70°00 / , measured on several different prepara- 



* Goldschmidt's two-circled goniometer with reduction attachment was 

 used. 



f The crystallographic orientation of Goldschmidt's "Winkeltabelle," Ber- 

 lin, 1897, pp. 286-287, has been followed in the notation. 



% Kurze Anleitung zur Mikroskopischen Kiystallbestimmung, Wiesbaden, 

 1898. 



