Optical Properties of Minerals
The optical properties of minerals are important for their identification. Optical properties are determined with the help of a ”polarizing microscope”. The study of optical properties of minerals is known as Optical Mineralogy. With the help of optical mineralogy, we study the minerals and rocks with the help of their optical properties. The rocks and minerals are studied by taking their sample in the form of a thin plate/ slide, which is then placed under a petrographic microscope. The mineralogical composition, origin, and evolution of geological materials are studied under the subject of optical mineralogy. We study the behavior of light by passing through a mineral or rock sample. Different types of rock and minerals influence the passage of light in different ways. The properties of minerals and rocks to influence the passage of light through them is known as the ”Optical Properties of the Minerals”. And the study of minerals carried out by means of light is known as ”Optical Mineralogy. We generally have two types of light. One is ordinary light, and the other is polarized light. Polarized light is induced by the help of prisms and polaroids. The polarized light clearly helps us identify the minerals, especially their crystal forms.
Ordinary Light: Ordinary light travels in straight lines with a transverse motion. It vibrates in all directions at right angles to the direction of propagation.
Polarized Light: When the vibrations of the wave motion are confined to a single plane only, the light is called ”polarized light”. The plane along which such vibrations take place is called the ”plane of polarization”. There are three ways by which the polarized light can be obtained: (i) by double refraction, (ii) by absorption, and (iii) by reflection. Nicol prisms and Polaroids are used in microscopes to produce polarized light.
Optical properties of minerals include:
(1): Refractive index of Minerals:
When a ray of light enters a medium, it deviates its direction. The deviation of light is because of the difference in the velocity of light in air and the denser medium. The more the medium is denser, the more will be the refraction. The amount of deviation depends on the angle of incidence and the relative velocity of light in the two media. The ratio of the velocity of light in the air (V1) and its velocity in the minerals (V2) is known as the ”Refractive Index”.
(2). Isotropic and an-isotropic minerals:
- Isotropic Minerals: In crystals belonging to the cubic system, the light travels with the same velocity in all directions and therefore each mineral has only one refractive index. Such minerals are called ”isotropic minerals”. Non-crystalline substances, such as opal and glass are also isotropic.
- An-Isotropic Minerals: The anisotropic group includes all crystals except those of the cubic system. In these crystals, the velocity of light and consequently the refractive index varies with the crystallographic direction. Anisotropic minerals generally show double refraction.
(3). Double Refraction:
When a ray of light passes through an an-isotropic crystal, it breaks into two polarized rays vibrating in mutually perpendicular planes. One ray which obeys laws of refraction is called ”ordinary ray” or ”O-ray”, and the second ray which does not obey these laws is called ”extraordinary ray” or ”E ray”.
Birefringence is another optical property of the minerals, which means the difference between refractive indices of the Ordinary-ray and Extraordinary-ray during the case of double refraction. For biaxial crystals, the numerical difference between the greatest and least refractive indices is the birefringence.
(5). Optic Axis:
The optic axis is an optical property that is associated with the an-isotropic minerals, in which there is no double refraction, which is called the ”optic axis”. The minerals crystallizing in the tetragonal and hexagonal systems have one optic axis and therefore, they are called ”uniaxial”. The minerals belonging to the orthorhombic, monoclinic, and triclinic systems are called ”biaxial” because they have two optic axises.
(6). Optical behavior of Uni-axial Minerals:
The crystals of the tetragonal and hexagonal systems possess only one optic axis, which is parallel to the c-axis. For this reason, they are called ”uniaxial”.
Optical Characteristics of Uni-axial Minerals
(7). Optical Behavior of Bi-axial Minerals:
Crystals of orthorhombic, monoclinic, and triclinic minerals have two optic axes, which is why they are called ”biaxial”. When these optic angles intersect each other, they form two types of angles, the acute and the obtuse angle. The acute angle between the two optic axes is called the ”optic axial angle” and is commonly designated as 2v. The plane containing the two optic axes is called the ”optic axial plane”. see detail
When a crystal is rotated in polarized light, it shows changes in the quality and quantity of its colour. Such minerals are known as pleochroic minerals. This change in colour is due to the change in the absorption of light vibrating in different directions of the crystal.
- Uniaxial crystals have two vibration directions. Such a crystal shows two different colours for these vibration directions. This property is called dichroism. For example, in tourmaline, the rays vibrating parallel to the length of the crystal are much less absorbed than those vibrating at right angles to the length.
- In biaxial crystals, there are three principal vibration directions, X, Y, and Z. Along these vibrations, three distinct colours are observed. This phenomenon is called ”pleochroism”.
Pleochroism or dichroism is described by noting the colour of light transmitted when each principal vibration direction fo a mineral coincides with the plane of the polarizer. For example, in hornblend, the pleochroism is described as follows: X-pale yello, Y-yellow green, and Z-green in the three directions of light vibration.
(9). Optical relief:
When a mineral grain is brought under the polarized light, it stands out in relief. In order to study the relief of mineral grain, the sharpness of its outline and the roughness of its surface is observed. The relief of a crystal is a function of its own refractive index and that of the cement.
- If the difference between the refractive indices of the mineral and cement layer is less, the crystal will appear flat and featureless with a faint outline. It is then said to have a ”low relief”.
- If the difference in the refractive indices of the mineral and the cement is high, the grain outline will appear bold and cracks on its surface will become conspicuous. In this case, the mineral is said to have a ”high relief”.
10. Extinction angle:
When vibration directions of an anisotropic mineral coincide with those of the polarizer and analyzer, the mineral appears dark. This position of an anisotropic mineral is called ”position of extinction”. During a complete rotation of the microscope stage, the position of extinction occurs four times at 90° intervals. In extinction position, the light from the polarizer passes through the crystal vibrating parallel to the plane of the polarizer and is eliminated on the analyzer.
The extinction position helps in locating the vibration directions of the crystal section under study. In this position the vibration directions of the crystal coincide with the cross-hairs of the microscope eyepiece, which are set parallel to the planes of the polarizer and analyser.
Extinction Angle: The extinction angle is the angle between the extinction position and some crystallographic direction of a crystal. Since extinction positions are always 90° apart, usually the angle between the cleavage planes or crystal boundary and the nearest extinction position is measured. Read more>>>
11. Examination of Minerals in Cross Polars:
Examination of Minerals in Crossed Polars: When the analyzer is placed in the position with its plane of polarization at right angles to that of the polarizer, the polars are said to be ”crossed”. If a section of a doubly refracting mineral is placed between crossed polars, the light is doubly refracted and polarized three times as discussed below:
- The light that enters the polarizer, is doubly refracted and polarized. The E-ray vibrates in the vibration direction of the polarizer and O-ray at right angles to it. Here O-ray is eliminated and only E-ray passes through the polarizer.
- When this ray strikes an anisotropic mineral section, it is divided into two rays: (i) E-rays, and (ii) O-rays. They vibrate at the right angle to each other in the vibration direction of the mineral. These two rays move upward to the analyzer.
- At the analyzer, the third double refraction takes place and the two rays are divided into four rays: (i) E”-ray, (ii) O”-ray, (iii) E”’-ray, (iv) O”’-ray. Here the two ordinary rays are eliminated and the two extraordinary rays are rays; E”-ray and E”’-ray pass through the analyzer. Since they vibrate in the same plane (plane of analyzer) and have a fixed phase difference, they interfere. If the phase difference of the two rays is zero or integral multiple of wavelengths, darkness results, and if this phase difference is one half wavelength or any uneven multiple thereof, maximum brightness is produced. read more >>>
12. Accessory Plates used with P.microscope:
There are some standard accessory plates used with a polarizing microscope to examine the optical properties of minerals. Some of the accessory plates are (i) quartz wedge, (ii) gypsum plates, and (iii) mica plates. read more>>>
(13). Becke line test:
In the Becke Line test, a higher power objective is used and the iris diaphragm of the polarizing microscope below the stage is partly closed. A tapering edge of a mineral grain is selected and brought into focus. If now the microscope tube is raised, a narrow line of light, called ”Backe line”, will appear at the grain boundary. The Backe line moves towards the medium of the higher refractive index when the tube is raised. This test is very useful for isotropic minerals, which have only one value of the refractive index.
In order to determine the accurate value of refractive index of a mineral, a set of suitable liquids of known refractive index is required. These liquids are called immersion media”. A mineral grain under examination is immersed in a drop of liquid of known refractive index. Then by the use of Becke line, the refractive index of the minerals and liquid are compared. Thus if the Backe line moves into the mineral grain, a new mount is made by using a liquid of a higher refractive index. This procedure is repeated (with different liquids) till an exact match between the liquid and mineral grain is obtained. In such a case, the refractive index will be equal to the refractive index of that liquid.
(13). Sign of elongation (Length fast vs. length slow):
In a thin section, the an-isotropic minerals contain two vibration directions, one ”fast” and the other “slow”. To identify these directions, the mineral section is turned from a position of extinction through 45°. Then through the slot in the polarizing microscope tube, the gypsum plate is inserted.
- If the X-direction of the gypsum plate and the mineral coincide, the interference colour will rise in its order.
- If the optical direction of the two are opposed to each other, the interference colour will fall.
Hexagonal and tetragonal crystals are frequently elongated on the c-axis. If such an orientation is known, the optical sign of the elongated grain can be determined. The sign of elongation is said to be ”positive”, when the slow vibration direction is parallel to the direction of elongation of a mineral grain. It is said to be “negative” when the fast direction coincide with the length of grain.
(14). Observations in convergent light:
On examining the sections of an an-isotropic minerals under a convergent polarized light, the ”interference figures” can be seen. Interference figures are observed in the sections of anisotropic minerals which are: (i) normal to an optic axis of uniaxial or biaxial minerals, and (ii) normal to bisectrix in biaxial minerals. Read in detail>>>
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