Cochise College Student Presentation

Roger Weller, geology instructor

wellerr@cochise.edu

Creating Thin Sections
by Jill Forsythe
text equipment process chart samples references

Text of Report

A thin section is a very thin slice of a rock which has been glued to a microscope slide.

A thin section is created to aid in the identification of minerals. Quality thin sections are

about thirty microns thick. They are then examined through both ordinary and polarized

light. As the light bends passing through the minerals it creates colors and patterns that

help to identify the minerals present in the thin section. The pattern created is distinct for

each mineral and is related to the specimen’s crystal system. This will only work for

transparent minerals.

All non transparent minerals, such as sulfides, are made into polished sections instead of

thin sections. Sulfides are never transparent and will show up as black on a thin section.

A polished section is viewed in refracted light because no matter how thin they get they

will never be transparent.

Thin Section Equipment

In order to make thin sections you need several specialized pieces of equipment. The

first piece of equipment used is a masonry saw. This saw has a constant stream of water

to aid in the cutting of the specimen. The diamond blades for the saw can vary in

thickness according to the specimen you are working with. Very thin blades are used for

more valuable specimens so as to cut down on the mineral waste. The regular size saw

blades will not cut fingers unless they are trapped between the blade and the deck of the

saw.

The second piece of equipment is a lapidary wheel. This wheel also has water available

and water close by to wash the specimen to check for progress. Most people have several

lapidary wheels for different stages of grinding. Polished sections also take other types

of lapidary wheels. The usual divisions of lapidary wheels for polished sections are 6, 3,

and 1 micron wheels. Again there are several wheels for the different stages of grinding

and finish. You use these wheels with oil instead of water.

Silicon Carbide, or grit, is used on the lapidary wheel to smooth the thin section to the

desired thickness. 400 grit sands to 40-50 microns. 600 grit sands to 20-40 microns.

Diamond paste can get to ½ micron. This is used so you are reflecting only the minerals

themselves. The desired thickness for a thin section is 30 microns.

A press is a machine used to press granular pieces together to create either thin sections

or polished sections.

A specialized hot plate is used to heat up the specimen and to remove all traces of water

from the specimen.

A polarizing microscope is very important for identifying the minerals present in the

specimen. A polarizing microscope has two polarizing filters, one below the stage and

one above it. This is important as polarized light consists of vibrations in one plane only

– the plane of polarization. This alleviates the glare and makes the crystal structure and

mineral pattern more obvious. A Petrographer reads the thin sections from these

microscopes. This is also an art and requires careful study of both the specimen and

charts.

The Thin Section Process

To make a thin section you first choose the rock specimen or core sample you are going

to work on, after documenting where the specimen was obtained.

You mark on the rock where you want the thin section to be cut or start cutting using the

masonry saw in order to remove a workable section of the rock. The piece you choose is

chosen for the sample of crystals, veins, and markings you want to view. Be sure to mark

the rock and all pieces cut from it so you can go back and get more sections if needed.

Using the masonry saw, you cut the rock into a piece about 4 x 2.5 centimeters. The

thickness at this point is about one centimeter.

After cutting, you grind your specimen down on the lapidary wheel to smooth the side

you plan to put on the slide. Carefully holding the section you have made, turn on the

lapidary wheel and sprinkle it with 200 grit silicon carbide. The 200 grit smoothes the

section down fairly quickly depending on the hardness of the mineral. Then move on to

another wheel and use 400 grit to smooth it down further. Rinse the specimen, and then

grind it again using 600 grit. By now you have a smooth specimen to glue to your slide.

It is still about the same thickness as when you started, but you have a smooth edge.

As the Silicon Carbide number increases the size of the grit granules are decreased to get

an increasingly smoother finish. To make polished sections you use 800 and 1000 grit.

You then finish the polished sections off with diamond paste, which also comes in

different grits.

The special hot plate is used to heat the rock sample up to 340 degrees. This removes all

water and heats the sample up enough to use the cement to mount it to a slide. Both the

rock and the slide are placed on the hot plate to heat up. After all the water has been

dried out and the specimen is heated, you use Thermoplastic Quartz Cement to adhere the

specimen to the slide. This thermoplastic quartz cement is used because it is totally

visual integrity is not affected which is very important. First put the glue stick on the

rock specimen and smooth it around to get a good surface. Then put the glue stick on the

slide and smooth it around. Place the two glued sides together so that you have a rock

sample, glue, then a slide. Push on the slide to remove all air bubbles.

After your rock specimen has cooled, you go back to the masonry wheel and cut the

specimen again to get it thinner. You use a guide to help hold the specimen steady. This

time the rock is sliced to about one eighth of an inch thick.

You then grind the rock again on the lapidary wheel. This is more of an art than a

science. You need to feel how fast the specimen is grinding down and how evenly it is

grinding. You need to get the pressure correct to get the desired thin slice. You continue

grinding up to the 600 grit in order to get a smooth clear thin slice. The desired thickness

is 30 microns which is about 1/1000 of an inch thick. Thirty microns is about single

crystal size. To help put pressure on exact areas you can use a pencil eraser. To polish

most rocks thinner than 30 microns creates a boring slide.

Check the slide in a microscope. Almost all specimens have quartz in them so you use

the quartz as the determining factor to decide if your sample is thin enough. If the

microscope shows too many colors in the quartz sample, you need to grind it some more

in order to thin the slide to 30 microns. Quartz at 30 microns is clear to grey to a light

yellow depending on the sample.

If you only have grains for a specimen, then you first press the sample together, then

slice, and grind on the wheel. An example of the need to check grains is when you are

checking tailings in an old mine. You check the tailings to see if enough mineral is

present to make it viable to reprocess.

When you have the correct thickness, then use perma mount to put a cover on the slide.

The perma mount is smoothed evenly before you put the cover over the slide; you then

press it to remove the air bubbles. You slide is now ready for saving and reusing.

Professional Use of Thin Sections

A petrographer is a professional who looks at thin sections of rock and identifies the

shape and color of the crystals, the presence of cleavages, and twinning or zoning of the

crystals. The petrographer uses the polarized microscope with the refracted polarization

and cross polarization to compare the colors. By doing this the petrographer decides

which minerals are present. When mineral identification has been completed, a decision

can be made to mine or drill further or to move to another location. Core samples from

one area can have different minerals present at different depths. Depending on the

mineral present and the concentration of the mineral, a company or person will know if it

is a money making area to work.

Thin Section Rock Specimens

Gabbro #4 from Portal, Arizona area. The thin section is mostly black and white. It is probably biotile and olivine. There is no quartz in it.

Basalt #9 from highway 80. This is quartz and augite.

Rhyolite #11 is from the Chiricahua Mountains. It is like granite and has been cooled in the earth. It has quartz and feldspar.

Scoria #2 road cut on highway 80. It is augite and glass.