SILICATES. 



19 



Topaz [(Al, F) 2 Si0 4 ]. 

 (Villa Rica Mines, Geraes, Brazil. Curves a and b, fig. 12.) 



The specimen from Brazil was a yellow rectangular prism with faces 

 1 by 2.5 cm. The natural face, which had a high polish, was examined. 

 The reflection curve a, fig. n, is low throughout the spectrum, except in 



the region of selective reflection, which extends from 10 to 11.5 // with 

 unresolved maxima at 10.05, 10.6, 10.9, and 11.3 /*. 



The white topaz examined, curve b, fig. n (in its highest part), is an 

 almost exact reproduction of the quartz curve, with the exception that the 

 latter is shifted to the longer wave-lengths. The white topaz, which was 

 ground and had a fairly high polish, has sharp maxima at 9.9, 10.45, an ^ 

 11 p. with a possible band at 12 \i. The band at 11 [i is in common with 

 that of several other silicates. 



Sodium Silicate (Na 2 Si0 3 ). 

 (Curve b, fig. 13.) 



The curve of liquid glass is similar to that of the glasses previously 



examined. This is one of the simplest obtainable chemical compounds of 



the silicates, and is further evidence that the silicon oxide radical is not so 



constant in its behavior toward heat-waves as was found in the C0 2 radical 



of the carbonates. 



Spodumene [LiA1(Si0 3 )2]. 



(From Pennington County, South Dakota. Curve a, fig. 13.) 



This mineral is of the same composition as kuntzite, the transmission 

 curve of which is given on a later page. The specimen examined was a 

 polished cleavage piece from a large gray crystal. The maxima of the 

 selective reflection bands occur at 9.18, 9.7, and 10.4 pt, respectively. 



