Antimony with Mitbidium. 277 



This salt differs from all the others of the series, since, it is 

 the only one on which tetartohedrism has been observed. 

 Whether the others are really tetartohedral, but have not 

 shown it owing to the absence of highly modified forms, can- 

 not be told at present. Also the basal cleavage, which is so 

 prominent on all of the others, could not be detected, while 

 the one half rhombohedron e was only observed on this salt. 

 A basal section was prepared, which in convergent polarized 

 light showed a normal, uniaxial interference figure, the double 

 refraction being negative, like the corresponding arsenic com- 

 pound. 



SRbBr . 2ShB>\. — Crystals of 

 this salt were prepared up to 7 mm 

 in diameter. The habit is gene- 

 rally that of six-sided plates, fig. 2, 

 haying the forms e, 0001, O ; r, 

 1011, 1 and s, 0111, -1. In one Fi s- 2 - 

 crop a, r and m were devel- 

 oped with some of the rhom- 

 bohedral faces predominating | 7 / e c | ^ 



to such an extent that the crys- ^— ^ — j 



tals looked like prisms, repre- 

 sented in basal projection by Fig- s - 

 fig. 3. 





Measured. 



Calculated 



r*c, 1011 *0001 



*52° 21' 





r^z, 1011*0111 



48° 



47° 57' 



None of the crystals show normal optical properties. Crys- 

 tals like fig. 3 showed for the most part an extinction parallel 

 to the direction c-d, sometimes with twinned lamellae promi- 

 nent at one end. In convergent polarized light no interference 

 figure was observed normal to c but some of the crystals like 

 fig. 3 could be tilted up on a rhombohedron face and showed 

 an acute bisectrix nearly normal to r. The axial angle was 

 small and the dispersion strong, the optical axes being in the 

 plane a-b for green and normal to that for red light, the inter- 

 ference figure looking like that of brookite. 



3RbI . 2SbI r — This salt unlike the corresponding arsenic 

 compound was obtained in crystals of considerable size some 

 over 10 mm in diameter. When prepared with an excess of 

 Sbl 3 usually lozenge-shaped crystals were obtained shown in 

 basal projection in fig. 4. These were often grouped in twin 

 position, penetration twins being prominent with a rhombo- 

 hedron as twinning plane. From solutions containing an 

 excess of Rbl, the rhombohedral habit was observed, some- 

 times with a negative scalenohedron s. 18*9'17, — yV — -?-, bevel- 

 ling its pole edges, fig. 5. 



