Orthorhombic system; monoclinic mineral examples

Orthorhombic system

The orthorhombic system has three crystallographic axes, mutually perpendicular but 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 (parallel to the axes), 3 diad axes (along a, b, c) and 1 centre of symmetry. The crystals are prisms with rectangular or rhombic cross-section. Minerals: topaz, barite, olivine, sulphur, aragonite.

Three monoclinic minerals

The monoclinic system has $a \ne b \ne c$, with $\alpha = \gamma = 90^\circ$ and $\beta \ne 90^\circ$. Examples:

1. Gypsum
2. Orthoclase feldspar
3. Mica (muscovite or biotite). Hornblende and augite are also monoclinic.