Orthorhombic System

Common minerals : Andalusite, Sillimanite, Olivine, Enstatite etc.

• Crystallographic axis : a  ≠ b ≠  c , a ^ b ^ c = 90⁰
  • Where,
  • c = Vertical axis, b = Macro axis (longer) , a = Brachy axis (shorter)
• Characteristic Symmetry elements :
  • Axis of symmetry : 3 of 2 fold

Classes of the System

1. Normal class/ Barite type (a : b :c = 0.815 : 1 : 1.31)
   • Plane of Symmetry : (3)
   • Axis of Symmetry : 3 axis of 2 fold
   • Centre of Symmetry : Present
2. Hemimorphic class/ Calamine type (a : b : c = 0.78 : 1 : 0.48)
   • Plane of Symmetry : (2)
   • Axis of Symmetry : 1 axis of 2 fold
   • Centre of Symmetry : no
3. Spheroidal class/ Epsomite type (Holoaxial) (a : b : c = 0.99 : 1 : 0.57)
   • Plane of Symmetry : no
   • Axis of Symmetry : 3 axis of 2 fold
   • Centre of Symmetry : no

Unit forms of Normal class :

Form	No. of faces	Miller Indices
Basal Pinacoid	2	001
front Pinacoid ‘a’ Pinacoid Macro Pinacoid	2	100
Side Pinacoid ‘b’ Pinacoid brachy Pinacoid	2	010
Prism	4	110
Macro Dome	4	101
Brachy Dome	4	011
Pyramid	8	111