Minerals are usually defined as a solid compound or mixture of compounds that have a clear distinction on the type of compositions present in them. The properties of these minerals are also defined based on the composition that they contain.
Isotropic and anisotropic are two such properties that are used to define the structure or composition of minerals. These properties are also used for defining the basis of other properties exhibited by these minerals. So to know the properties of a mineral, it is necessary to be able to differentiate between isotropic and anisotropic property.
Isotropic vs Anisotropic
The main difference between Isotropic and Anisotropic is that isotropic have invariable properties and equal composition throughout the mineral crystal. Whereas, anisotropic have variable properties that differ with direction and dimension. They also have different compositions.
Isotropic is the mineral crystal that exhibits equal and invariable properties throughout the material. This is because they have equal composition throughout and are not direction-dimension dependent.
Anisotropic is the mineral crystal that has different properties in different directions of the mineral surface. The different properties are due to the difference in composition. The properties of such minerals are direction-dimension dependent. They exhibit properties like double refraction.
Comparison Table between Isotropic and Anisotropic
| Parameters of Comparison | Isotropic | Anisotropic |
| Meaning | The crystals that have invariable properties. | They are the crystals that have different properties in different directions |
| Dependency | It is independent of the direction and dimension of the crystals and therefore has consistent properties throughout. | It is direction-dimension dependent and therefore the properties vary for each direction and dimension. |
| Light | Since they have equal composition, light does not pass through these minerals. | Due to the difference in composition, light/sunlight can easily pass through these minerals. |
| Chemical bonding | They exhibit consistent and uniform chemical bonding throughout the mineral crystal. | They exhibit inconsistent and different chemical bonding in each direction of the mineral crystal. |
| Double refraction | They do not exhibit double refraction and have only one refractive index throughout the mineral. | These crystals do exhibit double refraction and have a various refractive index for each direction. |
| Speed of light | The speed of light is consistent throughout the isotropic crystal. | The speed of light is different in each direction and is also dependent on the refractive index of that direction. |
What is Isotropic?
Isotropic crystals are the mineral crystals that have equal compositions and invariable properties throughout. So, the properties of the isotropic mineral are not dependent on direction and dimension.
This is implied by the factor that the chemical bonding is uniform throughout the mineral crystal as they have equal composition. Since light is not allowed to penetrate through these minerals, they appear dark when light is shown on them.
The word ‘isotropic’ has a Greek origin. It originated from two Greek words ‘iso’ meaning equal, and ‘tropic’ meaning direction. So, from this one can infer that isotropic means equal in all directions.
Isotropic materials are used in many industries. They are mainly found in the industries related to maths, physics, biology and chemistry.
What is Anisotropic?
Anisotropic crystals are mineral materials that have variable and non-uniform composition and properties. So, the properties of anisotropic minerals are direction-dimension dependent.
Anisotropic materials also have unequal and varying chemical bonding in each direction as the properties vary with direction. Light can easily penetrate through this material and therefore, they appear light-coloured when light is shone on them.
The word ‘anisotropic’ also has a Greek origin. It originated from two words ‘an’ meaning opposite and ‘isotropic’ meaning equal in all directions. So, from this one can infer that anisotropic means different in all directions as it is the opposite of isotropic.
Anisotropic materials exhibit several properties that isotropic materials fail to exhibit. Some are double refraction, dichroism, optical activity, etc. This is because they have different refractive indexes in different directions. They are also used in similar fields as isotropic materials.
Main Differences Between Isotropic and Anisotropic
- Isotropic crystals are the ones that have invariable and equal properties throughout the material. Whereas, anisotropic is the opposite. This mineral has different properties in different directions. This is the main point of distinction between the two minerals.
- Isotropic minerals are independent of the direction and dimension of the crystals and therefore have consistent properties throughout. On the other hand, anisotropic materials are direction-dimension dependent and this gives rise to different properties in different directions and dimensions of the mineral crystal.
- The penetration of light is also different for the two mineral crystals. Light, including sunlight, cannot penetrate through an isotropic mineral crystal. Since they have equal properties and no difference in composition, they do not allow light to penetrate. But in the case of anisotropic mineral, sunlight/light can penetrate through it easily.
- The chemical bonding of the two minerals has a clear distinction as well. Isotropic minerals exhibit consistent and uniform bonding throughout as they have the same composition throughout the material. On the other hand, the chemical bonding exhibited by an anisotropic mineral is inconsistent and non-uniform. They are different through different compositions of the anisotropic mineral.
- Isotropic mineral crystals have a single refractive index. This is a result of the uniform composition of the mineral. This also means that isotropic minerals cannot exhibit the property of double refraction. But anisotropic mineral has many refractive indexes. These refractive indexes are dependent on the composition and direction of the mineral. This also leads to anisotropic minerals being able to exhibit double refraction.
- The speed/ velocity of light in a medium is always dependent on the refractive index of a material. So according to this, the velocity of light in the isotropic mineral is constant throughout. Whereas, the velocity of light in the anisotropic mineral is different from a difference in the refractive index of the direction.
Conclusion
The terms ‘isotropic’ and ‘anisotropic’ are opposite in meaning. Isotropic minerals have equal composition and properties whereas the properties of anisotropic minerals differ with direction. Anisotropic materials exhibit different refractive indexes in different direction owing to the several properties exhibited by them that isotropic materials fail to show.
References
- https://iopscience.iop.org/article/10.1088/0264-9381/22/9/006/meta
- https://arc.aiaa.org/doi/abs/10.2514/3.10684