Geology on a Low Budget

Examining and photographing rocks under the microscope can be very expensive. A good petrological microscope will cost a very large sum of money. It is, however, possible for amateurs to achieve reasonable results on a lesser budget running into hundreds rather than thousands of pounds. This sections looks at ways of doing this.

Microscopes

Making thin sections

Photomicroscopy

Microscopes

An appropriate microscope is an indispensable tool for a geologist. The bare minimum specification is that it should have

• a rotating stage marked out for 360 degrees,

• which can be centred accurately so that the grain to be examined stays more or less in the same place when the stage is rotated. Without this accuracy it is impossible to measure extinction angles or pleochroism.

• The ability to use crossed polarised light.

• Several objectives, one at low magnification for general viewing, one at medium for close examination and checking the Becke line, and one at high magnification for viewing interference figures. Typical objectives are at X4, X10 and X40. These figures should be multiplied by the magnification of the eyepiece (usually X10) to get the actual magnification.

It is perfectly possible to obtain a microscope with the above minimum specifications for a few hundred pounds. The technically-minded can even adapt a cheaper biological microscope to take a rotating stage and polarising plates.

An inexpensive trinocular microscope can be used. Two eyepieces make for more comfortable viewing, and a digital SLR camera can be attached to the third tube. The basic microscope should come with rotating stage which must be able to be centred using built-in adjustment screws, and polarising plates (polariser below the stage, and analyser between the eyepieces and the objectives). The polariser is normally adjustable through 360 degrees. If the analyser is not adjustable, it may be necessary to dismantle the headpiece and adjust the angle of the analyser so that the polars cross accurately on a North-South axis. Cheap microscopes may have the disadvantage of having no slots for compensation plates or Bertrand lens.

Various accessories may be purchased and added to improve the microscope’s performance. For example…
• A X10 eyepiece with crosshairs and measuring graticule.
• A better sub-stage condenser with swing in third lens.
• A 20 bulb led light. To use this light, the base of the microscope with its built-in lighting system may have to be removed, and the microscope placed on a wooden base under which the led light may be centred. This can give a greatly improved viewing quality, make Becke lines much clearer, and is capable of producing interference figures.
• Mica (quarter wave) and gypsum (full wave) compensation plates. A slot above the objectives is not needed to use these. They can be inserted between the polariser and the bottom of the stage to give the same effect.

A simple phase telescope may be made, which is inserted into one of the eyepiece sockets (the eyepiece having been removed). This makes interference figures slightly larger and clearer, and replaces the need for a Bertrand lens.

Ingredients:

• A short length of 22mm copper plumbing pipe.

• Another tube (about 19mm) which fits snugly inside the previous one but so that it can be moved in and out. DIY stores usually stock a variety of metal tubes which will fit.

• A X10 eyepiece (to be cannibalised).

Method:

• Cut each tube to 8 cm lengths, and smooth the ends with a file.

• Remove the two lenses from the eyepiece. Note from which end of the eyepiece each lens comes.

• Glue the flat end of the upper lens to one end of the 22mm tube.

• Glue the flat end of the lower lens to one end of the 19mm tube.

• Place one tube inside the other and insert into the eyepiece socket so that the lens of the 19mm tube touches the base of the eyepiece socket. The outer tube can be pulled out or in to adjust the focus.

This image shows on the left the two elements of the phase telescope, and on the right, how they fit together and into the microscope eyepiece socket.

Making Thin-sections

Cutting thin rock sections to the required thickness of 0.03mm is essential. Unless you are lucky enough to have access to a high-tech professional cutting machine, the most important pieces of equipment are:

• a water-cooled tile cutter with a glass-cutting blade and an adjustable clamped ruler

• ear protectors because tile cutters are usually very noisy

• goggles to protect the eyes against splinters

• a wooden block to hold a microscope slide so that the rock can be cut accurately

• microscope blank slides and cover slips
• clear drying epoxy resin. A 5 minute epoxy can be used to save time but this does not allow enough time for all air bubbles to escape. For the best results use a slower epoxy.

A jeweller’s lapping machine with a variety of lapping plates (different grit sizes) takes most of the hard work out of grinding and polishing but is expensive. It is perfectly possible to replace this with a flat glass plate, a variety of powdered silicon carbide grits and plenty of hard work and patience. As long as the section has been cut thin enough to start with, it should be possible to polish down the section manually in an hour or so to the required thickness. Even when using a lapping machine, the final stages must always be done by hand.

The following methods seem to work:
• Cut off a piece of rock so that one side is reasonably flat.
• Polish the flat side with the lap or on the glass plate until it is smooth. 100-200 grits can be used for this. If you make it too smooth with finer grits it may not stick as readily to the glass slide.
• Abrade one side of the glass slide with 600 grit wet and dry paper.
• Wash the cut rock and the slide thoroughly and allow to dry.
• Mix up the required quantity of epoxy in room temperature. Spread on the abraded side of the slide and on the smoothed surface of the specimen. Press them together and leave to prove. Room temperature is important for this process. In cold workshop temperatures, the epoxy will become more viscous and will trap more air bubbles.
• Once the epoxy has proved, place the slide in the slot in the wooden block, adjust the ruler on the tile cutter, and cut off as much of the rough side of the specimen as possible. It should be possible, using this method, to render the section translucent which reduces the time needed for grinding, and to ensure that the section is of even thickness. Be careful not to cut so finely that the glass slide becomes damaged.
• Do not leave too long between gluing and polishing because the epoxy can become brittle.
• Now using either the lapping machine or the glass plate, polish down the section using progressively finer grits (start at about 150) until it is about 0.03mm thick at which point quartz should show a maximum interference colour under crossed polars of pale straw yellow and feldspars should be grey to white. If one part of the section is thicker than the rest it may be necessary to compensate by placing greater pressure on the thicker part.
• Use 2500 grit and 3000 grit wet and dry paper to polish out minute cracks and chips in the section. You can finish off with a polishing powder (cerium or tin oxide).
• Wash and dry thoroughly, and then glue on a cover slip.

If you are using a lapping machine, use the following techniques:

• Grip an end of the slide on both slides with two fingers and use a finger of the other hand to press the slide down on the lapping plate. Hold the slide firmly as the friction of the lap will try to pull it out of your hands. The finer the grit, the greater the pull.

• If necessary move the finger pressing down on the slide to compensate for uneven thickness in the section

• Never put the section to the lap for more than a few seconds. Then check for progress. It is very easy to wipe away the whole specimen. With experience and care it is possible to lap a section down very close to the required thickness.

• After each application of the section to the lap, turn the slide round and grip from the other end. This helps to achieve an even thickness.

Be patient! You will undoubtedly drop and break slides, polish sections off completely, and cut through the glass. It gets easier with practice.

Photomicroscopy

A digital SLR camera with removable lens and which is capable of remote control from a computer, gives the best pictures. The camera can be attached with its lens removed to one of the microscope eye tubes with the appropriate fittings and a low magnification eyepiece (X2 is ideal) so that a wide viewing angle is achieved. The camera is then linked by a special USB cable to the computer.

A USB microscope with built in camera for taking still photographs as well as movies can also achieve good results, if it is least of 5 megapixel resolution. A few USB microscopes with 9 megapixel resolution have become available. These give a result almost as good as a digital SLR camera. High resolution USB microscopes such as these are capable of showing up cleavage and fracture traces on a thin section which are not as obvious through other methods.

A laboratory retort stand can be used to hold the USB microscope in place. Alternatively, universal microscope stands with mechanical stages and inbuilt diffused lighting are also now available. These hold the microscope on a ratchet by which it can be moved up and down to achieve good focus. It is possible to add a home-made rotating stage which sits over the mechanical one.

The left-hand image shows the complete rotating stage. The right hand image shows the top plate removed and upside down to show how it fits together. This stage was milled in a school laboratory workshop. If access to specialist metal-working equipment is unavailable, a rotating stage may be improvised with several old CDs glued together, a short piece of 22m copper tubing glued underneath, and a couple of 22mm plumber’s olives glued to a stand into which the copper tube fits!

If LED lighting is used to illuminate the slide, a unit with many bulbs should be used. The LED lights readily available in DIY stores are not really suitable because they do not spread the light evenly. The lights should be shielded with a plastic diffusing material such as is used to frost windows.This can be purchased from DIY stores

In this image, diffusing material has been glued to a glass plate which can be inserted above the light.

The advertised maximum magnification of the USB microscopes can only be achieved if the plastic lens protector is actually resting on the object to be viewed. This means that the only way to get a thin section focussed is to file off a small portion of the plastic protector so that the lens is close enough to the thin section.

Polarisation can be achieved by using plastic polaroid which can be purchased cheaply online. If the USB microscope lens is already polarised, only one sheet of polaroid is needed which can be inserted at the correct orientation just above the light and below the stage. If the microscope lacks this, a piece of polaroid can be fixed between two thin pieces of glass. A small amount of superglue around the inner edges of two microscope glass slides will hold it together. This can then be fixed to a microscope cap and used as the analyser.

The polarising film has been glued between two microscope slides which have in turn been glued to a filed-down cap from the USB microscope.

No vestige of a beginning, – no prospect of an end

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