The study of rocks. Thin sections

In order to give a scientific name to a rock, it is essential to define its components with precision. Only in this way, in fact, and thanks to the help of constituent diagrams, is it possible to establish the correct petrographic denomination.
This type of investigation, carried out by a geologist who is specialized in petrography, takes place using a petrography microscope that allows the geologist to work in transmitted and polarized light, and considering, therefore, the various characteristics of the individual constituents of the rock (colour, shape, relief, pleochroism, cleavages, propagation of light within it…) he is able to recognize the genesis, the constituents and therefore give it a petrographic name.
In this phase of work, therefore, it is necessary to use very particular equipment which requires adequate preparation of the rock sample that cannot be studied as such unless in a macroscopic investigation, while for our microscopic analysis it must be in thin sections.
But what exactly is a thin section and how can it be obtained? It is a ‘portion’ of rock cut to a thickness of 0.03mm (so very thin!) in order to make sure the greatest number of its constituent minerals become transparent to light and therefore, thanks to the use of the polarizing microscope, it is possible to carry out a series of investigations that will lead to its recognition.
To obtain a thin section it is necessary to start with a fragment of stone that is representative of the petrographic characteristics of the rock to examine and if this is not possible, it is necessary to have more thin sections for the study of the material.
The thin section has a thickness of 0.030Ã‚Â±0,005 mm and standard dimensions equal to 33mm x 20mm, that can be increased should it be necessary to analyze lythotypes with grains of pluricentimetric dimensions. If the rock should result as not particularly coherent, it is necessary to start all the preparation procedure with its consolidation.
Whether it is carried out using special machines or by hand, in synthesis the procedure followed to obtain a thin section is as follows:
a) the starting point is the preparation of the “caramella”, i.e. the piece of rock with a rectified side of dimensions equal to 33x20mm and a thickness of a few mm;
b) the “caramella” is fixed on a slide using a special grade adhesive;
c) at the moment when the rock system and slide are well fixed, the procedure continues with a diamond wheel or by hand until the thickness is reduced to 1mm;
d) from this point onwards it would be preferable to finish the work by hand, and using Carborundum 100 ?m the thickness of the section is reduced to about 200 ?m (with these thicknesses it starts to be possible to appreciate the transparency of non opaque minerals);
e) work is continued using Carborundum of 60 ?m grain, bringing the thickness of the sample in preparation to 100 ?m (quartz and feldspars show second level light interference colours);
f) with Carborundum with 12 ?m grain, the final thickness of 30?m is reached. To be sure of this, the section is checked with the microscope: quartz and feldspar should present their typical grey interference colour, calcspar crystals, instead, are pink.

Having obtained the desire thickness (usually with great difficulty and the constant risk of “losing” the section), the procedure continues with its covering with a second slide, or with a transparent varnish.
It is then possible to proceed in the study of the material using the microscope, thanks to which its texture, constituents and irregularities will be analyzed (according to European regulation EN 12407). Thanks to this information, after also carrying out the macroscopic investigation of the sample, and using petrographic diagrams, the petrograph will be able to give his petrographic definition of the rock which will allow us to place it in an unequivocal way within one of the various families which make up the rock world.

Anna Maria Ferrari