Short essay on the Crystal Habits and Twinning

The crystal habit of a mineral is a term used to denote the relative development of faces and forms in its crystals; in other words, it refers to the form or combination of forms commonly occurring in crystals of that mineral.

When the external form of a crystal is column-like, bounded by a group of vertical faces, it is said to have a prismatic habit. Similarly, when a crystal displays maximum development of pyramidal faces, It is said to possess pyramidal habit and pyramidal crystals usually taper at two opposite ends. Whenever a crystal shows prominent flat surfaces because of the development basal plane the habit of the crystal becomes tabular.

Crystals occur in all possible sizes from the merest microscopic to crystals measuring in tens of metres in diameter. The variations in habit are due to the significant variations in the conditions attending crystallisation, including pressure, temperature, quality and/or quan­tity of foreign material.

It is usually observed that crystals of a particular mineral shows similar crystal habits, but in certain case, even though there are same faces, there is a difference in crystal habit, which is mainly because of the differential development of the faces.

Twinning:

Two or more crystals of the same or related mineral species may sometimes intergrowth in such a way that the individual parts are in reverse position to each other. Such crystals are called ‘twinned crystals’. Sometimes, it also appears as if one-half of the twinned crystal is produced by rotation of 180°, about some crystal axis common to both.

Twinned crystals are characterised by ‘re-entrant’ angles, and there are cleavages in different directions in different parts of what is apparently the same crystal.

(a) Twin-plane:

It is an imaginary plane which divides a twin- crystal into two halves such that one-half is a reflection of the other.

(b) Twin-axis:

It is an imaginary line about which rotation is necessary to bring the twin to its untwined state.

Twinning-centre:

When twinning is defined with respect to symmetry about a point, the referred point is called the ‘twin-centre.’

(d) Composition plane:

The plane by which the component crystals of a twin are joined is called the composition plane.

Types of twins:

(i) Contact twin:

It consists of two halves united by composi­tion plane such that one part is the mirror image of the other, e.g., ‘Orthoclase’.

(ii) Penetration twin:

Here the twin appears as if the indi­viduals were crossing each other. Staurolite, pyrite, fluorite etc. show this type of twinning.

In case of pyrite, this twinning called ‘Iron-Cross’.

(iii) Repeated twin:

(a) Polysynthetic or Lamellar twins. Here the twin planes are parallel to each other, e.g., plagioclase feldspars, calcite etc.

(b) Symmetrical or Cyclic twin:

Here the twin plane does not remain parallel but tends to produce circular forms as in spinel, rutile etc. Such twinning imparts as apparent symmetry of higher grade than that of the individuals and accordingly this is also known as mimetic-twin.

e.g., Aragonite appears to be hexagonal.

In case of rutile, the cyclic twin is known as ‘Geniculate lv, in\ In case of Gypsum, it is known as ‘Butterfly-twin.

In case of Gypsum, it is known as ‘Butterfly-twin.’