Spectral Library of European Pegmatites, Pegmatite Minerals and Pegmatite Host-Rocks – The GREENPEG Project Database
Abstract. The GREENPEG spectral database contains the spectral signature, obtained through reflectance spectroscopy studies, of European pegmatites and minerals, as well as their host rocks. Samples include Nb-Y-F (NYF) and Li-Cs-Ta (LCT)-type pegmatites and host rocks from pegmatite locations in Austria, Ireland, Norway, Portugal, and Spain. The database contains the reflectance spectra (raw and with continuum removed), sample photographs, and main absorption features automatically extracted by a self-proposed Python routine. Whenever possible, spectral mineralogy was interpreted based on the continuum-removed spectra. A detailed description of the database, its content and structure, the measuring instrument, and interoperability with Geographic Information Systems (GIS) is available in this database report. Moreover, examples of how the data can be used and interpreted are also provided. The advantages and added value of the presented dataset reside on its European scale with representative samples from pegmatites with distinct genesis, mineralogy, structure, and host rocks that can be used as a reference for pegmatite exploration at a global scale through satellite image processing, for example. The reported spectral mineral assemblages can also be of interest when considering resource estimation or ore processing. Thus, it is expected that this open dataset, available on the Zenodo platform https://doi.org/10.5281/zenodo.6518319 (Cardoso-Fernandes et al., 2022), will be a reference for distinct types of users ranging from academia to industry.
Joana Cardoso-Fernandes et al.
Status: final response (author comments only)
RC1: 'Comment on essd-2022-386', Anonymous Referee #1, 06 Feb 2023
- AC1: 'Reply on RC1', Joana Cardoso-Fernandes, 15 May 2023
RC2: 'Comment on essd-2022-386', Anonymous Referee #2, 29 Apr 2023
- AC2: 'Reply on RC2', Joana Cardoso-Fernandes, 15 May 2023
Joana Cardoso-Fernandes et al.
Spectral Library of European Pegmatites, Pegmatite Minerals and Pegmatite Host-Rocks – The Greenpeg Database https://doi.org/10.5281/zenodo.6518319
Joana Cardoso-Fernandes et al.
Viewed (geographical distribution)
The authors of this manuscript describe and propose a spectral library of LCT and NYF pegmatite samples taken from various localities in Europe.
Investigating the spectra of pegmatites is a highly relevant topic at the moment due to their importance for Li and REE exploration.
The overall structure and grammar of the manuscript are coherent.
I commend the authors for making their data freely available, as this is a pertinent aspect in research. Furthermore, by linking the spectral library to a GIS database is a great idea in order to visualise the sampling locations.
However, the major issues relate to the spectral data acquisition and processing:
I’m a little confused as to how the final spectrum of each rock was averaged. The authors say that “each measurement comprises an average of 40 scans with four additional measurements acquired in each analysed spot that were later averaged into a final spectrum.” ? - line 138
Did the authors take 40 point measurements on a small area on the surface of each sample (I would assume so)?
That being said, taking averaged spectra from a spot on a pegmatite rock might not be the most scientifically accurate approach. The average specra from that specific area will indicate some mica/clay/ateration mineral at that specific point, this will not be representative of the specific type of pegmatites. How do the authors ensure representativity and completeness?
In line 168 the authors say: "Our results show that the spectral mineralogy identified does not necessarily match the minerals identified by observation of hand specimens and optical microscopy. This is because some silicates do not present necessarily diagnostic absorption features (Spatz, 1997) or because the spectra are dominated by alteration minerals that are spectrally very active due to the presence of water/hydroxyl group and superimpose unaltered mineral domains (Hunt and Ashley, 1979)."
This statemetn is very true, adding to the issue of representativity.
I disagree with the following statement: Line 172: The representative reflectance spectra stored in the libraries can be utilised for satellite image processing, namely in the image classification tasks. To do so, the acquired spectra can be resampled to match the satellite sensors’ spectral resolution and used as a target for algorithm training instead of the image pixels.
Resampling the library spectra (with ~2000 channels) to the spectral resolution of multi-spectral satellite data (even WV-3; which is 10s of channels) would diminish the characteristic spectral absorption features to such an extent as to make the reference spectrum almost useless for satellite image classification. Large spectral features may be preserved when resampling, however, narrow and sharp diagnostic absorption features can be lost.
Additionally, doing a continuum removal over the entire spectrum can (and in most cases does) create artifacts and distortions. It is recommended to do continuum removal over specific parts of the spectrum the observer is interested in.
Furthermore, the authors only present spectra in the VNIR-SWIR range, however, the major rock forming minerals of pegmatites are active in the LWIR range (i.e., quartz and feldspar). This is a major issue, as the authors already showed that the spectra represents clays/micas/alteration minerals, which are not representative of only pegmatites.
Regarding the database, it is very misleading to show spectra in the SWIR where it indicates some mica or clay but naming the sample K-feldspar. Clearly the spectral measurement does not show feldspar, but probably a SWIR active mineral at that specific point on the rock sample.
All in all, the purpose of this library is unclear. As pegmatites are dominantly composed by quartz, felspars and micas in macro-crystals, it is required to determine a clear sampling strategy. Right now this library shows micas/clays/ ateration minerals and the mixure of those (which again is not unique to pegmatites). What is the purpose of this library? Mapping alterations? REE and Li minerals? Rock forming minerals? As long as this is not clarified and a bespoke sampling strategy adopted, I don’t see the utility of such a library.