Page 100 - Atlas of Sedimentary Rocks Under The Microscope

Appendix 1

Preparation of a

thi n section of a rock

It is sometimes believed that complex and expensive equipment is another glass slide.
required for making thin sections of rock of standard thickness of The alternative method is to use a material known a� Lake ide 70C
0.03 mm. but as the f o llowing instructions indicate. this is not the case. cement. 1 which is supplied in short rods and must be melted on a
Thin sections can be made by the amateur with a little patience and hotplate. This material begins to soften about 85 ·c. so a hotplate
perseverance. If a diamond saw is available to cut a slab of rock which reaches 10o·c is quite suitable. A flat piece of aluminium or
I 2 mm in thickness. the process is considerably speeded up. However, steel placed on a gas stove or on the clement of an electric cooker at
a chip of rock not more than 8 10 mm in thickness can usually be very low heat can be used for this stage. if !)O electric hotplate is
broken from a hand specimen with a small hammer and then a thin available. A glass microscope slide and the rock specimen should both
section can be made. be heated on the hotplate until they are just too hot to touch, then some
The operations required to prepare a thin section after obtaining the La kcside cement is melted on the flat surfaces of the rock and the slide
fragment of rock arc set out below. by touching them with the rod of Lakeside cement.
Using I 00 micron particle size ( 120 grade) carborundum abrasive. Whether the cold-setting epoxy resin or the Lakeside cement is used,
one surface of the rock fragment is ground flat on a piece of glass the procedure is the same at this stage in that thc flat surface oft he rock
measuring about 30cm x 30cm and up to I em in thickness; ordinary chip must be attached to the glass slide with no air bubbles between the
window glass is satisfactory if thicker glass is not available. Only a two surfaces. The rock chip is placed on the glass slide and, with a
small amount of carborundum (half a teaspoonful). just moistened �light pressure and circular movement. the excess mounting material
with water. is used for grinding. If too much water is present the and air bubbles are squceLcd out. The slide is then turned over to
carborundum tends to extrude from underneath the rock. and in observe whether any air bubbles have been trapped between the rock
con�cquence is much less etrective for grinding. and the slide: any bubbles must be gently extruded by pressure and. in
After grinding with a rotary movement for about half a minute, the the case of the Lakeside cement, this has to be done before the cement
noise of the grinding changes because the carborundum grains lose cools and becomes too viscous for the bubbles to escape easily.
their sharp cutting edges. The glass plate is washed clean and a fresh However. it can be reheated to render it fluid again. With the epoxy
slurry of carborundum made on the plate. The time spent on grinding a resin. since the hardening takes place over a period which depends on
flat surface will of course depend on how irregular the surface of the the variety, more time is av<Jilable for extruding the air bubbles. but in
rock chip was to begin with. this case the sample should not be heated because this only speeds up
When the surface of the rock is flat. the sample should be thoroughly the hardening process.
cleaned w1th a jet of water before gnnding with a finer grade of If a diamond saw is available the rock fragment can now be cut from
carborundum. The second stage of grinding should be carried out with II!> original thickness of 5 1 0 mm to about I mm. otherwise it must be
60 m1cron site (220 grade) carborundum and two periods of grinding. ground by hand. Its thickness :.hould be reduced to about 0.2 mm (200
for about a minute each. with a fresh quantity of carborundum is all micron:.) using 100 micron site carborundum: at this thickness it IS
that i� required at this stage. pos!>iblc to see through the transparent minerals. Carborundum of 60
After washing. a final grinding of one surface is made for about a micron size should be used to reduce the thickness from 0.2 mm to
minute with 1 2 micron size carborundum (3F grade). Again, after 0. I mm and at this stage quartt and fe ldspars should show bright
cleaning. the rock sample may be polished using cerium oxide (O.H second-order interference colours when examined under crossed
micron size) but this is not essential. polars.
The next stage is to glue the smooth surface of the rock to a The final stage of grinding from 0.1 mm to 0.03 mm is accomplished
microscope slide in one of two ways. It can be achieved by using a cold­ using 12 micron size carborundum. This is the stage in the whole
setting epoxy resin. which usually consists of two fluids which must be process of section making which requires the most skill. The grinding
thoroughly mixed. The maker's instructions for using these should be ha� to be done very carefully to ensure that the section is of uniform
fo llowed carefully because these matcriab should not be allowed to thickness over its "hole area. otherwise the edges tend tO be ground
come in contact \\ ith the skin and the vapour �hould not be inhaled. preferentially and become too thin. The slide must be examined
The refractive mdiccs of epoxy resin\ vary but most arc somewhat between each stage of grinding to ched.. on the uniform reduction of
1
higher than the value of . 54. For any work involving comparison of the interference colours.
the refractive index of mincrab with the mounting material, the
refractive index of the cold resin should be ascertained. The chief 1 l.akeside cellll'lil i.1 llw pmprit•1ary 11111/lt'.for a malerialmanufaclured in /he USA
disadvantage of using an epoxy resin is that it is very difllcult to a/111111//rl,ed !11 llw Uni1ed l\i11Kd11111 h.r f'roduC ' IitJII T e ch11iques Lui .. II ' lm•i.llock
remove. if, for example. it became necessary to transfer the rock chip to f<olf(l. Flee/. Hampshire.

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