In crystallography, the monoclinic crystal system is one of the 7 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 vectors are perpendicular (meet at right angles), while the third vector meets the other two at an angle other than 90°.
There is only one monoclinic Bravais lattice in two dimensions: the oblique lattice.
Two monoclinic Bravais lattices exist: the primitive monoclinic and the base-centered monoclinic lattices.
|Standard unit cell|
|Oblique rhombic prism
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; this is because the rectangular two-dimensional base layers can also be described with rhombic axes. In this axis setting, the primitive and base-centered lattices swap in centering type.
The monoclinic crystal system class names, examples, Schoenflies notation, Hermann–Mauguin notation, point groups, International Tables for Crystallography space group number, orbifold, type, and space groups are listed in the table below.
|#||Point group||Type||Example||Space groups|
|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|
Sphenoidal is also monoclinic hemimorphic; Domatic is also monoclinic hemihedral; Prismatic is also monoclinic normal.
The three monoclinic hemimorphic space groups are as follows:
The four monoclinic hemihedral space groups include