4 6 



INFRA-RED TRANSMISSION SPECTRA. 



PREHNITE (H 2 Ca 2 Al 2 (SiO 4 )s). 



(Stalactitic; light green, translucent, crystalline mass; = 0.78 mm. 



Curve a, fig. 32.) 



We have now to consider a series of hydrous silicates in which the 

 oxygen and hydrogen are supposed to exist as such in the molecule. It 

 will be noticed that the SiO 2 radical does not give sharp bands that are 

 in a fixed position e. g., at 2.9/x,. 



Prehnite has bands at 2.9, 4.15, 5.96, and 7.3/4, but none coincide 

 with the water bands. It so happens that in this mineral the bands 

 coincide very closely with those found in quartz. 



i 2 4- 5 6 



FIG. 32. Prehnite (a); Hydronephelite. 



(HNa 2 Al 2 (SiC>4) s +3H 2 O). 



(From Litchfield, Massachusetts. Massive dark-gray material ; subtranslucent in 



section of 0.12 mm. Curve a, fig. 32.) 



This substance is supposed to contain water of crystallization, but the 

 general outline of the transmission curve does not show it. Possibly it 

 is an hydroxide. The general appearance of the material did not appear 

 to warrant a further inquiry into this apparent exception to the rule. 

 There is a wide band at 2.9 ^. Water is expelled at a high temperature. 



PECTOUTE (HNaCa 2 (SiO 3 ) 3 ). 



(Translucent sections; / = i.o and 0.25 mm. Curves a and c, fig. 33. From Bergen 



Hill, New Jersey.) 



Curve a is for a monoclinic prismatic cleavage piece, while curve c is 

 for a mass of needles all of which came from the same piece. The two 



