Monoclinic System,
Symmetry and Forms

They symmetry of the Normal Class of the Monoclinic System is as follows: They crystallographic axis b is an axis of binary symmetry and the plane of the a and c axes is a plane of symmetry (see Figs. 167 and 168). The forms are as follows:

1. Pyramid
A monoclinic pyramid is a form consisting of four triangular faces, each of which intersects all three of the crystallographic axes. There are different pyramids, depending upon varying axial intercepts. There are, further, two independent types of monoclinic pyramids, depending upon whether the two faces on the upper half of the crystal intersect the positive or the negative end of the a axis. a unit pyramid of the first of these types is shown in Fig. 169 and ha for its symbol (111). A unit pyramid of the second of these types and has for its symbol (111). It should be emphasized that a monoclinic pyramid consists of only four faces, two of which are to be found intersecting the upper end of the c axis and the other two intersecting its lower end. The two types describe above are entirely independent of each other.

2. Prism
The monoclinic prism has four vertical faces, each of which intersects the a and b axes. There are various prism with different axial intercepts. A unit prism and has for its symbol (110).

3. Orthodome
And orthodome consists of two parallel faces, each of which intersects the a and c axes and is parallel to the b or ortho-axis. its name is derived from that of the axis to which it is parallel. There are different orthodomes with different axial intercepts. There are also two distinct and indepent types of orthodomes, depending upon whether the face upon the upper end of the crystal intersect the positive or negative end of the a axis. These two types of orthodomes are represented in combination, but it should be emphasized that they are entirely independent of each other. The symbol of the unit orthodome in front is (101) and that of the one behind is (101).

4. Clinodome
The clinodome is a form having four faces, each of which intersects the b and c axes and is parallel to the a or clino-axis. there are various clinodomes with differing axial intercepts. A unit form would have for its symbol (011).

5. Orthopinacoid
The orthopinacoid has two parallel faces, each of which intersects the a axis and is parallel to the b and c axes. It derives its name from the fact that it is parallel to the b or ortho-axis, its symbol is (010)

7. Basal Pinacoid
The basal pinacoid is a form consisting of two parallel faces, each of which intersects the vertical axis and is parallel to the a and b axes. Its symbol is (001).

It should be noted that the only monoclinic form that is absolutely fixed in its designation is the clinopinacoid. The other forms may vary with a variation in the choice of the directions of the a and c axes. For instance the orthopinacoid, basal plane and orthodome may be converted into each other by a change in orientation and the same is true of the prism, pyramid and clinodome.

Characteristic of Monoclinic Crystals
Monoclinic crystals are to be distinguished chiefly by their low symmetry. The fact that they posses but one plane of symmetry and one axis of binary symmetry at right angles to it would serve to differentiate them from the crystals of all other systems and classes. Usually the inclination of the crystal face which are parallel to the clino-axis is marked.