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EXAMINATION WITH THE PETROGRAPHIC MICROSCOPE


12.1 OVERVIEW

A petrographic microscope, also called a polarizing microscope, is best described as a compound transmitted light microscope to which components have been added to enable the determination of the optical properties of translucent substances. The designation of the microscope as a compound microscope indicates that it has an ocular that focuses on a virtual image of the subject produced in the tube of the microscope by the objective lens.

The professional petrographic microscope has a substage condenser that can be centered and focused. The substage has field and aperture diaphragms. The polarizing components are the upper and lower polarizing devices, the Bertrand lens and its mounting (between the upper polarizing device and the ocular), an accessory flip-in lens for convergent light mounted as the top element of the condenser, and a graduated rotating stage with a removable click stop that can be activated to indicate a 45░ rotation from any selected direction. The focus knob(s) and the stand are graduated to permit the determination of thickness by differential focusing. Figure 12-1 shows a petrographic microscope.


Figure 12-1 PETROGRAPHIC MICROSCOPE

The first polarizing devices were prisms fabricated from crystals of the mineral calcite specially cut and cemented back together according to the plan of Nicol (Nicol, 1828, as cited in Johannsen, 1968). Such devices were originally called nicol prisms (Johannsen, 1968). Now they are called nicols. Crystals of calcite, CaC03, were chosen because light that goes through this substance in certain directions is split into two distinctly different rays strongly polarized at 90░ to each other that travel at widely different indices of refraction; thus, calcite has a high birefringence. Nicol used the plane on which he cemented the crystals back together as a plane of total reflection for the ray with the index of refraction most different from that of the cement.

In modern petrographic microscopes, the nicols are polarizing plates fabricated in much the same way as are the lenses in polarized sunglasses. If two polarizing plates of sufficient thickness and quality are superimposed with their polarization directions at right angles to each other (thus,crossed nicols), no light can penetrate the pair. This is because the first polarizing plate excludes all light that is polarized perpendicular to the direction of the plate polarization, concomitantly polarizing the remaining light parallel to the plate polarization, and then the second plate does likewise. Together, all light is excluded. An indication of this effect can be observed by looking through two polarized sunglasses lenses superimposed at 90░ to each other. Polarized sunglasses work because all reflected light is (at least partially) polarized parallel to the substance from which it is reflected and the polarizing material prevents the passage of rays polarized in a horizontal direction, such as that reflected from puddles, snow, automobile surfaces, and pavement. The effect may be observed if you look through a polarized lens at a patch of reflected glare while turning the lens to various orientations relative to the polarization of the glare. (The sky is polarized by reflection from the molecules of the atmosphere.)

A properly adjusted petrographic microscope with nicols of sufficient quality allows no light discernable to the eye to penetrate the upper and lower polarizing devices when their polarization directions are at 90░ to each other and there is no birefringent substance between them. However, if a birefringent substance (such as a crumpled piece of cellophane) is placed between two polarizing plates that are positioned with their polarization directions at 90░ to each other, the birefringence of the inner substance polarizes the light that travels through the first polarizing plate in directions parallel to the optical directions of the substance and the optical system will transmit light. The intensity and color of this transmitted light are controlled by the birefringence, optical orientation, and thickness of the interior substance. The lower and upper nicols, or polarizing plates (called the polarizer and analyzer, respectively), of the petrographic microscope act as do the plates of polarized material. If both nicols are in the optical path and are oriented at right angles to each other and a birefringent specimen material is on the stage, the amount of the birefringence and many other optical properties may be determined.

In the petrographic microscope, the light is collimated by the condenser into a bundle of beams all parallel to the optic axis of the microscope. The specimens examined are transparent-to-translucent thin sections or grain mounts of the material under study (see 5.3 and 5.4). The light beams are polarized in one direction (by the polarizer) before the light reaches the specimen. This light is called plane polarized light.

The direction of polarization produced by the polarizer varies from microscope to microscope (usually north-south) and is often adjustable. The analyzer may be placed in the path of the light after it leaves the specimen whenever birefringence or optical directions are being determined. The analyzer is identical in nature with the polarizer, and for most work, the polarization of the analyzer is perpendicular to that of the polarizer (i.e., the analyzer polarization is east-west). In the standard orientation, the analyzer allows the transmission of light that is not polarized at right angles to the light from the polarizer.

When both nicols are in use in this standard orientation, the object on the stage is said to be viewed with crossed nicols. In addition, most petrographic microscopes are equipped with a slot at 45░ to the main polarization directions and various retardation plates that can be used in this slot in the determination of a number of optical properties. The most common and most useful of these plates is the 1/4 wave plate, or gypsum plate.

12.2 USES

The uses of the petrographic microscope include identifying translucent substances by means of their optical properties and by reference to the various charts and tables in the literature (Bloss, 1961; Kerr, 1959; Larsen & Berman, 1964; Rogers & Kerr, 1942). Thus, the composition and identity of these substances and the relationships between various phases of the material under study may be discovered. From these data, facts concerning the history and method of formation of the subject material can be deduced. The optical properties of substances given in texts, charts, and graphs have generally been accurately determined by the use of a universal stage on which the substance can be oriented at any desired angle to the optic axis of the microscope and the plane of rotation. In this manner, the optical properties have been determined for various substances. The concrete petrographer does not usually try to attain this degree of accuracy.

12.3 PROCEDURES

Courses on the use of the petrographic microscope are available at most colleges and universities that have departments of mineralogy or geology, in most departments of materials engineering, and in some departments of chemistry. The text books available on this subject vary widely in their emphasis. Certain texts concentrate on the theories of the behavior of light in various types of crystal structures, use of the Bertrand lens, and various optic axis figures. Some are oriented toward identifying and naming the minerals; others concentrate on teaching recognition of individual types of rock.

The recommended procedures vary from one author to another. For example, Bloss (1961) taught that birefringence is the most important property by which to identify a mineral substance. His book has charts and graphs that start out with the determination of this property and then branch to include other properties. The charts and lists of Larson and Berman (1964) have their first subdivision of lists of minerals on the number of optic axes possessed by the substance, the second subdivision on the optic sign, and the third on the index of refraction. Deer, Howie, and Zussman (1962) and Palache, Berman, and Frondel (1951-62) wrote important mineralogic reference books that list the optical properties of minerals in the part of the text that describes the mineral under discussion.

The standard texts written for geologists and mineralogists assume that all thin sections are 25 to 30Ám thick. With thin sections of HCC and similar materials, the grain size is so small that much thinner sections are often desirable (see 5.3.1). Unless highly specialized equipment is used, it is impossible to produce ultrathin sections that are the same thickness across their areal extent. Often, a thin section will vary in thickness from 20Ám in one area to nothing in another. This lack of flatness will, at first, seem objectionable to the average classically trained petrographer, but once having become accustomed to, the microscopist will realize that the Lack of flatness can allow a mineral substance to be viewed in a greater variety of ways than if the section were the same thickness everywhere.

The identification of aggregate minerals and rocks and concrete reaction products may usually be most efficiently accomplished by knowing which mineral substances are likely; noting the outward physical properties, color, cleavage, and hardness either in a hand specimen or with the stereomicroscope; and using the petrographic microscope to observe the general appearance in polarized light to determine the approximate birefringence, indices of refraction, and some of the other optical properties. The procedures involved include determining of some of the following optical properties, listed in order of most common usage:

Many identifications can be made from the general appearance, parting, color, cleavage, estimated index of refraction, and approximate birefringence. The experienced petrographer can accurately recognize a large number of minerals by observing the general appearance in plane polarized light and the birefringence as viewed with crossed nicols. Of course, unfamiliar substances will require the determination of a number of various properties before identification can be made.