of the Gtnus Gypsum-halolde. SOS 



Sectile. Thin laminae are flexible in a direction nearly 

 perpendicular to the edges between o, n, and P. 



Hardness = 2.0 . . . 2.5, nearer the latter. The perfect 

 faces of cleavage are even below 2.0. since they may be scratch- 

 ed by rock-salt, particularly when applied in a direction cor- 

 responding to the elongation of the crystals. Sp. Gr. = 2.730, 

 in a number of detached crystals. 



2. Dialomous Gypsum-haloide. 

 Form prismatic. Fundamental form, a scalene four- 

 sided pyramid. P- 133° 35', 123° 59', 75° 35' Fig. 5. 



a : b : c : = l : v 4.02 : v 2.83. 



Simple forms. (Pr.+l) »(mj - 137°41, 61° 27', 137° 35' ; 



(F+l) t(K) = 126 46', 59° 32', 121° 37'. P + x (f)=100° 0' j 

 Pr (0) = 126" 58' ; Pr+x (d) ; Pr— 1 (g) = 146° 53' ; Pr+1 

 (h) = 80° 8' ; Pr+2 (i) - 45' 36' ; Pr+x {f). 



Combinations. 1. Pr — 1. Pr. Pr+1. r + x . Pro:. 

 Pr+oo . Fig. 6. i 



2. Pr— 1. Pr. Pr + 1 (P+l)*- Pr+2. (Pr+1)'. P + x. 

 Pr+co. Pr+x. Fig. 7. 



Cleavage, highly perfect, and easily obtained in the dircc- 



tion of Pr+x • Surface, Pr. smooth, Pr. co smooth, or faintly 

 streaked in a longitudinal direction. The measurement of 



u 



the terminal edge of Pr is on that account much more ac- 

 curate than that of P+x , the faces of which are rather irre- 

 gularly streaked parallel to the axes of the crystals. The ho- 

 rizontal prisms, belonging to the short diagonal, are almost 

 all rough, particularly Pr — 1, which is at the same time 

 slightly rounded. The pyramids are rounded, though 

 smooth. 



Lustre vitreous. Colour white. Streak white. Trans- 

 parent, in small crystals, tranluscent. Double refraction ob- 

 servable through c, and the opposite face of J", making an 

 angle of 40°. The two indites of refraction are about 1.62 

 and 1.67. The less refracted image disappears, when the 

 axis of the tourmaline is perpendicular to the ^h^ of the re- 

 fracting prism. 



