REALGAR
Symmetry
Isometric | Tetragonal | Hexagonal| Trigonal | Orthorhombic | Triclinic | Amorphous

# THE MONOCLINIC SYSTEM

SOME MONOCLINIC
MINERALS

BRAZILIANITE

AEGIRINE

AZURITE

The monoclinic system is the largest symmetry system with almost a third of all minerals belonging to one of its three classes. This system contains two non-equal axes (a and b) that are perpendicular to each other and a third axis (c) that is inclined with respect to the a axis. The a and c axes lie in a plane. The a-c plane can be, but is not always, a mirror plane with the left side of the b axes a reflection of the right side. When viewed down the b axis, it becomes apparent that the b axis can be, but is not always, a two fold rotational axis. It can turn the crossed, but not perpendicular, a and c axes 180 degrees and repeat the identical structure; just like an X can be turned upside down and still be the same X. When the two symmetry operations are present together, the mirror plane is perpendicular to the two fold rotational axis and the symbology, 2/m (expressed 2 over m), is used.

If you followed the above discussion, then you have a good handle on the three classes possible in the monoclinic system. The lowest symmetry class has a mirror plane and lacks the two fold axis. The other low symmetry class has the two fold rotational axis but lacks the mirror plane. Which is lower in symmetry depends on the outcome of the argument: is a mirror a higher symmetry operation than a two fold rotation? The highest monoclinic symmetry class is the 2/m class with both operations being present. The 2/m class is a very popular class as more minerals form a structure with this symmetry than any other. By comparison, the other two classes of the monoclinic system are found lacking with a significant decrease in minerals representing them.

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## The Sphenoidal Class

MESOLITE

• Class: 4th
• Symmetry: 2
• Symmetry Elements: One two fold axis of rotation
• Axes: None are equal
• Angles: Alpha and beta angles = 90 degrees but gamma does not.
• Of Note: Crystals can be left or right handed, enantiomorphic, and hemimorphic, having different tops and bottoms.
• Common Forms: The sphenoid, pinacoid and pedion.
• Most Common Minerals Known to this Class: Amicite, boltwoodite, franklinfurnaceite, goosecreekite, halotrichite, joaquinite-(Ce), mesolite, miargyrite, pickeringite, remondite-(Ce), rinkite, uranophane, wollastonite-2M and many organic chemicals crystallize in this class including sugar and tartaric acid. There are roughly 70 minerals belonging to this class.
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## The Domatic Class

NEPTUNITE

• Class: 3rd
• Symmetry: m
• Symmetry Elements: One mirror plane of symmetry
• Axial Lengths: None are equal
• Angles: Alpha and beta angles = 90 degrees but gamma does not.
• Common Forms: The dome, pinacoid and pedion.
• Most Common Minerals Known to this Class:Alamosite, antigorite (serpentine), clinohedrite, natron, neptunite and scolecite Roughly 40 minerals belong to this symmetry class.
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CRYSTAL SYMMETRY EXAMPLES

ISOMETRIC

SPINEL

TETRAGONAL

AUTUNITE

HEXAGONAL

HANKSITE

TRIGONAL

DIOPTASE

ORTHORHOMBIC

CERUSSITE

MONOCLINIC

HEULANDITE

TRICLINIC

MICROCLINE

AMORPHOUS

PRECIOUS OPAL