Conclusions: crystal characteristics

Form and specific surface

Form is announced by a more or less equidimensional character of the crystal, that is, when the three main dimensions of an equidimensional crystal are not dissimilar. In the order of increasing non-sphericity, we can distinguish:

  • - equidimensional crystals: pyramids, rhombohedra, tetrahedra, hexahedra (cubes), octahedra (cube with truncated corners), dodecahedra;
  • - one dimension is slightly lower than the others (tablets);
  • - one dimension is significantly lower than the others (plates, scales);
  • - one dimension is slightly greater than the others (prisms);
  • - one dimension is significantly greater than the others: acicular (rods, needles, fibers).

Specific surface a increases with non-sphericity.

A high specific surface is:

  • - detrimental for permeability (filtration);
  • - favorable for drying;
  • - favorable for chemical reactivity.

Specific surface is acquired directly by filtration.

The intermediate dimension of the crystal is acquired by screening.

The volume is provided by a Coulter counter.

Finally, by microscopic examination of fifteen crystals, we can fix a mean for the ratios:

Knowledge of these two ratios allows us to evaluate the a and в Nyvlt coefficients in order to calculate both volume and surface:

Specific surface a can be estimated easily if the particles can be assimilated to solids of a simple geometrical form. For example:

 
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