Environmental Engineering Reference
In-Depth Information
10.1.1.
Microscopy and physicochemical analyses
With traditional chemical analysis techniques, such as wet chemistry (sample put
in solution to look for cations and anions with appropriate indicators
1
) or dry
chemistry will not be used because they are increasingly being replaced by
instrumental methods that are faster and much more accurate.
10.1.1.1.
Optical microscope
Microscopic observation under polarized (natural) light provides a general view
of the morphology of the external surface of the stone sample (notably through the
use of binoculars). It allows us to identifying some minerals and detect the presence
of living organisms that would be not visible with the naked eyes (e.g. algae that
become fluorescent under UV radiation [NUG 93]). This kind of observation,
however, is far from providing as much information as observation under a
petrographical microscope using analyzed and polarized light (polarized light).
It is worth remembering that light, according wave theory, is an electromagnetic
wave that has both an electric and magnetic field at perpendicular angles and in the
direction of light wave propagation. When light is polarized, both fields vibrate in a
haphazard way (direction, amplitude and phase). When the light is polarized and
analyzed, due to crossing a polarizer, the electric and magnetic fields vibrate in the
same, constant direction. The direction of polarization corresponds to the direction
of the electric field [CHA 97].
Examination of materials under polarized and analyzed light requires the sample
to be cut into thin slices, stuck to a glass plate and reduced to a thickness of 30 µm
by abrasion. This allows observation under light transmission.
For examination, the section is set up on the rolling plate of the microscope
between two polarizing filters, one located under the rolling plate (the polarizer) and
the other above (the analyzer). Two modes are used for observation:
- observation under so-called natural light or polarized-not-analyzed light, where
the direction of the polarizer (historically termed
nicols
) and the analyzing direction
are parallel (parallel
nicols
). The analyzer may also be removed;
- observation under so-called polarized-analyzed light (PAL) where both
polarizers are present, their directions being perpendicular (crossed
nicols
).
These two modes display specific morphological and optical properties of
minerals. Under natural light, these properties are: crystal shape, relief (linked to the
1 The Italian standard “DOC NORMAL-13/83: Dosagio dei Sali solubili” which deals with
the quantification and the extraction of soluble salts from the stones
