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What is a geological thin section?
In many areas of geological study such as mineralogy, petrography, sedimentology and so on, thin section preparation is needed in order for samples to be examined microscopically in order to analyse the characteristics of the soil or rock.
This analysis is normally carried out using transmitted polarised light which creates a need for thin sections of known and exact thickness.
Reflected light microscopy is also widely used for certain applications and this technique requires the surface of the sample to be flat and highly polished for the best results to be achieved. The increasing use of electron microscopes is also contributing to the need for top quality thin sections of a wide range of materials.
Thin Section Preparation Systems
Traditionally, this time consuming and highly skilled process was done by hand. Logitech have revolutionised the process by introducing technology that automates and simplifies the preparation of thin sections - all with much better quality results :
- produce the highest quality thin sections - consistently
- substantially increase thin section preparation output
- virtually eliminate the need for hand lapping & polishing
- make thin sections to widely differing dimensions with equal ease
Geological Thin Section Preparation Process Route
Step 1 : Slabbing and Trimming
If the geological material is solid and not liable to disintegrate, the first operation is to trim the bulk material down to dimensions suitable for slide mounting.
Step 2 :
Impregnation (If required)
Where geological material is soft or friable, and therefore liable to disintegrate, it is first impregnated with resin. THE IU30 allows the sample to be outgassed for as long as is required in the right hand chamber. The resin can then be outgassed seperately in the left hand chamber and poured onto the specimen which remains under vacuum. Once the resin has cured, the material can be trimmed as at stage 1.
Step 3 : First Face Lapping
A standard thin section is 30µm thick uniformly over the rock slice area. To achieve a flat reference surface for subsequent bonding to the slide, one face of the chip prepared at stages 1 and 2, is lapped flat in a conditioning ring under a pressure block and a 3.5 load on a grooved, cast iron lapping plate.
Step 4 : Preparing Slides to Uniform Thickness
The glass slide for supporting the geological specimen material must be parallel and of a known thickness. One face of the slide is lapped flat and parallel on the PLJ2, PLJ7 or CJ30 which can be set to stop the lapping action automatically at the required thickness.
Step 5 : Bonding Specimens to Prepared Slides
The lapped geological specimens may now be bonded to the glass slides. Epoxy resin with an appropriate refractive index is used (e.g. Epoxt-Pack 301). A thin layer of resin is spread on the dried, lapped face of the chips and the lapped side of the slide is placed on the rock surface. Once bubbles have been eliminated, the slide and rock chip are placed in a bonding jig such as the BJ 6.
Step 6 : Thinning Bonded Specimens
At this point, the chips are too thick for final lapping and they require to be trimmed down to a thickness of 300-500µm. This is achieved on saws such as the GTS1, Model 15 or CS30.
Step 7 : Lapping Geological Specimens to Final Thickness
The thinned bonded specimens can now be mounted on a PLJ2, PLJ7 or CJ30 Precision Lapping jig and lapped to the final desired thickness - 30µm for a standard thin section or 40µm if the section is to be polished.
Geological Application Areas
In addition to standard thin section preparation, Logitech systems are versatile enough to accommodate materials requiring special techniques such as :
Soil Thin Section Preparation
The delicate nature of soil demands careful thin section sample preparation to avoid structural damage and disintegration.
Before any processing can begin, water must be removed from the soil. Normally, the sample is left in a well ventilated area for several days until a constant weight is achieved and then dried on a hotplate at 40°C for 48 hours.
The soil is ready for impregnation on the IU30 Vacuum Impregnation Unit (Step 2 of the standard process route). Once the resin has cured, the sample is trimmed on the GTS1 cut-off saw or CS30 saw. A non-aqueous solution, for example ethylene glycol, can be used as a coolant to avoid damaging the soil. Water should NOT be used.
Concrete Thin Section Preparation
Thin sections of concrete are prepared for a number of different tests: porosity determination, inclusion analysis, mineral composition etc.
To make analysis easier, in particular for porosity testing, the pores in the concrete are defined more clearly by impregnation them with dyed resin. Normally the dye is mixed with the resin before being used to impregnate the specimen in the IU30 Vacuum Impregnation Unit (Step 2 of the standard route).
As a result of impregnation, the dyed resin fills all the pores and makes them easily distinguishable from the surrounding material. Fluorescent dyes can be used if required. Whether dyed or not, however, concrete specimens must be either fully or surface impregnated to permit further processing.
Ultra Thin Section Preparation
Ultra thin sections require to be polished on both faces. Trimmed rock chips are first free lapped according to the standard process route up to Step 3. Then they must be "free" polished under load in a conditioning ring on one of the following systems:
When polishing is complete, the rock chips can be mounted, polished side down, on a prepared glass slide (Step 5), thinned (Step 6) and lapped to 25-30µm (cf. Step 7). Thereafter, the lapped section is polished using a PP5 Precision Polishing Jig on a soft metal plate to the required final thickness, down to less than 13µm in some cases.
Fluid Inclusion Studies
Specimens used for fluid incision analysis require to be polished on both sides as the inclusions are so small that they would be obscured by the larger surface features of the lapped surface.
First the chips are trimmed and free lapped (as in steps 1 and 3 of the standard route). After lapping, the chips are also free polished (i.e. under load on a polishing pad or soft metal plate). They can be mounted, polished side down, on a prepared glass slide, either with resin if the finished section is not to be de-mounted (as per Step 5) or with wax or other 'temporary' adhesive if the specimen is to be demounted later. Final thinning, lapping and polishing can then be carried out, exactly as for a standard thin section (Steps 6-8) if the finished thickness requirement is in the normal 80 to 250 micron range or using the techniques described for ultra-thin section preparation.
Coal Thin Section Preparation
Coals are particularly difficult materials to process. They are generally friable, prone to distortion under small stresses, heat sensitive and opaque, thus requiring a section less than 10µm thick to define the structure clearly under the microscope.