162 Watanabe — Bdbingtonite from Japan., 



The crystal habits and the zonal relations of the faces 

 are illustrated in the accompanying figures. 8 



Among the observed forms, both b and k are generally 

 well developed and give a prismatic habit to most crystals, 

 although some platy crystals are produced by the pre- 

 dominance of b. These two forms are usually character- 

 ized by vertical striations, caused by their mutual oscil- 

 latory combination. The forms a, h and g all appear as 

 narrow faces, which truncate the acute edges between b 

 and k. Among the terminal faces, c is best developed in 

 size, though it is striated parallel to the axia a, owing to 

 the oscillatory combination of c and d. All other forms 

 are represented as either narrow or minute faces. 



Physical Properties. 



The mineral is black in color and has a brilliant vitre- 

 ous luster. Crystals are commonly opaque, but in thin 

 sections transmit light and gives deep color. The pleo- 

 chroism is very marked, the axial colors being as follows : 



X deep emerald-green, 



Y purple-brown, 



Z , deep brown. 



In this particular point, the mineral is distinguished 

 from other triclinic members of the pyroxene group, 

 which have the similar crystal forms. The absorption 

 is strong in the order of X > Y > Z, and the dispersion 

 is marked. Hence, it is difficult to accurately determine 

 the extinction position. As the averages of repeated 

 measurements on several sections, the extinction angle 

 on 6(010) is given as about 37° from the vertical axis c, 

 and that on c(001) is about -5° or -6° from the axis a. 



8 In drawing these figures, a gnomonic projection of the crystal on the 

 plane normal to the prism zone was first made (fig. 1). Each edge in the 

 plans on this plane (figs. 2b and 3b) is normal to the zone line, which con- 

 nects the poles of the faces intersecting at that edge. Next, to make the 

 elevations on the plane of fc(010) (figs. 2c. and 3c), the intersection of this 

 plane with the plane of projection and its angle point were found on the 

 projection. In this particular case, they are shown as the line LL' and the 

 point B respectively. Each edge in the elevations is normal to the line con- 

 necting this point B and the intersection between the line LL' and the zone 

 line, which passes the poles of the two faces intersecting at that edge. Like- 

 wise, the line LL' and its other angle, point B', determine the directions of 

 the edges of elevations on the plane of b' (010) (figs. 2a and 3a). (See 

 Boeke: Die gnomonische Projektion, 1913, pp. 40-44.) 



