Environmental Engineering Reference
In-Depth Information
and hence its identifi cation (Brindley & Brown
1980). However, for samples with a particle diam-
eter less than 50
Phyllosilicate mineral nomenclature is based on its
structure, the presence of cations and interlayer
expansion capability, with differentiation obtained
mainly from basal distance (c) (Brindley & Brown
1980; Caillère
et al.
1982). For instance, 1 : 1 clay
mineral structure, such as kaolinite, has a 001 invari-
able basal distance of about 7 Å, despite many treat-
ments (Churchman
et al.
1984). For 2 : 1 clay
minerals, as in micas, the 001 structure has an invari-
able basal distance of about 10 Å with solvation and
heating treatments (Moore & Reynolds 1997);
whereas vermiculites and smectites vary in their basal
distance 001 at about 14 Å, depending on hydration
in the presence of cations in the interlayers (Robert
& Tessier 1974).
The formation of X-rays is based on the idea that
they are produced when electrically charged particles
are suddenly stopped (Schulze 1989). When these
particles are electrons, they can be accelerated, and
are halted by collision with other electrons (anti-
cathodes), and produce X-rays. The electrons become
excited because of the collision, and can pass from
K to L layers or from K to M layers, after the emis-
sion of photons called
K
α
and
K
β
, respectively. The
most commonly found anti-cathodes are Fe with
wavelength
K
α
=
1,935 Å, Cu with
K
α
=
1,540 Å, and
Co with
K
α
= 1,788 Å. The emission of
K
β
energy is
fi ltered using monochromators.
When interacting with crystals, X-rays are sub-
jected to diffraction, refl ection, and refraction,
among others. Bragg's law relates the position of
diffracted peaks between atomic planes in the crystal
(Schulze 1989). If the planes are coherent to X-ray
diffraction, then high-intensity peaks in a region cor-
responding to the inclination angle of to the X-ray
emitter will be produced. Bragg's law is expressed as:
m, all ionic planes will be repre-
sented on the X-ray diagram and may result in
interference. In this case, if the objectives of the study
are phyllosilicates (plate form) the sample must be
orientated so the other 00l planes are more likely to
interact with the X-rays (Robert & Tessier 1974),
for example by drying the clay suspension over a
glass blade.
Thus, X-rays rely on sample pretreatments as
shown in Fig. 3.2. Variation in basal distance by
means of various treatments, such as saturation with
ethylene glycol, formamide, and heating at 200, 300,
and 550 °C, assists in the production of X-ray dia-
grams, and the identifi cation of the mineral species
required (Brindley & Brown 1980).
Figure 3.2 shows that basal distances can be
altered owing to the opening or collapse of minerals
interlayers. Such behavior is a diagnostic character-
istic in the identifi cation of clay mineral species
(Brindley & Brown 1980). Fig. 3.3 presents an X-ray
diagram of the less than 2
μ
m fraction of fl uvial sedi-
ment from a watershed in Rio Grande do Sul, Brazil
(Bortoluzzi 2004) after various pretreatments.
There are intense peaks between 2 and 16° of 2
μ
θ
at room temperature (trace N), which indicates that
smectite (15 Å) and kaolinite (7.2 Å) are present.
After treatment at 200 and 550 °C, it is mainly smec-
tite that reacts. The identifi cation of clay minerals in
this way is reasonably easy; however, there is no
information on the ratio between species identifi ed.
Thus, posttreatment of X-ray diagrams with the aid
of mathematical models, such as deconvolution anal-
ysis (Lanson 1997), or simulation of interstratifi ed
clay minerals can be used (Reynolds & Reynolds
1996). Posttreatment has been used in mineralogical
studies for semi-quantitative analyses (Inoue
et al.
1989; Lanson & Benson 1992; Moore & Reynolds
1997; Bortoluzzi 2004; Bortoluzzi
et al.
2005), so
that as well as being able to identify the clay miner-
als, their relative ratios can also be elucidated.
However, sediment particles not only have size
and form but also varied mineralogy, according their
source material (Hsieh 1984). Characterization of
sediment using XRD requires careful sample prepa-
ration, knowledge of the minerals present and grain
size distribution, as well as consideration of
posttreatments.
dn
=
λ
(
2sin
θ
)
(9)
where
is the wavelength of the X-ray beam (in
ångströms);
λ
is the angle of incidence on atomic
planes (in degrees);
n
is an integer determined by the
order given;
d
is the spacing between the planes in
the atomic lattice (in ångströms).
At a certain inclination angle, X-rays incident on
coherent planes will refl ect Bragg's law and vibrate
in phase amplifying the resulting emissions, namely
a DRX peak. Published reference X-ray diagrams of
clay minerals with known basal distances enable a
comparison to be made with the unknown sample
θ
