58 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 66. 



Fro. 6.— Epidote. Crtstallographic drawing 



IN CLINOGRAPHIC AND ORTHOGRAPHIC PROJECTION 

 OF EPIDOTE OF THE HOURGLASS TYPE. 



occur minute crystals of epidote of a peculiar type. These are also 

 found entangled in the asbestiform amphibole of a number of speci- 

 mens. These peculiar epidote 

 crystals were observed in all of the 

 specimens of this lot, but this is 

 not an isolated occurrence of the 

 mineral as shown by the fact that 

 numerous crystals of the same 

 type occur in specimens of preh- 

 nite collected in the quarry by 

 Doctors Merrill and Wherry some 

 eight years previously. 



The crystals are flattened on 

 the front pinacoid a (100) in the 

 orientation adopted and are 

 moderately elongated on the h 

 axis, the habit and development 

 being uniformly as shown in or- 

 thographic and clinographic projections in figure 6. The angles meas- 

 ured on two different crystals with the elongation vertical are com- 

 pared with the calculated angles for these forms in the following table. 

 The agreement is as 

 good as can be ex- 

 pected when the 

 minute size and thin- 

 ness of the crystals is 

 taken into considera- 

 tion. The dimen- 

 sions of the two meas- 

 ured were 0.25 mm. 

 long by 0.12 mm. 

 wide by 0.01 mm. 

 thick, this being the 

 average maximum 

 size. 



The drawing, fig- 

 ure 8, is an ortho- 

 graphic projection 

 on h (010) show- 

 ing the optical orientation of the flattened crystals. Their prin- 

 cipal peculiarity, optically, is an "hourglass" pattern which makes 

 them beautiful objects under the microscope between crossed nicols. 

 The appearance of the birefringence pattern is shown in the drawing 



Fig. 7. — Epidote. Sketch showing optical directions and hour 



GLASS structure. PROJECTION ON a (100). 



