Environmental Engineering Reference
In-Depth Information
refraction index of minerals), splitting plans (fracture plans or plans of weaker
strength inside crystals), and pleochroism (change in color or hue due to
modification of orientation of the mineral with the polarizing plan).
Under PAL, the properties used are essentially birefringence (due to the
anisotropy of most minerals that have two or three refraction coefficients), and the
extinction angle, which is the rotation angle used to turn the observed mineral black
under PAL.
By means of these properties we are able to determine almost all minerals,
except metallic minerals that do not have transparent phases [ALB 76, DEE 93,
MAC 96, ROU 82]. Petrographic studies carried out on thin sections under natural
and polarized light provide information about the composition of the stone and its
superficial alterations concurrently [BRO 92b].
When magnification is weak, the general macro-texture of the stone will be
visible and it will be possible to detect microcrack networks. By increasing the
magnification, this can no longer be seen, however it will be possible to observe the
possible filling of cracks by neoformed minerals such as gypsum [GRA 93].
Sections perpendicular to the altered surface and observed under polarizing
optical microscopy show the alteration versus depth in such way that it is possible to
evaluate: the mean thickness of gypsum and soot types forming a black crust [FAS
93a, VER 93]. It is also possible to determine the thickness of the calcite matrix
replaced by gypsum at the surface of limestones or marbles (epigenic gypsum; see
Chapter 8, Figure 8.23).
Arnold proposed combining microscopy under polarized light with
microchemistry (the use of reactants) as a simple and rapid method for identifying
the most common soluble salts and carbonates [ARN 84]. This method is well
adapted for very small amounts of matter and in situations where there is an
association of complex minerals or hydrated salts [BER 94].
Observation of a surface stone sample polished after being impregnated with
liquid resin that polymerizes at ambient temperature (e.g. epoxy resin from Struers)
under vacuum, makes it possible to study porosity and its quantification using image
analysis. This partial observation of porosity, made in two-dimensional space, has
often led to lower porosity values than those given with other methods, such as
mercury porosimetry [FIT 93]. It must be underlined that the value obtained depends
on the magnification used; as the magnification increases, the microporosity is
increasingly taken into account, but the representativity of the medium decreases.
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