# Monoclinic crystal system

In crystallography, the **monoclinic** crystal system is one of the 7 lattice point groups. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal length, as in the orthorhombic system. They form a rectangular prism with a parallelogram as base. Hence two pairs of vectors are perpendicular, while the third pair make an angle other than 90°.

There exist two monoclinic Bravais lattices: the simple monoclinic and the centered monoclinic lattices, with layers with a rectangular and rhombic lattice, respectively.

- Monoclinic.svg
Simple monoclinic

- Monoclinic-base-centered.svg
Centered monoclinic

The crystal classes that fall under this crystal system are listed below, followed by their representations in international notation and Schoenflies notation, and mineral examples.

Name | International | Schoenflies | Example |
---|---|---|---|

monoclinic normal | C
_{2h} |
gypsum, orthoclase, mica | |

monoclinic hemimorphic | C
_{2} |
halotrichite | |

monoclinic hemihedral | C
_{1h} |
hilgardite |

The number of space groups for each crystal class is 6, 3, and 4, respectively.

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.

## See also

## References

- Hurlbut, Cornelius S.; Klein, Cornelis, 1985,
*Manual of Mineralogy*, 20th ed., pp. 65 - 69, ISBN 0-471-80580-7

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