1. Optical mineralogy.
2. Properties of minerals.
3. Study of mineral groups.
Feldspar., -Amphiboles. -Chlorites. – Garnets,
Quarz, -Pyroxenes, -Micas, – Carbonates.
Optical-Mineralogy
Principles of optics:
In the context of optical mineralogy, it includes refraction, double-refraction and total internal reflection.
(i) Refraction. As we know, when light passes from one medium to another, there is. in genera], an increase or decrease in its velocity results and accordingly there is also a change in the direction of its propagation. When light travels from a rarer to a denser medium, it is bent towards the normal on entering the latter and vice versa.
Refractive index is a ratio between the sine of the angle of incidence and the sine of the angle of refraction, which is always a constant for the two media concerned. The index of refraction depends upon the nature of the substance and the kind of light used and increases with density and decrease in wavelength. Thus, since the wavelength of red light is greatest and that of the violet light is the least, the index of refraction for the violet light is greater than that of the red. The difference of R.I. between red and violet light, is called ‘Dispersion’.
Methods of determining the refractive index:
‘There are several methods of determining the refractive indices of minerals as follows :
(a) With test liquids (Immersion method),
(b) By Prism methods,
(c) By Refractometers, and
(d) By Polarising Microscope.
In case of determining the refractive index by polarizing the relative index of refraction of a mineral with respect to the immersing medium-The Canada Balsam ; or the two adjoining minerals in contact is determined. The methods commonly employed, are as
(a) Central illumination or Becke-line method.
(b) Oblique illumination method.
(a) Becke-line method:
The principle involved is the total reflection of light incident at more than the critical angle when passing from a mineral of greater index to that of a lesser index in thin section. Accordingly a portion of the beam is deflected towards the mineral with greater index which results in a thin band of light visible just inside the boundary of greater index. This band of light is called the Becke-line, which moves towards the mineral of greater refractive index when the microscope-tube is slightly raised and the reverse effect is produced when the tube is lowered.
(b) Oblique-illumination method:
Oblique illumination is made by cutting off half of light by using a finger or card below the stage. Thus half of the field remains illuminated and one side of the mineral will be dark and the; opposite side lighted. It has been observed that of the rays pass from a mineral of higher index into a mount of lower index, they are concentrated by refraction and form a light band; if they pass from the mount of lower index into the higher mineral they are spread out by refraction and so produce a shadow. In general “if the shadow appears on the side away from the dark half, the refractive index of the mineral in question is greater than that of the adjoining medium and vice versa”
(ii) Birefringence:
Excepting minerals which crystallize in the isometric system or are amorphous all the minerals are anisotropic i.e. away of light striking the surface normally or obliquely, breaks into two rays vibrating along planes which are perpendicular to each other. These two rays travel with different velocities and are differently refracted. Both of these rays are plane polarized, in other words they vibrate in definite directions or in definite planes, Such as phenomenon where by there occurs a division of the refracted “got into two rays is called ‘Double Refraction’.
If a transparent calcite rhomb is placed over a dot, two images of the dot will be seen while observing from upwards. Moreover on other crystal one of the ‘mages remain stationary, while the other words around the stationary image. The stationary image is called the ordinary image’, produced by the ray called ordinary ray which has passed through the crystal as if it were an isotropic medium. The other is known as the ‘Extra ordinary’ image produced by the Extra-ordinary ray.
The extraordinary ray vibrates in a plane containing ‘c’- crystallographic axis, i.e., within the principal section and the ordinary ray vibrates perpendicular to the principal section.
In every anisotropic mineral there is a direction in which both the extraordinary and ordinary ray travels with the same velocity. This direction is called the ‘Optic axis’. In an anisotropic mineral the velocity of ordinary ray is constant for all directions but that of the extraordinary ray varies with direction, becoming minimum or maximum at right angles to the optic axis.
Birefringence is a measure of the difference between the maximum and minimum refractive indices of a particular mineral; in other words, it is the difference between the refractive indices of the two-rays, i.e., extraordinary and ordinary ray. Calcite shows the highest birefringence, which is 0.142, because the refractive indices of the extraordinary and ordinary rays are 1.0516 and 1 .658 respectively.
According to the number of directions along which no double refraction occurs, anisotropic minerals are classified into two groups:
(a) Uniaxial minerals. Minerals of teragonal and hexagonal system«which possess only one optic axis.
(b) Biaxial minerals. Minerals of the orthorhombic, mono- clinic and triclinic system possess two optic axes, along which no double refraction occurs.
Birefringence also determines the optic sign of the minerals. “In any mineral if the extraordinary ray is faster than the ordinary ray, i.e., the refractive index of the ordinary ray is more than that of the extraordinary ray, the mineral is said to be negative.” Similarly when the ordinary ray is fast, i.e., the refractive index of the extraordinary ray is greater than that of the ordinary ray, the mineral is said to be positive. Thus Calcite is optically negative and Quartz is optically positive.
Birefringence is used in determining the thickness of the section.
It has also been observed that greater the birefringence, higher the interference colour.
The minerals showing double refraction are also called as birefringent minerals.
Nicol prism:
This is a device to produce and analyze polarized light through the principle of double refraction. It consists of a rhombohedron of calcite whose length is three times the breadth. The top and bottom surfaces are ground down to give an angle of 68° with the long edge. The block is then cut along the smaller diagonal and cemented together again with the help of a layer of Canada Balsam whose refractive index is 1.537, which acts as a rarer medium for an ordinary ray (R.I. 1.658) and a denser medium for the extraordinary ray (I.516).
Since the ordinary ray travels from denser to rarer medium, it is totally internally reflected, as it is so arranged that the angle of incidence at the Canada Balsam layer is greater than the critical angle for the Ordinary ray. But in case of extraordinary ray, since it travels from rarer to denser medium it passes through the nicol with little deviation and vibrates parallel to the short-diagonal of the crystal.
These are called nicol prisms which are used in the petrological microscopes both for producing and analyzing the polarised light and thus aids in the identification of minerals.
Pleochroism:
It is an important optical properties of minerals in thin- section, in which the change in quality and quantity of colour is observed on rotation of the stage through an angle of 90°.
The colour shown by a mineral in this section results from the absorption of certain ‘colours’ (wavelengths) from the incident white light; the resulting transmitted light being complementary in colour to that absorbed. Thus pleochroism is defined as the variation in colour resulting from differential absorption of wavelength in different directions.
1. Isotropic substances:
In isotropic substances, the absorption of light is same in every direction and there is no variation of intensity and colour of the light during the rotation of the stage. Accordingly isotropic substances are non-pleochroic.
2. Anisotropic substances:
These minerals show distinct pleochroism, but it depends on the crystallographic orientation of the section cut from the mineral. Different sections of the same mineral show different degree of pleochroism.
(a) Uniaxial minerals:
Basal sections of (tetragonal and hexagonal) uniaxial minerals are isotropic ; only prismatic sections snow pleochroism. The greatest degree of pleochroism occurs when crystallographic directions of the section are either parallel or Perpendicular to the vibration plane of the polarizer, in other words, coincides with those of the ordinary and extraordinary rays. Thus the pleochroism is seen only in two directions and accordingly minerals are said to be ‘dichroic.’
(b) Biaxial minerals:
If a section of biaxial mineral, cut right angles to an optic axis be examined; it will be found that the absorption is the same in every direction, and it will behave as 2 isotropic sections.
In other sections of the biaxial minerals, the variation of colour occurs according to the three optic directions ‘X, Y and z’ of which X’ is the fastest and ‘Z’ is the slowest direction 0f transmission of light. Thus absorption is three-fold in biaxial minerals and biaxial minerals are, therefore, known as trichroic.
It should be noted that pleochroism is the property of coloured minerals only but all coloured minerals are not pleochroic in colourless minerals there is no question of pleochroism.
Pleochroic haloes:
These are circular areas around minute inclusions, which are dark-t coloured than the remainder of the crystal and commonly exhibit pleochroism. The inclusions, in these cases, are the product of radioactive disintegration and themselves are radioactive and produce the ionisation effect on the surrounding portion.
Pleochroic haloes are observed in muscovite, biotite, tourmaline, cordierite, andalusite, pyroxene, amphibole etc. and the included minerals belong to zircon, apatite, rutile, sphene etc. These haloes, however, are destroyed above 500° and thus help in determining the upper limit of formation of such minerals.