Dihexagonal Alternating Type. '207 



age lines. Repeated attempts were not successful in etch 

 ing the unit prism, which appeared quite glassy and 

 rough, so that a 20-percent cold citric acid was tried, 

 which at first showed no trace of etching, and satisfactory 

 figures were obtained only after an immersion of 20 min- 

 utes (see fig. 3). They appear well scattered over the 

 face of the crystal ; the primitive and mature figures are 

 very much alike. The more distinct etchings reveal eight 

 sides, and six bounding faces. One end of the figure is 

 very narrow, almost a point, from which the four larger 

 faces extend outward and upward. The three grooves 

 formed by the intersection of these four larger faces 

 appear to meet in a common point, forming triangular 

 faces of the two central and lowermost faces, the apex of 

 which measures approximately 27° very near the angle of 

 the basal form of the HC1 figure and apex of the actual 

 scalenohedron face. The narrowest portion of the figure 

 is decidedly the deepest, similar to the HC1 figure. Also 

 the narrow end is occupied by a four-sided face, slightly 

 grooved vertically in the center, and dipping abruptly to 

 the pit of the figure. In outline the figures are fan- 

 shaped, and are symmetrical with regard to a vertical 

 plane. The figures of adjacent faces indicate by their 

 relative positions an alternating vertical axis, which is 

 characteristic of the type. Several of the figures possess 

 but six bounding lines but this only indicates an absence 

 of the two small lines at the apex, and does not alter the 

 shape of the figures to any very noticeable degree. 



Another solvent was prepared by mixing the HC1 and 

 HN0 3 , used for etching, in a 1:1 mixture. One of the 

 crystals was immersed for about 20 seconds, with fairly 

 good results. The figures produced upon the unit prism 

 1010 are very similar to the HNO M forms, except that they 

 are more elongated with the lateral boundaries less 

 curved. The apex of the triangular basal plane, as in the 

 HNO. { figures, measure between 12° to 14°. The primitive 

 forms are not all symmetrical, in fact many of them 

 appear hooked at the narrow end, which is due to unequal 

 development of the lateral planes. As development con- 

 tinues, however, the figures become symmetrical to a ver- 

 tical plane and reveal by their relative positions an alter- 

 nating vertical axis. 



On the whole, then, four different solvents were used 

 for the purpose of etching the unit prism, and four differ- 



