Tetragonal system & crystal forms

Common minerals : Zircon, Cassiterite, Rutile are the common minerals

• Crystallographic axis : a₁ = a₂  ≠  c , a₁ ^ a₂ ^ c = 90⁰
• Characteristic Symmetry elements :
  • Axis of Symmetry : Unique 4 fold symmetry or a 4 fold axis of 7 fold axis of inversion

Classes of the system

1. Normal class/ Zircon type (a : c = 1 : 0.64)
   • Plane of Symmetry : (5) 3-axial, 2-diagonal
   • Axis of Symmetry : (5) 1 of 4 fold & 4 of 2 fold
   • Centre of Symmetry : Present
2. Hemimorphic class/ Iodosuccinimide type (Artificial)
   • Plane of Symmetry : (4)
   • Axis of Symmetry : 1 axis of 4 fold
   • Centre of Symmetry : no
3. Tripyramidal class/ scheelite type (a : c =1 : 1.535)
   • Plane of Symmetry : (1)
   • Axis of Symmetry : 1 axis of 4 fold
   • Centre of Symmetry : Present
4. Pyramidal hemimorphic class/ Wulfenite type (a : c = 1 : 1.577)
   • Plane of Symmetry : no
   • Axis of Symmetry : 1 axis of 4 fold
   • Centre of Symmetry : no
5. Sphenoidal class/ Chalcopyrite type (a : c = 1 : 0.985)
   • Plane of Symmetry : (2)
   • Axis of Symmetry : 3 axis of 2 fold
   • Centre of Symmetry : no
6. Trapezohedral class/ Nickel sulphide type (Holoaxial class, Artificial)
   • Plane of Symmetry : no
   • Axis of Symmetry : (5) 1 of 4 fold & 4 of 2 fold
   • Centre of Symmetry : no
7. Tetartohedral class/ Ca, Al, Silicate (Artificial)
   • Plane of Symmetry: no
   • Axis of Symmetry : 1 axis of 2 fold
   • Centre of Symmetry : no

Unit forms of Normal class :

Form	No. of faces	Miller Indices
Basal Pinacoid	2	001
Tetragonal prism of 2nd order	4	100
Tetragonal prism of 1st order	4	110
Ditetragonal prism	8	hko
Tetragonal pyramid of 2nd order	8	101
Tetragonal pyramid of 1st order	8	111
Ditetragonal pyramid	16	hkl (123)