28 Dana and Wells — New mineral, Beryllonite. 



Hardness 5*5-6. Specific gravity = 2*845. Luster vitreous 

 and brilliant, but on a natural basal face {c) sometimes pearly. 

 Colorless to white or slightly yellowish when not perfectly 

 clear. Transparent. 



Optical characters. — The axes of elasticity coincide in position 

 with the crystallographic axes. The axial plane is parallel to « 

 and the acute bisectrix normal to c, so that a cleavage fragment 

 shows the axes on the border of the field of the polariscope. The 

 dispersion is small, /><>. The double refraction is negative, 

 in other words a is the bisectrix. Sections* cut normal to the 

 bisectrices gave the following for the axial angles : 



Also 



Also 2Y 2/ =67° 34'. 



A prism afforded by a crystal whose edge was parallel to the 

 axis a and whose faces were formed by the planes d (023) gave 

 tolerable values of two of the refractive indices ; the faces, 

 however, were not quite smooth, so that no very high degree 

 of accuracy can be claimed for them. The results for yellow 

 (Na) are : ft= 1*5580, and f= 1*5630. Another prism was ob- 

 tained having the same edge but the faces did not make quite 

 equal angles, as was intended, with the axis b, which should 

 have bisected the prismatic edge ; the values of ft are, there- 

 fore, fairly good, while those of y are somewhat too small. 



Another prism with an edge just parallel to the axis c gave 

 good values of the index a. 





Red (Li) 



Yellow (Na) 



Green (Tl) 



2 E 



120° 26' 



121° 1" 



121° 24' 



2H 

 a 



72° 35' 



72° 47' 



73° 01' 



2 H 



125° 13' 



124° 59' 



124° 30' 





Red (Li) 



Yellow (Na) 



Green (Tl) 



a 



1-5492 



1-5520 



1-5544 



P 



1-5550 



1-5579 



1-5604 



y 



1-5604 



1-5608 



1-5636 



It will be seen from these results that the refractive power 

 of the mineral is not especially high, varying but little from 

 that of quartz, which has co = 1*54:42, e = 1*5533 for Na. 



Etchings. — It has already been remarked that the crystalline 

 faces are almost always dull, and in some cases show natural 

 etching figures as the result of the action of some solvent upon 

 them. These figures have often great regularity and beauty 



* The sections and optical preparations used in our work were made for us very 

 satisfactorily by Mr. H. Hensoldt, School of Mines, Columbia College, New York 

 City. 



