Optical

Optical properties: basic principles

The optical properties of minerals are most reliable properties available to distinguish and identify minerals. “The study of interaction of light with minerals is known as ”. The optical properties depend on the manner that visible light is transmitted through the crystal, and thus are dependent on crystal structure, crystal symmetry, and chemical composition of the mineral.

In order to describe the interaction of light rays with mineral, we must understand something about light and its properties.

Nature of Light

Light can be thought of both as a wave phenomenon (electromagnetic theory) or a particle phenomenon (quantum theory). The wave theory of light describes light as a longitudinal wave, with the direction of propagation and the direction of energy transfer being perpendicular. In optical mineralogy the direction of propagation is referred to as the ray path and the energy transfer direction as the vibration direction. The geometrical relationships between ray path, vibration direction, and a mineral constitute one major portion of the optical study of minerals, basically a geometrical optical phenomenon. Results from wave theory are used to explain how light is refracted by a mineral.

Figure 1 Nature of light

Department of Geology 1

Optical Mineralogy

Several techniques for the measurement of optical properties of minerals use observation of light refraction. The particle theory describes light as composed of photons of different energies with these energies related to the wavelength in the electromagnetic theory. A description of the interaction of photons with the bonding electrons in minerals can be used to explain such phenomena as , color, and Pleochroism, and to interpret most spectroscopic studies.

The human eye can only see a small part of the large spectrum of electromagnetic radiation, namely the spectral domain between about 400 and 800 nm (visible light). A single wavelength light is known as Monochromatic Light.

Figure 2 wavelengths of light waves

Department of Geology 2

Optical Mineralogy

An ordinary light ray travels in a straight line through the vibration of particles of the medium along all the direction but at right angle to its direction of propagation, known as non-polarized light.

Figure 3 Non-ploarized light A polarized ray of light travels by vibration of particles of the medium only in one plane. The phenomenon of conversion of un-polarized light to polarized light is called and the particular plane to which vibration of particles confined are known as Plane of polarization.

Figure 4 Polarized light

Department of Geology 3

Optical Mineralogy

Methods of polarization- light may be polarized by selective absorption, double refraction, reflection and scattering. Unpolarized light can also undergo polarization by reflection off of nonmetallic surfaces. The extent to which polarization occurs is dependent upon the angle at which the light approaches the surface and upon the material that the surface is made of.

Figure 5 reflection method for polarizing light ray

Polarization can also occur by the refraction of light. Refraction occurs when a beam of light passes from one material into another material. At the surface of the two materials, the path of the beam changes its direction. The refracted beam acquires some degree of polarization.

Figure 6Refraction method for polarizing the light ray

Department of Geology 4

Optical Mineralogy

The Nicol consists of two specially cut calcite bonded together with an adhesive known as Canada balsam. This prism transmits waves vibrating in one direction only and thus produces a plane- polarized beam from ordinary light.

Figure 7 working of Nicol prism Nicol prism may act both as well as analyzer.

Figure 8 Nicol prism as polarizer and analyzer

Department of Geology 5

Optical Mineralogy

Double Refraction- also called is an optical property in which a single ray of unpolarized light entering an anisotropic medium is split into two rays, each traveling in a different direction. These two polarized rays are mutually perpendicular to each other.

Figure 9 Double refraction Ray with constant refractive index travels with the constant velocity in all directions through the crystal is known as ordinary ray or O-ray. While with variable refractive index, the ray whose velocity changes with the variation of its path through the crystal is known as Extra-ordinary ray or E-ray.

In an anisotropic mineral, there is a direction along which no double refraction occurs, known as Optic axis.

All non-opaque minerals are divided into two classes- a) Isotropic Minerals- Materials whose refractive index not depends on the direction that the light travels. Minerals in which light travels in all direction of vibration with the same velocity. Isotropic materials have a single, constant refractive index for each wavelength. Minerals that crystallize in the isometric system, by virtue of their symmetry, are isotropic. The most likely ones to see in thin section are garnet and spinel. b) Anisotropic Minerals- Materials whose refractive index does depend on the direction that the light travels are called anisotropic materials. In this type of minerals the velocities of light varies with the direction of its vibration.

Department of Geology 6

Optical Mineralogy

Figure 10 Isotropic and Anisotropic crystal Anisotropic materials can be further divided into two subclasses. a. Uniaxial Minerals- all minerals that crystallize in the tetragonal and hexagonal crystal systems, because they have a single optic axis. In these minerals when a ray of light passes along the direction of optic axis, there is no double refraction occurs and hence ordinary ray and extra ordinary ray are not produced. When the direction of ray is other than the optic axis, O-ray and E-ray produced. The difference between the refractive indices of E-ray and O-ray is called “Birefringence”. When the velocity of O-ray is greater than the E-ray, the uniaxial mineral are said to be “positive” and if the velocity of E-ray is greater than O-ray called “negative”.

Figure 11 Uniaxial Positive and Negative

Department of Geology 7

Optical Mineralogy b. Biaxial Minerals- minerals which have two optic axes are known as biaxial mineral. Minerals of Orthorhombic, Monoclinic and Triclinic system are biaxial in nature. Similar to uniaxial mineral when ray of light travel through the direction other than optic axis, splits into two rays E-ray and O- ray whose vibration direction are mutually perpendicular to each other. The velocities of ray’s changes with the crystallographic axis, and in the direction of Z velocity is slowest while X direction have highest velocity and hence they have refractive indices α, β and γ respectively in X, Y and Z direction. Refractive index α along X direction is lowest while γ along Z direction is highest. The optic sign of biaxial minerals depends on whether the β refractive index is closer to that of α or to γ. A mineral is biaxial positive if β is closer to α than to γ. A mineral is biaxial negative if β is closer to γ than to α.

Figure 12 Biaxial positive Figure 13 Biaxial Negative

The acute angle between the two optic axes is called “optic axial angle” and is commonly designated as 2V. The difference between the greatest and least refractive indices is called “Birefringence”.

Department of Geology 8

Optical Mineralogy

Polarizing Microscope In optical mineralogy we use a microscope called a polarizing microscope. Such a microscope is equipped with two that are normally oriented so that their polarization directions are perpendicular to one another. To study the mineral under microscope, their thin sections are prepared. The thickness of thin section is approximately 0.03 mm. The microscope must be equipped with both a polarizer, positioned in the light path somewhere before the specimen, and an analyzer (a second polarizer), and placed in the optical pathway between the objective rear aperture and the observation tubes or camera port. Different parts of polarizer microscopes are as follows a) Eyepiece- it has two cross-hires which help in locating a particular mineral grain for detailed examination. One cross-hire is in E-W direction and other is in N-S direction. b) Circular stage or Rotating stage- This is the specimen stage and it can rotate 360° to facilitate the correct orientation of the specimen with the objective plane. In several stages, a Vernier scale is also provided to provide an accuracy of 0.1° in the rotational angle of the stage. c) Polarizers- Polarizing filters are the most critical part of the polarized light microscope. There are usually two polarizing filters: the polarizer and the analyzer. The polarizer is located below the specimen stage and can be rotated through 360°. It helps to polarize the light which falls on the specimen. The analyzer is placed above the objective and is removable. It combines the different rays emerging from the specimen to generate the final image. d) Condenser- it is used to collect and focus the light from the illuminator on to the specimen. It is located under the stage often in conjunction with an iris diaphragm. e) Iris Diaphragm- it controls the amount of light reaching the specimen. It is located above the condenser and below the stage. f) Bertrand Lens - A specialized lens mounted in an intermediate tube or within the observation tubes, a Bertrand lens projects an interference pattern formed at the objective rear focal plane into focus at the microscope image plane. g) Plano-concave Mirror- it is located below condenser and used to control the amount of light.

Department of Geology 9

Optical Mineralogy

Figure 14 Polarizing Microscope

Figure 15 Polarizing microscope

Department of Geology 10

Optical Mineralogy

Figure 16 Polarizing Microscope 2

Figure 17 Polarizing Microscope 3

Department of Geology 11