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USGS Digital Spectral Library Purity

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SPECTRAL PURITY

Each spectrum has a purity code in its header. In this version of the spectral library, the code is: W1R1Bx 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 letters).

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 other contaminants.

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 this way.

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 available.

?:

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 information.)


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.


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This page is maintained by: Dr. Roger N. Clark rclark@usgs.gov
Last modified November 13, 1998.