139 



which are tabular parallel to the second pinacoid. The com- 

 bination-edges between the pinacoids in the longitudinal zone 

 are sometimes truncated by the prism of the first order x, 

 whose faces are ahvays narrow (Figs. 5, 9, Plate VII). The prism 

 (pinacoid) of the second order d occurs distinctly developed only 

 on a small number of crystals (Figs. 5, 7, 8, 9, Plate VII). Its 

 faces are almost always smooth and brilliant. Occasionally they 

 are tolerably large. The prism of the third order m occurs, 

 or is at least present as traces on every crystal of leuco- 

 sphenite; its faces are, however, in most cases very small and 

 indistinct (Figs. 4 — 6). Often they occur only as step-like 

 formations on the edge between n and d. The prism of the 

 fourth order p is a form of tolerably common occurrence, and 

 its faces are occasionally rather large. In the majority of 

 cases, however, Ihey are small (Fig. 6) and uneven with no 

 high lustre. The two forms g and r occur as extremely nar- 

 row truncations on the obtuse combination -edge between n 

 and с (Figs. 6, 7). Often the two forms occur together. The 

 faces are brilliant, but their edges are frequently irregularly 

 rounded off. 



The forms a and s have been observed only on a single 

 crystal. This crystal, which measures a little more than 1™" 

 in length and ^^з™"^ in thickness, is represented by Fig. 9. It 

 was found associated with minerals quite different to those 

 which generally accompany leucosphenite , and it also differs 

 in habit from other crystals of this mineral. Like them it 

 is prismatically elongated in the direction of the a-axis and 

 bounded in the zone of this axis by the second and the third 

 pinacoids, Avhich are almost equally well developed. Their com- 

 bination-edges are truncated by narrow faces belonging to the 

 form X. The crystal is developed only at one end and is there 

 colourless and perfectly clear. The other end, by which the 

 crystal has been attached, is opaque. As on this crystal the 

 prismatic form w is very subordinate, the wedge-like termination 



