CRYSTALLINE SYSTEMS 143 



II. Monoclinic system. 



1. Hemimorphy: one double axis of symmetry. Ex- 

 ample: cane sugar (C 12 H 22 O n ). 



2. Hemihedry : one plane of symmetry. Example : 

 potassium tetrathionate (K.,S 4 O 6 ). 



3. Holohedry : one plane of symmetry and a double axis 

 of symmetry at right angles to it. Example : sulphur (S 8 ). 



III. Rhombic system. 



1. Hemihedry: three double axes of symmetry at right 

 angles. Example: magnesium sulphate (MgS0 4 . 7H 2 O). 



2. Hemimorphy : one double axis and two planes parallel 

 to it and at right angles to one another. Example : struvite 

 (MgNH 4 P0 4 .6H 2 0). 



3. Holohedry : three planes of symmetry at right angles, 

 and three double axes of symmetry at right angles. 

 Example : sulphur (S 8 ). 



IV. Tetragonal system (one quadruple axis). 



1. Sphenoidic tetartohedry : one plane of compound 

 symmetry at right angles to the quadruple axis. Not 

 yet actually observed. 



2. Hemimorphic hemihedry. Example: wulfenite 

 (PbMo0 4 ). 



3. Sphenoidic hemihedry. As in class i ; in addition, 

 two double axes of symmetry at right angles to one another 

 in the plane of compound symmetry, and two planes of 

 symmetry which cut in the quadruple axis and bisect the 

 angles of the double axes. Example : (CuFeS 2 ). 



4. Trapezohedric hemihedry: four double axes in the 

 plane at right angles to the quadruple axis. Example : 

 nickel sulphate (NiSO 4 . 6H 2 O). 



5. Pyramidal hemihedry: one plane of symmetry at 

 right angles to the quadruple axis. Example: scheelite 

 (CaW0 4 ). 



6. Hemimorphic holohedry : four planes of symmetry 

 cutting one another in the quadruple axis. Example : silver 

 fluoride (AgF . H 2 O). 



7. Holohedry: in addition to the above, four double 



