In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic system. They form a rectangular prism with a parallelogram as its base. Hence two pairs of vectors are perpendicular (meet at right angles), while the third pair makes an angle other than 90°.
Bravais lattices
Two-dimensional
There is only one monoclinic Bravais lattice in two dimensions: the oblique lattice.
| Bravais lattice | Oblique |
|---|---|
| Pearson symbol | mp |
| Standard unit cell |  |
Three-dimensional
Two monoclinic Bravais lattices exist: the primitive monoclinic and the base-centered monoclinic lattices.
Rectangular vs rhombic unit cells for the 2D base layers of the monoclinic lattice. The two lattices swap in centering type when the axis setting is changed.
| Bravais lattice | Primitive monoclinic |
Base-centered monoclinic |
|---|---|---|
| Pearson symbol | mP | mS |
| Standard unit cell |  |
 |
| Oblique rhombic prism unit cell |
 |
 |
In the monoclinic system there is a rarely used second choice of crystal axes that results in a unit cell with the shape of an oblique rhombic prism; it can be constructed because the rectangular two-dimensional base layer can also be described with rhombic axes. In this axis setting, the primitive and base-centered lattices swap in centering type.
Crystal classes
The table below organizes the space groups of the monoclinic crystal system by crystal class. It lists the International Tables for Crystallography space group numbers, followed by the crystal class name, its point group in Schoenflies notation, Hermann–Mauguin (international) notation, orbifold notation, and Coxeter notation, type descriptors, mineral examples, and the notation for the space groups.
| # | Point group | Type | Example | Space groups | |||||
|---|---|---|---|---|---|---|---|---|---|
| Name | Schön. | Intl | Orb. | Cox. | Primitive | Base-centered | |||
| 3–5 | Sphenoidal | C2 | 2 | 22 | + | enantiomorphic polar | halotrichite | P2, P21 | C2 |
| 6–9 | Domatic | Cs (C1h) | m | *11 | polar | hilgardite | Pm, Pc | Cm, Cc | |
| 10–12 | Prismatic | C2h | 2/m | 2* | centrosymmetric | gypsum | P2/m, P21/m | C2/m | |
| 13–15 | P2/c, P21/c | C2/c | |||||||
Sphenoidal is also monoclinic hemimorphic; Domatic is also monoclinic hemihedral; Prismatic is also monoclinic normal.
The three monoclinic hemimorphic space groups are as follows:
- a prism with as cross-section wallpaper group p2
- ditto with screw axes instead of axes
- ditto with screw axes as well as axes, parallel, in between; in this case an additional translation vector is one half of a translation vector in the base plane plus one half of a perpendicular vector between the base planes.
The four monoclinic hemihedral space groups include
- those with pure reflection at the base of the prism and halfway
- those with glide planes instead of pure reflection planes; the glide is one half of a translation vector in the base plane
- those with both in between each other; in this case an additional translation vector is this glide plus one half of a perpendicular vector between the base planes.