Methods


Although it is possible to study sands obtained directly from the usual mechanical analysis, better results are obtained if a specific extraction procedure is carried out in order to obtain a larger quantity of sands.
Starting with a sample of 1 of 2 kg of soil, the steps below are followed:

 

Extraction
1. Sieving, using a 0.2 mm mesh sieve, under running water and collecting the fraction retained.
2. Treating with H2O2 .
3. Mechanical agitation for 8 hours in water with sodium hexametaphosphate.
4. Final sieving using sieves sized 2 and 0.2 mm and collecting the fraction that has passed through the sieve 2 mm and not passed through the sieve 2 mm.

 

Concentration
1. Separation of lighter and heavier fractions using bromoform.
2. Separation of magnetic and non-magnetic fractions with a magnet.

 

Mounting
Coarse sands in soils have a grain size that has undoubtedly been a serious inconvenience for the mineralogical study of this fraction. Loose grains cannot be mounted directly onto a slide to be studied in the microscope, as fine sands can, since, due to their size (2-0.2 mm) they will be opaque to the light transmitted and will be seen as black. On the other hand, as they are loose grains, petrographic techniques cannot be applied if they are not previously agglomerated in a compact block. Nowadays, this difficulty can be overcome easily with the aid of plastic resins that harden on polymerisation.

1. Inclusion in polyester resin of the sand samples, using some small plastic beakers with a flat bottom as the containers.
2. After the samples have hardened, the next stage is to cut, trim and polish the plastic blocks containing the sands (the same way as for rock samples) and to obtain the corresponding microscopic preparations.

Mineralogical study

The following are used:
1. binocular loupe
2. petrographic microscope
3. X-ray diffraction, for grains with a doubtful nature.

 

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