Amorphous and Crystalline Solids

Cleavage Property of Crystalline Solids

Differentiation between Crystalline and Amorphous solids based on the Cleavage Property

If a crystalline solid is cut with a sharp object, it would always give parts with smooth edges whereas an amorphous solid would cut into surfaces with rough, uneven edges. Crystalline solids are therefore said to have cleavage property, and amorphous solids do not show cleavage property.

Do amorphous solids show cleavage property

Imagine you are passing a raw potato through a slicer blade; what you get is thin slices of potato with smooth edges. But if you pass a boiled potato through a slicer, it would crumble and fall apart giving small uneven pieces with rough edges. The raw potato represents a crystalline solid and the boiled potato acts like an amorphous solid.

Why Crystalline Solids show Cleavage Property?

The cleavage property is shown by crystalline solids because they possess cleavage planes. In a crystalline solid, the cells are neatly stacked. The cleavage planes are areas where the crystal structure is the weakest. It is only along these planes that a crystalline solid can be cut. Therefore, a cut from a sharp object would give two smooth parts. Amorphous solids do not show any cleavage planes.

crystalline solid cleavage planes

If you look at the crystal structure, you will notice a constant arrangement of the unit cells. In the two-dimensional diagram, it is seen as a lattice. Usually, the cuts made in the direction of the linear sequence of points shown in green are preferred over other cuts. The cuts along the direction of red are not allowed. These are the cuts that can shatter or disintegrate the crystal. In other words, the cuts that preserve the arrangement of the particles are preferred in a crystalline solid.

cleavage planes crystalline solid
An example is of the diamond, a crystalline solid that when cut along the cleavage planes gives small diamonds with smooth edges having the same arrangement of the particles as that of the parent diamond.

cleavage planes in crystals

Let me give you another example. A NaCl crystal structure has a cubic arrangement. Its cleavage plane is parallel to the cube faces. If cut along any other plane as shown in red, the crystal structure will shatter.

NaCl cleavage plane
Every crystal has a unique cleavage plane depending on the arrangement of the particles. For example, crystals gypsum, feldspar and calcite have one, two and three cleavage planes.
The amorphous solid quartz does not have well-defined cleavage planes. It does not show cleavage property and breaks unevenly giving rough edges.

examples cleavage planes crystalline amorphous solids

In summary, crystalline solids show cleavage property and is the reason why crystalline solids are smoother than the amorphous solids. Cut along the cleavage plane results in getting crystalline solid with smooth edges. Cleavage planes exist due to the ordered arrangement of the atoms thereby giving smaller crystalline solids of same geometric arrangement as the parent. On the other hand, the constituent particles of the amorphous solids are randomly arranged and do not show cleavage property. They break into uneven pieces with rough edges when cut.

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Crystalline Versus Amorphous Solids- Anisotropy and Isotropy

Anisotropy and Isotropy

Amorphous solids are said to be isotropic, and crystalline solids are anisotropic for their physical property measurements.

Isotropy comes from the Greek word; iso means same and tropos means direction. The name rightly indicates that for the amorphous solids; the physical property measurements are same in all the directions. The same correlation applies for anisotropy that means no same direction. It means that for the crystalline solids, the physical property measurements are not same in all the directions.

The physical properties that depend on direction for taking measurements and therefore affected by the nature of the solid are- refractive index, electrical conductivity, thermal conductivity, photoelasticity and many more.

For the crystalline solids; the arrangement of particles is ordered and periodic, then why does the physical property measurement changes? How does it become anisotropic?

isotropic and anisotropic crystals

What it implies is that, if physical property measurements are taken along the x-axis, its reading will be different than if measured from another axis, say y-axis or the z-axis. Different direction will give different measurements. But this is not the case with amorphous solids. For the amorphous solids, same values will be obtained irrespective of the direction of the measurement.

An example will help us to understand this better. A refractive index measurement is taken for crystal calcite and amorphous solid glass at a single wavelength keeping the direction fixed at the x-axis. For the crystal calcite, the values ranged from 1.4 to 1.6 but for the glass the values ranged from 1.50-1.52 only. When the direction of the measurement changed, the values changed drastically for calcite but remained the same for glass at 1.50-1.52.

isotropic and anisotropic solids examples

Anisotropy is observed in crystalline solids because the concentration of the atoms is different in different directions of the unit cell. If you look along the X-axis, the concentration of particles around it is different than the distribution of atoms around the y-axis and same is for the z-axis. As the concentration of the particles in a particular direction of the crystal changes, therefore, the measurement of physical property changes depending on the direction of the measurement.

example anisotropy in crystalline solids

Amorphous solids have a tightly packed random arrangement of the constituent particles, unlike the crystalline solids that have a fixed arrangement of atoms in a crystal lattice. Due to the random arrangement, the distribution of particles would be widely different along each axis. Hence, an average value of the measurement is taken.

arrangement particles crystalline and amorphous structure of matter

But what if the crystalline solid has an equal perfect distribution of atoms in a unit cell like in a cubic structure, then would it be isotropic for all the properties? The answer is no. A perfectly arranged cubic crystal structure would be isotropic for some properties like refractive index but would be anisotropic for other properties like photoelasticity.

Example of anisotropy in crystalline solids

Therefore, in general, we can say that all crystalline solids are anisotropic for some of their physical properties and all amorphous solids are isotropic.  

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