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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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