a Xezc Mineral Species. 333 



Although this mineral is evidently a compound one from 

 the distinctness of the re-entering angles, yet this composition 

 is not seen when examined by polarised light, through the 

 feces perpendicular to the axis. This circumstance would of 

 itself have been sufficient to show that it has only one axis of 

 double refraction, but I determined this to be the case, by the 

 direct examination of the polarised rings. Its double refraction 

 is negative, like that of calcareous spar, and other obtuse 

 i-homboids, and though not great, yet the images may be easi- 

 ly separated. Its ordinary refraction is a little greater than 

 that of almond oil, and very nearly the same as that of Pri- 

 mitive Chabasie. 



I have sent a specimen, containing a few minute crystals 

 of this substance, to M. Berzelius for analysis, but I have not 

 yet received the results which he has obtained from them. 



It is not soluble in acids, nor does it gelatinise with them. 

 It whitens and intumesces with heat like Chabasie and Meso- 

 type, and, according to Mr Haidinger's observations, it yields 

 with salt of phosphorus a transparent globule, which contains 

 a skeleton of silica, and becomes opaque on cooling. 



For the following crystallographic observations I have been 

 indebted to Mr Haidinger. 



Rhombohedral. R = 79° 29'. 

 a = v/ 8.38. 



Simple forms. R — od (o) ; R — 1 (g) = 106° 4/ ; R 

 (PJ; §R+l(n) = 7Q°7'. 



Character of Combination. Rhombohedral. 



Combination. R — cc. R— 1. R. Fig. 4. of Plate 

 VIII. represents two individuals composed parallel to R — qd, 

 the individuals being continued beyond the face of composi- 

 tion, as in Chabasie. Inclination of o on g = 136° 1', of u 

 on P = 117° M, of o on n = 109° 13'. 



Cleavage, indistinct, parallel to R. Fracture imperfect 

 conchoidal. Surface, R — 1 and 11 streaked parallel to their 

 common edges of intersection. It — od uneven, and often 

 curved, so that the opposite faces are often inclined on each 

 other at an angle of 2° — 3°. 



Lustre vitreous. Colour white. Streak white. Semi- 

 transparent. 



Brittle. Hardness — 4 0. 



