The prefix “ortho” means perpendicular, or
with 90° angles, so the orthorhombic system keeps the 90º angles
between the a, b, and
c, but allows all the axis lengths to
vary:
lengths a ≠ b
≠ c, and
angles α =
β =
γ = 90º.
The resultant shape is relatively rare in minerals,
because objects with orthorhombic symmetry can have nothing higher than the
2-fold rotation axes along a,
b, and c
(Figure 9), but many minerals
crystallize in this system. Typical orthorhombic shapes include a cereal
box and a brick, assuming they have no writing on them. The formal
geometrical shapes are called rhombic prisms or rhombic dipyramids.
Examples of minerals that crystallize in this system are olivine and some
of the amphiboles and pyroxenes.
Figure 9.
Representations of the orthorhombic crystal system. For this crystal system a,
b, and c
can differ in length, while the interaxial angles are still constrained to be
90° (left): A 3-D perspective sketch showing the relationship between the three
crystallographic axes. (center): A solid composed by these three axes showing one
possible morphology in this crystal system. (right): A stereographic projection
showing the angular relationships between the three crystallographic axes.
