THE PETROGRAPHIC MICROSCOPE

Basically, a polarizing microscope in transmit light includes:

• an illuminator, source of a parallel beam of white non-polarized (circu-lar po(circu-larized) light;

• the lower polarizer (or Nicol) that transforms the incident beam into plane polarized light. This first polarizer may be rotated. The cross hairs of the reticle (at right angle) materialize the polarization plane of the polarizers. Depending on the construction of the microscope, polariza-tion plane of the first polarizer may be E–W or N–S;

optic axis ε

optic axis ε

circular section ω

U+ U−

Figure 2.3 Indicatrix: uniaxial minerals.

• a stage bearing the thin section. The stage can rotate freely around the axis of the microscope and can carry a mobile holder (mechanical stage) for moving the thin section and precisely locate a point. The edge of the stage is graduated so that the angles can be measured; most models are also equipped with a vernier for precise measures;

• the objective, actually there are a series of objectives with different mag-nifications placed on rotating nosepiece. The observations are made from the lowest magnification to the highest one. You must take care to handle the nosepiece at the base and not by objectives, which may decenter the objectives;

• the upper polarizer, or analyser, is removable and whose polarization plane of is at right angle to that of the lower polarizer. You verify that the two polarizers are at right angles by observing that no light emerges from the analyzer when the two polarizers are crossed;

• the ocular.

Various removable accessories are installed on the microscope:

1 a diaphragm placed over the light source used to modulate the size of the light beam; it is used for increasing the contrast between two fields of the image (for instance for a Becke line);

2 removable auxiliary condenser lenses which converts the incident paral-lel light beam (orthoscopic illumination) into a conical one (conoscopic illumination); it is used to make interference figures in convergent light;

3 accessory (compensator) plates, that are introduced into a slot located between the objective and the analyzer. The accessory plates have a known birefringence. The most useful is a quartz plate whose birefrin-gence is equal to δ = e(nγ − nα) = 0,018 (for a thickness of 30 microme-ters) (which corresponds to purple/indigo color that marks the boundary between the red at end of the first order and the blue of the beginning of the second order in Michael Lévy’s interference color chart). The acces-sory plate is used to determine the sign of elongation of a section or the optical sign of a mineral;

4 the Bertrand lens, removable, is used to observe interference figures in convergent light.

An objective lens must be centered so that the axis of the microscope coincides with the rotation axis of the stage. It is centered either through rotating rings located on the objective or small screws at the lens collar or in the rotating nosepiece. The method involves: identify a distinctive point in the thin section, place it at the intersection of the cross hairs of the reticle and rotate the stage of 180°. If the objective is not centered, the identified point is no more located at the intersection of the cross hairs. Then move the

objective so that the intersection of the cross hair moves halfway between its initial position of the identified point and its position after rotation, and do it again as many times as necessary. The exercise may be especially irritating.

Generally, a rock is made of several minerals. A thin section shows dif-ferent sections of each mineral. The most difficult task for the beginner, is to recognize the different sections of the same mineral: for instance, a muscovite cut parallel to the cleavage does not look the same as muscovite cut perpendicularly to the cleavage. The observations intend to reconstruct, from these different sections, a 3D image of the mineral, its structure (forms, cleavages, twins), its indicatrix and the position of the indicatrix in relation to its structural elements.

For this purpose, there are three types of observations:

1 Observations with one Nicol, the lower polarizer, the incident beam is plane polarized, these observations are called observations in parallel polarized light (here abbreviated into PPL) (it is called in French “obser-vation in natural light” or NL).

2 Observations with two Nicols, the lower and upper (analyser) polarizers crossed at right angle. In the absence of a thin section, no light should be observed through the ocular. When a thin section is interposed between the lower and upper polarizers, the vibration coming out of the ana-lyser is not nul. The theory of this phenomenon will be explained in the following paragraph. These observations are called observations in cross polarized light (here abbreviated into CPL) (it is called in French

“observation in polarized light” or LP).

Ocular

Upper polarizer (analyzer) Nicol prism the plane of polarization is

orthogonal to the one γ is at 45° of the plane of polarization

of the lower polarizer with a known e (γ − α) (most used is a quartz wedge

with e (γ − α) = 0,018 boundary between 1st and 2nd order) stage with

the thin section plane

polarized light condensing lens

(transform the normal incidence beam of parallel light

(orthoscopic illumination) into a conical converging beam

(conscopic illumination)

Figure 2.4 The petrographic microscope.

3 Addition of the condenser converts the incident parallel beam into a cone beam and creates an interference figure that can be observed by either removing the ocular or by adding the Bertrand lens. There is no particular name for such observation in English; in French it is called

“observations in convergent light” (LC).

Although opaque minerals are not determined with the transmit light microscope we may still have some indication in considering the form of opaque minerals (cubic habit for magnetite and pyrite, elongated habits for ilmenite, for instance) and also by removing the lamp from its slot and illu-minating the thin section from above: One recognizes the yellow brass color of pyrite, bright yellow to orange of chalcopyrite, the grays of ilmenite and magnetite. This does not replace a serious determination with the reflected light microscope.

2.3 CRYSTALLINE PLATE WITH PARALLEL