Orthorhombic system; isometric mineral examples

Orthorhombic system

The orthorhombic system has three crystallographic axes at right angles to each other, all of different lengths.

$$a \ne b \ne c, \qquad \alpha = \beta = \gamma = 90^\circ$$

        c
        |
        |_____ b
       /
      a        a, b, c unequal, all 90 deg

Symmetry of the normal class: 3 planes of symmetry, 3 diad axes and 1 centre of symmetry (7 elements). Typical forms are rhombic prisms and pyramids. Minerals: topaz, barite, olivine, sulphur, aragonite.

Three isometric minerals

The isometric (cubic) system has $a = b = c$, all axes at 90°. Examples:

1. Galena (PbS)
2. Pyrite (FeS₂)
3. Halite (NaCl); fluorite, garnet, magnetite and diamond are also isometric.