a new Cornish Mineral. 55 



metraJ ratios a : b : c= 1 : 0-4389 : 0-8932. The forms met with 

 on the different crystals examined arc 



a, 1 ; c, 1 ; m, 110; ft lllj cl ; 101; 



*, 2 01; #,201; Z, J 1 ; r, 1 1 2 ; p, 1 1 1. 



The face <j is a prominent face with vitreous lustre, and is 

 parallel to the only distinct cleavage. It presents fine stria- 

 tions parallel to its intersections with the faces q. The face a 

 is small, usually triangular, and remarkable for its lustre : the 

 faces of the form q are the most developed after the faces e, 

 often equally developed with the latter, and then imparting to 

 the crystal a rhombohedral aspect. They are furrowed with 

 striation-planes parallel to the edges they form with the clea- 

 vage-plane. 



The faces of the form m are brilliant but very narrow, repla- 

 cing the acute edges in which a face c meets a pair of the 

 faces q. They are somewhat rare. The faces of the form cl 

 truncating those edges of the faces q which lie in the plane of 

 symmetry are very small, but occur not unfrequently ; the 

 composite character of the crystal, however, renders a good 

 measurement of the angle ad very difficult. The faces I, t, 

 and p have been only once observed. 



Owing to the difficulty of taking accurate measurements 

 between the furrowed faces of the form q and the faces of the 

 forms c or a, a good measurement of the angle 111, 111, 

 involving also a fair determination of the angle 111, 101 on 

 one of the crystals, was employed for the third datum in deter- 

 mining the elements of the crystal, as given above — the other 

 two elements being the angles 1 0, 1 and 10 0, 110, as 

 given by two other crystals. Angles obtained from the mea- 

 surement of four crystals were as follows : — 



Calculated angles. 





Measured angles 



r C tc = 79 27 



79° 26i' 



| 79° 27' || 80° 2( 



ca' =100 33 







col = 46 23 



45° 56' 





da' = 54 9i 



54° 33 r 



| 54° 9' || 54° 3' 



ad =125 50i 



125° 2V 





at =26 31i 







tc = 52 55i 







eh = 69 2i 







ha! = 31 30i 



31° 37' 





-am = 65 55 \ 



65° 56' 



65° 55|' 



mmf-= 48 9 



48° 8' 





m / w = 131 51 



131° 52' 





