350–2500 nm wavelength of the electromagnetic spectrum.
Measurements were done on small portions of air-dried soil
samples quartered from the whole sample to ensure representativeness.
Time spent to acquire the reflectance spectra of soil
samples was incomparable with that which will be spent for the
conventional geotechnical engineering tests that will take
several weeks’ time.
3.5. Mapping of clay minerals and alteration zones
The alteration of hydrothermal deposits may result in an
extension from tens of centimeters to tens of kilometers. It is
generally represented by the spatial arrangement that is
divided from the central potassic zone to the outward direction
of the phyllite or phyllic (sericitic) zone, argillite or argillic
zone, and propylitic zone in that order (Lowell and Guilbert,
1970). These zones are characterized by an assemblage of
minerals that exhibit distinctive spectral absorption features.
In the phyllite or phyllic zone, mainly quartz, sericite, and
pyrite exist, but sericite is predominated. The absorption of
sericite (muscovite) is strongest at a wavelength of 2.2 lm
(ASTER band 6). In the argillite or argillic zone, kaolinite
and montmorillonite are present, whereas pyrophyllite and
alunite are abundantly formed in advanced argillic zones.
With the exception of montmorillonite, these minerals display
absorption features near 2.16 lm (ASTER band 5). Calcite,
chlorite, and epidote mainly exist at the propylitic zone.
These minerals have the strongest absorption features at the
2.33 lm (ASTER band 8).
In the present study, a new spectral library was established;
derived from the signatures taken from field samples to