144 MOLECULAR GROUPING 



axes of symmetry and one plane of symmetry, all at right 

 angles to the quadruple axis. Example : tinstone (SnO 2 ). 



V. Trigonal system (triple axis of symmetry). 

 i . Hemimorphic tetartohedry. Example : sodium per- 

 iodate (NaIO 4 . 3H 2 O). 



2. Rhombohedric tetartohedry: the triple axis is also 

 the sextuple axis of compound symmetry. Example : 

 dioptase (CuH 2 SiO 4 ). 



3. Trapezohedric tetartohedry: three double axes in a 

 plane at right angles to the triple axis. Example : quartz 



4. Bipyramidal tetartohedry: a plane of symmetry at 

 right angles to the triple axis. Not yet actually observed. 



5. Hemimorphic hernihedry : three planes of symmetry, 

 cutting one another in the triple axis. Example : tourmaline. 



6. Rhombohedric hemihedry : besides the foregoing, 

 three double axes of symmetry in the plane at right angles 

 to the triple axis. Example : corundum (A1 2 O 3 ). 



7. Bipyramidal hemihedry : like No. 5, but in addition 

 a plane of symmetry and six double axes of symmetry, 

 all at right angles to the triple axis. Not yet actually 

 observed. 



VI. Hexagonal system (sextuple axis of symmetry). 



1. Hemimorphic hemihedry. Example: nepheline 

 (Na s Al 8 Si 9 34 ). 



2. Trapezohedric hemihedry : six double axes in a plane 

 at right angles to the sextuple axis. Example : antimonyl- 

 baric tartrate 4- potassium nitrate ((C 4 H 4 O G SbO) 2 Ba . KN0 3 ). 



3. Pyramidal hemihedry : one plane of symmetry at 

 right angles to the sextuple axis. Example : chlorapatite 

 (Ca 5 Cl(P0 4 ) 3 ). 



4. Hexagonal hemimorphy : six planes of symmetry 

 cutting one another in the sextuple axis. Example : silver 

 iodide (Agl). 



5. Holohedry : like No. 4, but in addition one plane of sym- 

 metry and six double axes of symmetry, all at right angles 

 to the sextuple axis. Example : beryl ( AL,Be 3 (SiO 3 ) ). 



