USGS Digital Spectral Library Purity
Each spectrum has a purity code in its header. In this version of the
spectral library, the code is:
The "W" stands for wavelength region followed by the region
measured. All spectra in this version cover the nominal range of
0.2 to 3.0 microns which is region 1. The digital data
for the wavelength set are located in splib04a, record 6.
The "R" stands for resolution, followed by the resolution index.
All spectra in this version of the library were measured using
resolution set 1 in wavelength region 1. Figure 1 shows the
resolution function, and the digital data for the resolution
are located in splib04a, record 8.
The next letter signifies the instrument used. All spectra in this
version of the library were measured on the USGS, Denver
Spectroscopy Laboratory, Beckman 5270 spectrometer, and are
designated by the letter "B".
Following the instrument letter is a lower case letter signifying
the spectral purity of the spectrum for this wavelength range and
resolution (the "x" in the above example is one of the following
a: The spectrum and sample are pure based on significant
supporting data available to the authors.
The sample purity from other methods (e.g.
XRD, microscopic examination) indicates essentially no
b: The spectrum appears spectrally pure. However, other sample
analyses indicate the presence of other minerals that probably affect
the absolute reflectance level to a small degree, but do not add any
spectral features. The spectral features of the primary minerals may be
slightly less intense, but the feature positions and shapes should be
representative. For example, in this wavelength region (W1), quartz
would tend to increase the reflectance level and decrease absorption
band strength, but would not add any measurable features to the
spectrum. Such a sample would rate a "b." In a few cases, where we have
little support data, but the spectra for that mineral are well known, we
assigned the spectral purity based on the spectra data along with a
microscopic examination of the sample. There are a few "b" classes done
c: The spectrum is spectrally pure except for some weak features with
depths of a few percent or less caused by other contaminants. For
example, some minerals may have some slight alteration that is apparent.
Spectroscopic detection of alteration is easier for more transparent
minerals. For example, some of the albite spectra show weak 2.2-microns
features due to alteration. From the knowledge of the mineral formula,
you can often tell which features do not belong to the mineral. Albite,
for instance does not have OH in the formula, so water features (1.4,
1.9, 2.2 microns) are not due to albite. However, you could argue that
incipient alteration due to weathering is common in minerals at the
Earth's surface. Thus, spectral bands due to weathering are somewhat
characteristic of many samples (e.g. feldspars), even if they are not a
property of the pure mineral. Thus these alteration spectral features
might be useful in some cases.
d: Significant spectral contamination. The spectrum is
included in the library only because it is the best
sample of its type currently available and the primary spectral
features can still be recognized. However, the spectrum
should be used with care. The sample description should
be consulted as a guide to what features are a part of the
actual mineral. This sample may be purged from the
database in future releases as better samples become
?: There are insufficient analyses and/or knowledge of the spectral
properties of this material to evaluate its spectral purity. In general we
have included such samples because we believe their spectra to be
representative. These are samples for which we are concentrating
future analyses in order to resolve the purity issue. Updates to the
spectral purity and sample documentation will be placed online for
anonymous ftp as the information becomes available.
(See the section below on availability on how to
electronically access the data and obtain further
Commenting on the spectra in general, reflectance tends to decrease in
the UV and beyond about 2.7 microns. Some of the spectra show minima
in the UV. We have taken careful measurements of scattered light and
believe all these features are real. Beyond 2.7 microns, even anhydrous
minerals show absorption due to water adsorbed onto the surfaces of the
mineral grains. Our experience has shown that these water absorptions
are still present in dry nitrogen purged environments, although slightly
weaker. Spectra of similar samples obtained at other facilities, like
those in Hawaii or the east coast of the US. have shown us that the
water absorptions in the spectra from relatively dry Colorado are really
quite small in comparison. Placing the sample in a dry nitrogen
atmosphere or a vacuum oven has little effect on the water absorption as
water from the atmosphere will readsorb onto the sample by the time it
reaches the spectrometer. Experiments by the senior author when he was
at the University of Hawaii have also shown that most of the adsorbed
water remains even under a strong vacuum at room temperatures. We
decided in general not to heat our samples in order to avoid any
temperature induced alteration.
The overall spectral purity is high for this library. Seventy-one
percent of the spectra have a purity code of either a or b (36% a,
and 35% b), while only 17% have c, and 2% have d. Ten percent are
yet to be classified.
U.S. Geological Survey,
a bureau of the U.S. Department of the Interior
This page URL= http://speclab.cr.usgs.gov/spectral.purity.html
This page is maintained by: Dr. Roger N. Clark email@example.com
Last modified November 13, 1998.