The petrographic microscope

A petrographic microscope is used to observe a series of characteristics in a mineral which reflect its properties and allow us to identify it. The petrographic microscope is a compound microscope which can work with plane polarised light, meaning that it has some peculiarities. This is always done with transmitted plane polarised light, meaning that the polariser must be inserted. The type of illumination varies according to the feature to be studied, and may be orthoscopic (parallel, without the condenser) or conoscopic (convergent, with the condenser incorporated). The size of minerals that allows for optical identification is not samaller than 0.010 mm. Identification of cryptocrystalline and amorpous materials can be achieved using submicroscopic techniques such as a scanning electron microscope.

The petrographic microscope

A petrographic microscope is used to observe a series of characteristics in a mineral which reflect its properties and allow us to identify it

The petrographic microscope is a compound microscope which can work with plane polarised light, meaning that it has some peculiarities

This is always done with transmitted plane polarised light, meaning that the polariser must be inserted

The type of illumination varies according to the feature to be studied, and may be orthoscopic (parallel, without the condenser) or conoscopic (convergent, with the condenser incorporated)

The size of minerals that allows for optical identification is not samaller than 0.010

mm Identification of cryptocrystalline and amorpous materials can be achieved using submicroscopic techniques such as a scanning electron microscope

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This is the physical support to the other elements of the microscope

Attached to it are the mechanisms which move the stage and focus the sample

Other accessories are also joined to it by the same bracket as the stage, such as the condenser and the polariser

Illumination system(Hệ thông chiếu sáng)

This is in the base of the stand at the foot of the microscope In research microscopes,

it is made up of a Light source (1), a set of Lenses (2) which allow a parallel beam of light to be obtained to avoid the loss of intensity by dispersion, an anti-thermic filter (3) which prevents the other elements from overheating, a set of chromatic filters (4) which allow the chromatic characteristics of the light to be modified, a mirror (5) to orient the light beam in the correct direction and an iris diaphragm (6) to regulate the

light intensity and breadth of the beam.

Polariser( Kính phân cực)

This is situated immediately above the illumination system and below the condenser, although it is connected to the stage and the condenser by the same upright bracket

Its function is to convert the natural light from the illumination system into plane polarised light

The plane of vibration of the light in the polariser can be turned in some kinds of microscopes but in normal working conditions it is always fixed at 0°, often

coinciding with what could be called the "East-West" direction

The polariser is always positioned in the pathways of the light rays for the study of any optical property

Polarised light is produced by the polariser and analyser, both of which in modern microscopes consist of a sheet of plastic (polaroid) which absobs all light except that vibrating in one direction Older microscopes employed an ingeneous combination of calcite prism to produce polarised light (described first by W Nicol and known as Nicol prisms)

Condenser(Tụ sáng)

Situated between the polariser and the object, it has a removable lens (1) in such a way that when it is fitted in the "ON" position it makes the light rays converge onto the thin section placed on the stage of the microscope In this situation one speaks of

convergent light or, more normally, conoscopic illumination

By contrast, when the convergent light lens is in the "OFF" position, the light rays no longer incide convergently but rather folow an approximately parallel path and all of them incide perpendicularly on the slide In this situation one speaks of parallel light

or, to be more correct, orthoscopic illumination

An iris diaphragm allows the illuminated area to be varied (known as the aperture diaphragm) For observing relief and Becke line it is usually necessary to partially

close this diaphragm

Stage(Bàn soi)

This serves as a support to the thin section (1) which is to be studied It may be

heightened or lowered (2) to allow the object to be focussed

It is round and can be rotated on a vertical axis which passes through its centre

It is graduated and on the outside it has a fixed goniometer (3) which allows the value

of the angles turned to be measured accurately

Objetives

These are the lens used for magnifying the specimen on the stage Four or five are normally supplied (x4, x10, x25, x50)

This allows a real and inverted image to be obtained of the object under examination

It lets the polarised light pass through without affecting the polarisation plane

Objectives are often mounted on a revolving objective holder (1), which allows

quick and easy change of magnification

When the stage is rotated, the axis of rotation should coincide with the centre of the field of view (the axis of the microscope) This is achieved on some microscopes by

adjusting a collar on the barrel of each objective (2)

To centre a microscope, the point about which an object is seen to rotate when the stage is rotated must be brought to the centre of the cross-hairs by adjusting the centring screws

El enfoque de la imagen en el microscopio se realiza separando el objeto a estudiar de los objetivos Mediante unos anillos (1 y 2) se puede subir y bajar la platina para buscar el foco (en los microscopios antiguos la platina permanece fija, siendo los objetivos los que se desplazan) En este microscopio existe un amilla "macro" de desplazamiento brusco (1), para aproximar el enfoque, y uno, llamado "micro" (2), para ajustarlo

La operación de enfoque requiere seguir unos determinados pasos para realizarla con seguridad El procedimiento correcto es el siguiente Mirando por fuera del

microscopio se lleva el objetivo junto a la preparación y mirando luego por el ocular

se va separando lentamente hasta obtener la imagen Si se busca el enfoque acercando

el objetivo a la preparación se corre el riego de producir fracturas (en la preparación o,

en lo que es mucho peor, en la lente frontal del objetivo) si nos pasamos del plano de foco

Slot for inserting compensators

This is immediately below the analyser and its greatest dimension forms an angle of 45º with the direction of vibration of the Nicols (polariser and analyser)

The main directions of vibration of the compensating plate and those of the polariser and analyser are situated at 45°

Compensators(Bù màu)

These are plates of anisotropic substances, whose planes of vibration coincide with

their two dimensions, length and width

Accessory plates consist of mineral sections of a thickness such that they produce a known amount of retardation

They may be wedge-shaped so that retardation (and thus colour) will depend on the thickness acting at any moment in time

The effects that these compensators introduce are superimposed on the effects that the minerals on the microscope stage introduce

They are used for studying interference figures and the retardation produced by mineral specimens

When required, they are inserted into the microscope tube in a slot between the

objetives and the analyser

Analyser(Phân tích)

This is above the objective, and is made up of a polarising plate, whose height may

be adjusted at will, using a graduated dial (1)

Unlike the polariser, the analyser does not always have a part to play in the passage of the light rays and can be installed or removed at will It is used to study certain

properties but is not necesary for others.

The polarising plane is generally N-S, and it is always perpendicular to the polariser,

such that if there is no object in the way, no light passes and Extinction occurs

Below it is a slot where compensating plates can be inserted

When only the polariser (1) is being used, a normal image is observed, but when the analyser is in place (2), an extinction of light occurs

If an anisotropic substance is in the light's path, the light splits into two rays which vibrate perpendicularly and do not necessarily coincide with the directions of the polariser or analyser When these rays reach the analyser, two components come into being which vibrate, one on the plane of the analyser and the other on a perpendicular one The former is responsible for the grain being seen, whilst the latter is annulled A false colour appears, known as an interference colour (3)

Amici Bertrand lens

This is found immediately below the ocular It may be incorporated (1) or removed (2) at will It can only be used when convergent light is used to observe the property called the Interference figure (3).

The Bertrand lens magnifies and focusses interference figures but the Bertrand lens does not produce the interference figure but modifies the focal plane of the image formed by the objective

to allow it to be focussed and amplified by the ocular.

An alternative means of viewing interference figures is to remove the eyepiece and look down the microscope tube at the highe-power objetive lens, preferably wiyh the aid of a pin-hole stop inserted at the top of the tube.

Eyepieces(Thị kính)

This is a system of lenses fitted to the top of the microscope and whose function is to form a virtual and amplified image from the real image created by the objective The eyepiece assembly contains two cross-hairs, and is slotted into the microscope tube so that the cross-hairs are orientated E-W and N-S, i.e parallel to the vibration directions of the polariser and analyser

Most eyepieces have a magnification of x8 or x10.

The focal plane of the new image is about 25cm from the upper lens, the normal distance of vision of the human eye

Thin sections

In order to observe a sample it is necessary to previously prepare it The method

depends on whether we are dealing with a coherent material (rock) or a loose material (soil and sand).

Preparation of a rock

Cut.

The first step is to cut the original rock in order to obtain a fragment with a flat

surface similar in size to that of the preparation which we require

Once the flat surface is achieved, it must be polished to remove the roughness of the cut and make it as smooth as possible

Cementation

After the surface has been polished, it is cemented onto a glass slide with resin or Canada Balsam

Cut

It is then recut, trying to make the second face parallel to the first and as thin as

possible

Lapping

Next, the sample is lapped until it is only 50µm thick, so that with a final polish its thickness is between 20 and 30µm

Cover

The last stage is to cement a cover glass on top with the same material used as before, trying to ensure that no air bubbles remain trapped in between

Preparation of a soil

In order to obtain microscopic preparations of soils, it is necessary to give coherence

to the soil samples so that they can be cut, lapped and polished

In order to achieve the hardness required, the soil samples are vacuum packed in liquid polyester resin

The resins harden as they polymerise, thus producing blocks which include the soil sample and conserve its natural structure unchanged These blocks can now be treated

as if they were rock samples

Preparation of sands

The way to prepare sands differs according to their grain-size

Fine sands

For sizes smaller than 200µm, which are known as fine sands, this can be done

directly

Coarse sands (>200 µm)

In this cases it is necessary to place them in a resin and follow a similar procedure to that of a rock (2),

Las arenas gruesas tienen un tamaño (>200 micras) que las hace opacas si se montan directamente (1), por lo que es necesario incluirlas en una resina para darle

coherencia Para ello se colocan en un pequeño recipiente de fondo plano y se

incluyen al vacio como se hace con las muestras de suelos (2)

Oscillating theory of the light

Light is a form of radiant energy, and although its precise nature involves very

complex Physics theories, all the phenomena relating to minerals can be explained by

exclusively considering the oscillating theory, i.e for our purposes, light is

propagated as a consequence of a vibration of particles

In the figure, a light wave is represented in diagram form along a rectilinear path that includes the state of vibration of the different points, successively reaching different displacements, from a state of rest The result of this vibration of adjacent points is the propagation of the wave

We therefore have a very important point and one that we must keep in mind: that is,

the directions of vibration and of propagation are perpendicular This is strictly

true for all isotropic media, although in certain anisotropic ones the angle may be different from 90° However, for our purposes we can always consider them as

perpendicular (such an assumption simplifies the explanations without detracting from the essential principles)

We shall now review some very simple concepts relating to this oscillating

phenomenon

Wave

This is a sinusoidal movement caused by the group of vibrating particles

Ray

This is the rectilinear path followed by the wave (the path followed by the light)

Wave length

This is the distance between two points in phase with each other, which are those which are vibrating in the same manner, are at an equal distance from the state of rest and move in the same direction

The different wave lengths are translated by the human eye into different colours:

violet = 410 mµ yellow = 580 mµ blue = 480 mµ orange = 620 mµ green = 530 mµ red = 710 mµ

Frecuency

This is the number of wave oscillations per second, which gives the wave velocity The part of the wave between two points in phase is called oscillation

Velocity of propagation

This is a characteristic of the medium in which light is propagated, and is determined

by the Refractive index (n), or the relation between the velocity of propagation in a vacuum (c) and in the medium under consideration (v)

n=c/v

For this reason, the "n" of minerals is always greater than one (ranges from 1.43 to 3.22) The index of refraction of a vacuum is one, and it is considered the same for air

as well

The velocity and the refractive index are inverse (a high velocity corresponds to a low index)

In anisotropic mediums, as in most minerals, velocity (and thus "n" as well) varies with direction

Natural light and plane polarised light

Natural light (from the sun) vibrates in all directions in space (infinite directions of vibration), and its axis is defined by the ray (which we shall consider as being

perpendicular to all directions of vibration and coinciding with the direction of light propagation)

Polarised light vibrates in a single plane at any one moment in time, but the direction

of the plane of vibration changes with time When it always vibrates on the same plane, it is called plane polarised light (which we shall simply call polarised light)