Ill A \(i()N.\l. SVS'I I \1 



97 



I )i hexagonal pri-m, Tihkio). 



ll< xa^oual prism first order, T(hohl)._ 



II.Aaiiniial pri-in second order, r(hh2ho). 



Hexagonal b:i>r. T(0001). 



Examples. - There has been as yet no mineral assigned to this 

 type ; iri fact, the trapezohedron has never been observed on any 

 crystal. There arc, however, several salts included here, as barium 

 stiluotartrate, Ba(C 4 H4O6) 2 SbO2, KNO 3 , which from the symmetry 

 of its etching figures must crystallize with an hexagonal holoaxial 

 symmetry. 



CLASS, RHOMBOHEDRAL TETARTOHEDRONS 

 TYPE 15, HEXAGONAL ALTERNATING 



Symmetry. Crystals of this type 



possess an alternating hexagonal axis, .- \ xao / \ 



the c axis, and a center of symmetry, / / a 



but no planes of symmetry, Fig. 182. 

 The forms of this type are tetarto- 

 hedrons, derived by superimposing 

 type 17 upon type 19, and extending 

 the faces not selected by these two 

 types to produce a new form. If the 

 dihexagonal pyramid is rolled out, 

 the faces being numbered ; the equa- 

 torial edge represented by a horizon- 

 tal line, and the terminations of the a axes marked by a vertical 

 line, as here represented: 



Oio 



X* 4 - 



FIG. 182. Type 15, Hexagonal 

 Alternating. 



Fio. 183. The Plus 

 Rhombohedron of the 

 Third Order, (hkil). 

 H 



If a line be drawn through those faces 

 which are extended by the method of selec- 

 tion used to produce forms of type 19, and 

 a line drawn under those selected in type 

 17, then it will be seen that there are three 

 faces above and three faces below the equa- 

 torial plane not marked. 



The faces 2, 6, and 10 do not lie sym- 

 metrically above the three faces, 16, 20, 

 and 24, neither do these faces lie symmet- 



