182 



Robinson — Octahedrite and Brookite. 



form. The faces are all of small size, yet the measurements 

 from five give an average error of but 1-J 7 . On one crystal 

 three out of the possible four forms were observed at one 

 extremity, on a second only two. 



2. Brookite. 



Five crystals of this mineral were examined and they are of 

 special interest because they possess a habit quite different 

 from that ordinarily observed. 



The habit of all the crystals is prismatic. Their develop- 

 ment on the a and b axes is well shown in the plans of figures 

 3-6. The length in the direction of the vertical axis is from 

 four to six times their greatest width. Being relatively so long 

 and slender, it did not seem best to represent them as doubly 

 terminated. 



5(a) 



4 fa) 



\--~ 





r\ 





\ 



m 





a 





rn- 



The unusual feature of these crystals is the prominent devel- 

 opment of the pyramid s (322). This form was first observed 

 by Kokscharow* as a very small face, and the only notice of its 

 having a prominence at all like that on the crystals from this 

 locality is in an article by Streuver, "Sulla Brookite di Buera 

 nelPOssola."f The habit of the crystals described by him is, 

 however, quite different in that it is short and tabular parallel 

 to a (100), the proportion between the width and thickness 

 being as 1 : -§-. 



*Vh. Min. Ges., St. Petersburg, 1848-49. - 

 \ Proc. R. Ace. d. Line. (4), vi, 71. 1890. 



