Monoclinic crystal system
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In crystallography, the monoclinic crystal system is one of the seven lattice point groups. 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°.
Contents
Bravais lattices and point/space groups
Two monoclinic Bravais lattices exist: the primitive monoclinic and the centered monoclinic lattices, with layers with a rectangular and rhombic lattice, respectively.
| Name | Primitive | Base-centered |
|---|---|---|
| Pearson symbol | mP | mC |
| Unit cell |  |
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Crystal classes
The monoclinic crystal system class names, examples, Schönflies notation, Hermann-Mauguin notation, point groups, International Tables for Crystallography space group number,[1] orbifold, type, and space groups are listed in the table below.
| # | Point group | Type (Example) |
Space groups | ||||
|---|---|---|---|---|---|---|---|
| Name | Schoenflies notation (Schön.) | Hermann–Mauguin notation (Intl) | orbifold (Orb.) | Coxeter notation (Cox.) | |||
| 3–5 | Sphenoidal [2] | C2 | 2 | 22 | [2]+ | enantiomorphic polar (halotrichite) |
P2, P21 C2 |
| 6–9 | Domatic [2] | C1h (=C1v = Cs) | 2 = m | *11 | [ ] | polar (hilgardite) |
Pm, Pc Cm, Cc |
| 10–15 | Prismatic [2] | C2h | 2/m | 2* | [2,2+] | centrosymmetric (gypsum) |
P2/m, P21/m, C2/m 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.
Specific chemical examples
An example of a monoclinic crystal is elemental sulfur (which can also occur in a rhombic form).[3]
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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The 7 crystal systems
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