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The analysis of the principal components is one
of the numerical analysis methods to obtain
information based on multivariable data. The
method is basically a technique of variable
reduction, using the original information
structure to search for n-dimensional
combinations with one number less for the “new
variables”, which explain the maximal variance
of the system. This is reducing the number of
original variables for others, which are lineal
combinations of the first ones but contain
substantially the same information.
• Spectral considerations
The electromagnetic spectrum of a substance
can have absorption factors in one or various
bands, and inclusing in none. The three cases are
useful. For example, for the case of iron oxides
and iron hydroxides, their absorption ranges
from 0.4 to 1.0 pm. The phenomene can be
explained due to the two types of
electromagnetical energy absorption (HUNT &
SALYSBURY 1970): (a) Load transfer:
generalized absorption which is presented with
its major intensity in the ultraviolet region and
which diminuish progressively towards the
visible region and the close infrared, (b) Effect
of the crystalline field: absorptions of little
amplitude but well localized, situated at 0.45,
0.65 pm and between 0.85 - 0.95 pm.
The analysis of the experimental spectrals for
iron oxides and hydroxides show that each of
these minerals has characteristical absorption
zones (Table 1, Fig. 2).
Table 1: Absorption regions for iron oxides
and hydroxides
Mineral
Absorption region (pm)
Jarosite
0.43, 0.90
Hematite
0.80
Goetite
0.65, 0.92
Another group of important minerals in
alteration zone are those with hydroxile
(OH-) molecules, such as caolinite,
pyrofilite, alunite, montmorillonite, sericite-
muscovite (illite), buddingtonite, chlorite,
epidote and dickite. These species show
strong energy absorptions within the interval
from 1.4 to 2.2 pm.
WAVELENGTH (Aim)
Future 2 Reflectance spectra of jarosile. hematite and goctite
(From Hunt and Salisbury. 1970)
The absorption characteristics of the 2.2 pm
region are caused by combinations of the
vibrational tones of the Al-OH bonds
(HUNT 1979). This permits to distinguish the
alteration hydroxides from other hydroxides
where Mg-OH bonds are presented, such as in
amphiboles, tri-octahedric phyllosilicates and
carbonates. The window of 2.2 pm represents a
window for the alteration mineralogy. As an
example are shown the reflectance spectrums for
caolinite and muscovite. Note the
characteristical inflexions located at 1.4, 1.9 and
2.2 pm (Fig. 3).
• Hydrothermal mapping by principal
components (CP)
The use of the principal components technique
(CP) for the reconnaissance of mineralogical
patrons is based on the premise that the spectral
channels are treated as variables, which are
subject to algeabric transformations to obtain its
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