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Optical mineralogy
Primarily aims ï To learn the language (meaning and dentition of the used terms) of optic and optical mineralogy ï To Learn some principle of optic such as property of light (polarization, interference, reflection, refraction, dispersion and wave length competent) the definition of the optic sign of uniaxial crystals ï To Learn the different effects minerals and materials on light under polarizer microscopes (these effects such as index of refraction, double refractions, polarization, inference color and figure ï Get to learn what is the biaxial and uniaxial indicatrix, its various axes, planes, and the 2V angle. ï Learn to determine the. Indices of refraction, optic sign, and 2V angle of biaxial crystals in addition to sign of elongation.
Final aim: to learn how to identify minerals and the rocks with differentiating their economic
values
Method of the study
The study of mineral are done with thin section under polarized microscopes by and follow the below subdivision: 1- General aspects of minerals: Mineral composition, crystal symmetry, and optics 2- Orthoscopic mode of the mineral study / plane-polarized light: relief and refractive indices, form, cleavage, color 3-Orthoscopic mode of the mineral study / crossed polarizers: birefringence, sign of elongation, extinction, twinning 4- Conoscopic mode of the mineral study: optic character and optic sign, optic axial angle
Relation of the optical mineralogy with other branches of Mineralogy
Mineralogy includes Visual study of hand specimen by eyes
Chemical mineralogy Optical mineralogy Minerals Crystal morphology (crystallography)
Optical mineralogy: that branch of geological sciences that deal with optical properties of minerals. Optical mineralogy mainly deals with three things: 1- Polarizing microscope (petrographical microscope) 2- Visual light (white light) 3- Thin section
1-polarizing microscope Polarized microscope is of possible interest to many sciences that are concerned with crystalline materials such as : geology, mineralogy, crystallography, materials science, biology, forensic science
Polarizing microscope has four differences as compared to ordinary microscope, they are:
a) It has rotating circular stage which graduated in to 360 degrees b) It has Polarizer and analyzer plates: In modern microscopes polaroid sheets are used as polarized and analyzer but in old microscopes Nicol prism or Ahern prism are used the polarizer and analyzers used in our microscopes are made of Polaroid sheets which consist of a sheet of cellulose fed with crystals of quinine iodosulfate which is absorbing light in one direction but transmit in the other direction. c) It has Accessory plates : such as mica plate , gypsum plate and quartz wedge d) It has Bertrand lens: located between analyzer and ocular, which brings the image of interference fiure in to the focal plane of the ocular. e) Objectives : they are used for magnification of the original object on the stage, two numbers are engraved on the tube of objective lenses: 1- Magnification of the original object e.g: (1x, 3x, 10 x, 40 x and 100 x) the objective lens in the diagram has magnification power of (10) time more than original object on the stage.
2- Numerical apertures (N.A) N.A = sin u
Modern students polarizer microscope (normally used after, 1970) on which the most important parts are indicated (Also called petrographic microscope)
Simple polarizer microscope (normally used before, 1970) on which the most important parts are indicated
Some application of Polaroid sheets (plates) in human life:
1- LCD watches (liquid crystal display watch) Television, mobile, computer and digital watch screens consist of a two Polaroid sheets (one act as polarizer and other as analyzer) which are called LCD (liquid crystal display) sheets or pieces. These sheets are called Polaroid sheets which are first made by Polaroid Company in USA. The most common liquid-crystal displays (LCDs) in use today rely on picture elements, or pixels, formed by liquid-crystal (LC) cells that change the polarization direction of light passing through them in response to an electrical voltage. As the polarization direction changes, more or less of the light is able to pass through a polarizing layer on the face of the display. Change the voltage, and the amount of light is changed. The sheets are consisting of small crystal embedded in certain liquid. as shown in the cross-section diagram in the diagram. when there is no battery in the watch, the watch screen appear clear (bright 1) but when battery used some area of the liquid crystals became dark because the crystals of these area acts as analyzer and absorb light as shown in position.
Digital LCD watches work by polarizer and analyzer, the blue (grey) segments are those through which voltage or current is applied changing direction of the polarization of the crystals and black which is visible.
Adjustment of polarizing microscope: 1- Centering the objective with the microscope field of view. 2- Crossing the polars (or Nicols) 3- Testing the cross hairs 4- Determination of the vibration plane of the lower polar (polarizer) 1- Centering the objective: The objective is (centered) when its lens axis concede with the vertical axis about which the microscope stage is rotating For centering the objectives a simple procedure is followed: 1- Fined an easily recognized point and then rotate the stage. the point must has a concentric circle of rotating about the intersection of the cross hair but it has no concentric circle of rotation , the following procedure is followed:- 1-rotate the stage until the point is farthest from the intersection of the cross-hairs 2-bring it in half way by means of centering screws 3-then bring it to the center of the cross hairs to the center of the field of view) by actually moving the slide by hand 4-rotate the stage and repeat the operation if the centering has not been completed in the first time
2-Crossing the polars (polarizer and analyzer) Every time before putting the slide on the stage , the polars must be crossed as shown on the page (1 and 2) (cross polars diagram) the polarizer located below the stage which has N-S polarization direction the analyzer located between objective lenses and the ocular which has E.W vibration direction. Which both polarizer and analyzer are inserted in the light path of microscope the field of view become dark this mean that both polars are at right angle to each other 2- Testing the cross hairs: the cross hairs are lines engraved on a glass plate in the ocular. It is important that the hair lines be parallel to the planes of the vibration of the two polars. This is done by the manufacturer company, but some time necessary to test the setting of the cross hairs with planes of the polars. This done by natrolite crystal with elongate and rectangular section. The natrolite crystal becomes dark between crossed polars when the edges of the crystals are parallel to the vibration direction ñA slide containing a small natrolite crystal may be placed up on the stage between crossed polars. If the cross hairs are in adjustment, the hair lines should be parallel or at right angles to the straight lines of the crystal boundary (crystal of face) 3- Determination of the vibration plane of the lower polar (polarizer) :- After the other adjustments have been made, the vibration direction of the polarizer can be determined with either fibrous tourmaline fragments or a rock section containing biotite showing cleavage. Biolite is used because of availability of biotite. Biolite is brown under polarized light and has maximum dark color when the cleavage is parallel to the vibration of the polarizer. So the stage
is the rotated until biotite attains maximum dark brown cooler, in this case the direction of the biolite cleavage is direction of polarizer vibration.
2- Visual light (traverse wave motion) The vibration light represent limited band of wave length with in electromagnetic spectrum, ranging from 3900 to 7700 A (angstrom). If light of all wave length simultaneously reach the human eyes, the light is interpreted by the brain as white light. But light with one wave length called mono chromatic light, for example, sodium vapor lamp is a source of mono chromatic light with wave length of 5890 A but the tungsten lamp is a source of white light (ordinary light).Traverse wave terminology and shape in which polarization and interference occur. Only sound wave is longitudinal which means that the vibration direction is parallel to direction of propagation in which polarization and interference do not occur
3- Thin section It is a fragment of rock or mineral mechanically reduced to thickness of (0.03mm) by grinding and polishing of thickness makes most rocks and minerals transparent which finally ready for optical study.
Procedure for making thin section A thin section is a 30 μm (= 0.03 mm) thick slice of rock attached to a glass slide with epoxy. Typical thin section slides are 26 mm x 46 mm, although larger ones can be produced. They are generally covered by another glass slide, a cover slip also attached to the rock with epoxy. The epoxy ideally has an index of refraction of 1.54, although our epoxy is slightly higher, perhaps 1.56.
The sections may be left uncovered for chemical analysis on the SEM or electron microprobe. If so, temporary cover slips may be weakly attached with glycerin.
1- Sawing a hand specimen to get a flat chip which has 4 square cm in area. 2- Polishing one of the two surfaces and then mounting on glass slide using Canada balsam. 3- Grinding of the chip, which is mounted on the glass slide, to a thickness close to 0.03mm with (1000) grade carborundum. 4- Then the slide covered with thin cover slide by liquid Canada balsam. 5- During polishing the chip must be examined several times under polarizer microscope to determine the standard thickness of the thin section (0.03mm). This done by the interference color of the quartz or feldspar which they give first order grey when the thickness reaches (0.03mm).
Index of refraction of material can expressed as the ratio between the velocity of light in the air and it is velocity in the denser material n= velocity in air/ velocity in material The velocity of light in air is considered here as equal to the reciprocal of the velocity N=1/v = 1/ velocity of light in materials Generally index of refraction of two substances in contact with each other are n1/n2 = v2/v The precise relationship of the angle of incident (i) to the angle of reflection (r) is given by Snells law: which state that ratio of sin I / sin r = constant and this constant is index of refraction (n). { sin i / sin r = n } Which is regarded as constant property for each transparent material. N1/ n2 = 1v1 / 1I v2 n1/ n2 = v1-1^ / v2-1^ v2/v
Measurement of refractive index of minerals
Refractometer
A refractometer is an absolute necessity for gem identification. It measures the refractive index of your gems. Besides the RI, a refractometer will also give you the birefringence and optic sign. When possible, it is easier to obtain the optic sign from a polariscope. However, that canít always be done, so you need the refractometer as an alternate means to determine the optic sign when necessary. Refractometers costs in the neighborhood of $500 to $1000. In North America the primary source is the Gemological Institute of America. In Europe the primary supplier is Kneuss Instruments. Used refractometers occasionally come available on eBay. Inexpensive models are now available from China for under $200. Their reliability varies, so if you purchase one of these make sure it is from a reliable company that will exchange it. For instructions on how to check the accuracy, see Gem Lab Refractometer.
Two photos of two jeweler's refractometers as seen from different directions (see the diagram below for parts) showing light reflected at the critical angle (C.A) for mineral spinel
Ray path illustrating reflection and refraction, Rays 1, 2, 3, are incident from the lower left and each ray has reflected and refracted portion. The angle of the reflection is equal to the angle incidence (ei) can be found by application of the Snells Law. Ray 3 shows the case of the total reflection when Ët=90 degrees
1- By refractometer
Finding of critical angle is a quick and easy method for determining the refractive index of minerals and liquids. The instrument used is refractometer which consists of a polished hemisphere of high refractive index glass. a crystal face or a polished surface of the mineral is placed on the equilateral plane of the hemisphere but separated from it by a film of a liquid. The liquid must has a refractive index higher than that of hemisphere slightly convergent light is directed up ward through the hemisphere and depending on the angle of incident, is either partly refracted through the unknown mineral or totally reflected back through the hemisphere. If a telescope is placed in a position to receive the reflected ray, we can observe a sharp boundary between the portion of the field intensely illuminated by the totally reflected light and the remainder of the field, when the telescope is moved so that its cross hairs are precisely on the contact, the critical angle C.A is red on the scale on the hemisphere. By knowing this angle and the index of refraction of the hemisphere (n) we can calculate the index of the mineral (n) of the mineral = sin critical angle ◊(n) hemisphere Index of mineral = sin critical angle ◊ index of hemisphere n= N ◊ critical angle The hemisphere must be made of high index glass of known index. Nr/ni = sin i/ sin r Q1 / why the refractometer does made in the form of hemisphere Q2/ how can you prove that (f) not change when light pass through high density (high n) material.
Three steps of the de Chaulness method for measuring the refractive index of unknown mineral.
The differential reading of drum obtained in step 1 and 3 are respectively proportional to t and ta , so this two value can be substituted directly in the above equation as below N=t/ta this equation can be derived from the fig (4) Tan u = ox/op-^ and tan Ë= ox/op and op-^ = ta so op =t Tan u / tan Ë = op/op-^ = t / ta since rays px and xz obey Snells law, so n= sin u / sin Ë for small angles the ratio of their sins approximately equals that of their tangents, so n= sin u / sin Ë ≈ tan u / sin Ë = t / ta
3-Finding refractive index by immersion methods
This method is one of the most convenient methods of measuring the refractive index of a transparent solid (mineral fragments). This is done by immersing fragments of mineral (or any substance) in a series of liquids of known refractive index. The immersion liquids used should at least span (include) the refractive index range between 1.430 and 1.740 at intervals of 0.005 .such sets of immersion liquids are obtainable Commercially or can be prepared in the laboratory. This method depends on the observing the relief of the grains (fragment) in the liquids several grain of unknown (n) of the minerals are prepared and all put on a glass plate. Then each grain immersed in a drop of the known liquid (known index) when the grain cannot distinguished in the liquid, this means that the index of grain is equal to that of the liquid Q1/ why immersion method is more convenient than other method The observation of mineral and liquid is done by microscope if the grains are very small but when the grains are large the observation can be done by eye or hand lens. When a grain immersed in liquid (or oil) there is two possibilities 1- The index of mineral is more than the liquid 2- The index of mineral is less than the liquid. In this case how we can know the right relation between the mineral and liquid? This is done by the beck line method
Beck line: It is a bright line which separate substances (minerals) of different refractive index and this line can be visible under microscope Beck line method: Beck Line method is done under polarizing microscope for comparing the refraction index of two substances in contact with each other. The two substances may be: 1- Two mineral with common boundary. 2- A mineral and immersion oil or liquid 3- Mineral and Canada balsam which surrounds the mineral in thin section. If these substances have different refractive indices, they are separated by a beck line which moves toward the center of high refractive substance when this stage lowered. But moves away from the substance when the refractive index is less. When the mineral can be seen in the oil it means that two substances have different relief. Relief: it is visibility of outline (boundary), cleavage and surface feature of the mineral in the field of the microscope under (ppl) which depends on the index of refraction. 1- Low relief: has smooth surface and boundary cannot be seen but when analyzer used boundary can be seen. I.R = C.B or IR=OIL e.g: quartz and Canada balsam 2- Moderate relief: boundary and surface can be seen I.R of mineral > I.R of C.B e.g.: Muscovite and Canada balsam 3- High or very high relief: the boundary and cleavage of the Mineral can be seen clearly and the surface of the mineral is Rough. e.g: olivine and C.B or olivine and nepheline. Refractive indices and relief of some common minerals minerals Indices of refraction relief C.B 1. Fluorite N= 1.434 Low relief Quartz N∫ = 1.553 , nw = 1.544 Low relief Calcite Nw = 1.658 , ne = 1.641 High relief (twinkling) Apatite Nw = 1.646 , N∫ High Grossularite (garnet)
N=1.771 Very high relief
Sphalerite N = 2.369 Very high relief Aragonite N = 1.530 , nB = 1. N = 1.
High relief (twinkling)
Opal 1. diamond 2.
A-Isotropic group: includes 1- noncrystalling substances such as gases, liquids, and glass these are called (amorphous materials) 2-crystals that belong to the isometric (cubic) system. In these material light moves in all directions with equal velocity and hence each isotropic substance s has single refractive index (n) A- Anisotropic substance: Which include all crystals, except those of isometric system, the anisotropic crystals are belonging to one of these systems:- 1- Orthorhombic 2- tetragonal, 3- hexagonal 4- trigonal 5- monoclinic and 6- triclinic system. In these crystals velocity of light varies with crystallographic direction and thus there is a range of refractive index. In general, light pas through anisotropic crystal is broken in to two polarized rays vibrating in mutually perpendicular planes. Thus for a given orientation, a crystal has two indices of refraction, one associated with each polarized ray. Now we are in position to discuss polarization by double refraction. Polarization by double refraction was the method by which the first efficient polarizer is made by William Nicol, and this polarizer called Nicol prism, as shown in the diagram, which made of clear calcite crystal called Iceland spar. An elongate rhombohedral crystal cut at a certain angle and then the halves are rejoined by Canada balsam. When light enter the prism from below, it resolved in to two ray o- ray (ordinary ray) and E- ray (extra ordinary ray). Because of the greater refraction of the O- ray it is totally reflected of the Canada balsam surface. The E- ray with refractive index (n) close to that of the Canada balsam continue to pass the prism and emerge as a plane polarized light (PPL). Nicol prism was the only polarizing device in the old microscope but now Polaroid sheets are used.
Nicol prism used as polarizer and Analyzer device in old microscopes (before, 1970).
3-polarization by reflection:
Light reflected from a smooth, non metalic surface is partially polarized with vibration direction parallel to the reflecting surface is the degree of polarization depends on the 1- angle of incident, 2- index of refraction of the reflecting surface. The maximum polarization happens When the angle between the reflected and refracted ray is (90) degree this is called (Brewsters law). The fact that the reflected light is polarized can be easily shown by viewing it through a polarizing sheet. When the vibration direction of the sheet is parallel to the reflecting surface the light pass through the sheet with only slight reduction in intensity but when the sheet is turned 90 only small percentage of light reach the eye and all absorbed.
Brewster's angle (also known as the polarization angle) is an angle of incidence at which light with a particular polarization is perfectly transmitted through a surface, with no reflection. When unpolarized light is incident at this angle, the light that is reflected from the surface is therefore perfectly polarized. This special angle of incidence is named after the Scottish physicist, Sir David Brewster (1781ñ1868).
