Triclinic System, Crystallographic Axes, Symmetry and Forms

Crystallographic Axes. The crystallographic axes of the Triclinic System are three in number. They are of unequal lengths and make oblique angles with each other. The axial directions for each tricline mineral are chosen arbitrarily, but in such a way as to yield the simplest relations. Any one of them may be taken as c, the vertical axis. the longer of the other two is designated as the b or macro-axis, while the shorter is called a or the brachy-axis. The trelative lengths of the three axes and the angles which they make with each other have to be calculated for each mineral from appropriate measurements. The angles which the different axes make with each other are designated respectively as alpha, beta and y . For example, the crystal constants of the triclinic mineral axinite are as follows: a:b: c=0.492:1: 0.480; alpha= 80° 54’, b=91° 52’; gamma= 131° 32’

Normal class

Symmetry and Forms.
The symmetry of the Normal Class of the Triclinic System consists only in a center of symmetry. It has no axes or planes of symmetry. All forms of the Triclinic System consist of two similar and parallel faces. In this respect all triclinic forms might be spoken of as pinacoids. They are, however, usually designated as pyramids when they intersect two axes and as pincaoids when they intersect but one axis. it is to be noted, however, that by a change in the choice of axial directions any one of these forms could be converted into any other.

1. Pyramid.
A triclinic pyramid consists of two parallel faces, each of which intersects all three crystallographic axes. There are four possible types, depending upon the octants in which the faces lie.

2. Prisms
A triclinic prism consists of two parallel faces, each of which intersects the a and b axes and is parallel to the c axis. There are two possible types, a combination of which is shown in Fig. 185.

3. Domes
A triclinic dome consists of two similar parallel faces, each of which intersect the c axis and either a or b axes and is parallel to the other. They are spoken of as either macro-or brachydomes, depending upon the axis to which they are parallel. There are two types of each. Fig. 186 represents a combination of the two types of macrodome and Fig. 187 a combination of the two brachydomes.

4. Pinacoids
A triclinic pinacoid is a form consisting of two parallel faces, each of which intersects one crystallographic axis and is parallel to the other two. They are designated as the macropinacoid with the symbol (100), as the brachypinacoid with the symbol (010), and as the basal pinacoid with they symbol (001). Triclinic Combinations are also possible.

Characteristic of Triclinic Crystals
There are only a few triclinic minerals and they seldom show distinct and well-developed crystals. When such crystals do occur they are to be recognized by the fact that they have no plane or axis of symmetry by the fact that each form consists of only two similar and parallel faces.