Brief notes on the Morphology of Crystals

Face:

The most important attribute of a crystal is the presence of flat surfaces known as faces and their arrangement in a distinctive pattern. These are the external expression of the internal atomic structure.

The common faces are normally parallel to net-planes containing maximum number of lattice points (ions/atoms). The faces are of two types, like and unlike. Like faces have equal properties i.e. they are similar in shape, size and orientation. NaCl commonly crystallizes in the form of cube. One such cube is shown in, which is bounded by six square faces of equal size and shape. ABCD, BCFG and CDEF are three like faces of the cube.

Unlike or dissimilar faces are characterized by different properties i.e. they are unequal in size and shape. A crystal shown has two types of faces, square (PQRS, RWXY, QUVW) and equilateral triangular (PQU, PTS, QRW, SRY). The square faces are bigger in size than the triangular faces.

Form:

The forms of crystals can be of different types depending on the nature and number of faces. Due to paucity of naturally occurring crystals, models made up of wood, plastic or glass are generally used in practical classes.

Simple form:

A crystal bounded entirely by like (similar) faces is termed as a simple form. The crystal shown in is a simple form because it is made up of six square faces, which are all alike.

Combination form:

A crystal having two or more types of faces is called combination form. The crystal shown in a combination form because it is made up of two types of faces i.e. square and equilateral triangular. In naturally occurring combination forms two or more sets of faces may be present.

Closed form:

A crystal may be termed as a closed form (solid) when it encloses some space. It can occur independently.

Open form:

An open form does not enclose any space. Since the crystals are three-dimensional solids, an open form cannot occur independently. Occurs in association with other form(s), giving rise to a combination form.

Polyhedral form:

The form having maximum number of faces as per demand of the highest degree of symmetry possible in a system is known as polyhedral form. Octahedron of isometric system with 8 faces is an example.

Hemihedral form:

This is the form with half of the number of faces present in a polyhedral form. The number of faces decreases consequei upon decline of the symmetry in comparison to polyhedral form. The faces are evenly distributed throughout the crystal. Tetrahedron with 4 faces is the hemihedral form of octahedron of isometric system.

Hemimorphy form:

This is the form with half of the number of faces present in a polyhedral form. The faces are present at one extremity of the vertical crystallographic axis. Hemimorphy forms do not possess centre of symmetry.

Tetartoidal form:

These forms have a quarter of the number of faces corresponding to the polyhedral form. They lack both the plane and centre of symmetry.

Enantiomorphism form:

These are the forms with half of the number of faces of a polyhedral form. They lack both plane and centre of symmetry. They occur in two positions, which are mirror images of each other but not superposable i.e. one cannot be converted to other by any rotation, Because they are related to each other as hands of human being, they are commonly designated as right- and left-handed.

Common forms:

A closed or open crystal form having a set of like faces is designated by a name. Some common forms are cube, tetrahedron, prism etc. Common forms occurring in a crystal class are indicated by different names. Their definition and description are given in later part of this chapter.

Edge:

An edge is formed by the intersection of two adjacent faces. Therefore, it is parallel with the rows of atoms occurring at the intersection of net-planes. BC edge in is formed by the intersection of ABCD and BCFG faces. Similarly, in PQ edge is formed by the intersection of square face PQRS and triangular face PQU.

Solid angle:

A solid angle is formed by the intersection of three or more faces. In the circle at C indicates a solid angle formed by the intersection of three faces ABCD, CDEF and BCFG faces. Similarly, in, the circle at R indicates a solid angle formed by the intersection of four faces PQRS, QRW, WRYX and SRY.