302 Mr Haidinger on Two Newly determined Species 



Art. XVII. — On Two Newly determined Species of the Genus 

 Gypsum-haloide of the System of Mohs. By William 

 Haidinger, Esq. F. R. S.E. Communicated by the Author. 



I. Hemi-prismatic Gypsum-haloide. 



r orm, hemiprismatic. Fundamental form, a scalene four- 

 sided pyramid. P = { \f° \Q , 129° 21', 97° 8'. . Inclina- 

 tion of the axis = 24° 56', in the plane of the short diagonal. 

 Plate VII, Fig. I. 



a : b : c : d = 2.15 : 2.24 : 1.49 : 1. 



Simple forms. | (Z) = 139° 17; P + co (/) = 117° 24 ; 



— £=!<«)= 141° 8' ; (P + oo )' (g) = 157° 5' ; — 2=1 ( ) 

 = 83°14'; Pr + co (P). 



p" r y p i y 



Combinations. 1. — ' "a - " P + co . Pr + oo . Fig. 2. 



2. -v — ^=-\ — ~i. P + co . (P + co y Pr + co Fig. 3. 



The crystals are lengthened in the direction of the edges 

 of combination between o, n, and P, and attached at one of 

 the extremities, in most cases several together, so as to form 

 stellated or divergent groups. On the disengaged termina- 

 tion of the crystals, one of the faces of f is enlarged, so as 

 generally to make the other entirely disappear. The appear- 

 ance of Fig. 4 is then produced. 



Cleavage, parallel to Pr -f- oo , highly perfect and easily ob- 



Pr — 



tained; traces in the direction of — 5- = 54° 55', and Pr + co , 

 very faint, fracture uneven. 



Surface, the prisms parallel to the axis streaked in the 



direction of that line ; Pr parallel to its intersections with 



■pr p r \ P— 1 



— Y' The faces of — - 2 _ and — — are deeply streaked 

 parallel to their common edges of- combination. 



Lustre vitreous. Pr + oc slightly inclining to pearly, 

 both upon faces of cleavage, and of crystallization. 



Colour white, inclining to yellowish. Streak white. Trans- 

 parent or translucent. Index of refraction nearly 1.6, mea- 

 sured through f and P ; no separation of images. 



