Guanacoite from Las Minicas (Benahadux, Almería, Spain)
3rd Oct 2017 18:07 UTCAdolf Cortel
https://www.mindat.org/forum.php?read,6,420445,420608#msg-420608
About one year ago José F. Castro, a very active member of this group, sent me a sample of bluish radiated crystals,with the appearance of aurichalcite, collected in Las Minicas area (Benahadux, Almería, Spain). This area, with a lot of old mines, has been described (in Spanish) in:
http://www.foro-minerales.com/forum/viewtopic.php?p=114433&highlight=minicas#114433
The crystals, shown in picture made by Josep A. Soldevilla, have been identified as a Zn-rich guanacoite. The scarcity of the sample has prevented by now the confirmation by XRD.
The preliminary analysis made by LIBS and Raman revealed that it was not aurichalcite (or another carbonate) but an arsenate containing As, Cu, Zn and Mg.
Further analysis by SEM/EDS confirmed As, Cu and Zn, but, as it is well known, in EDS the presence of As prevents the observation of the only spectral line of magnesium which remains hidden under the AsL line. Therefore, magnesium was confirmed from the strong emission lines in the UV region by LIBS by means of many shots in the region containing the needles and in the nearby region. The SEM-EDS analysis was made thanks to the Grup Mineralogic Catala within the agreement with the University of Barcelona.
The presence of magnesium was indirectly confirmed comparing the ratio of the intensities of the EDS AsL and AsK lines in different samples of arsenates with or without magnesium (all of them without Pb, which interferes with the AsK line) made with the same instrument and the same conditions (20 kV). These samples are from Pedro Mingueza, Joan Vinyoles and Josep Carreras and correspond to analysis I made for members of GMC in the University of Barcelona. The ratio AsL/AsK is bigger for the Mg bearing arsenates due to the Mg line overlapped and hidden under the AsL line, as shown in the following figure.
At this point if it is considered that the small content of Zn is due to substitution Zn=>Cu and that this element is not essential in the composition of the mineral, a search in mindat by the elements gives only ahrbarite (physically very different), erikapohlite (contains Ca, which is not the case) and guanacoite (which looks very similar).
The Raman spectrum of the sample is weak but agrees well with the one of guanacoite from the RRUFF database as shown in the following spectra. Therefore, from the available data and thanks to the effort of the APGA group, it seems clear that the mineral is a Zn-bearing guanacoite.
For those interested in the paragenesis, zincolivenite (ratio Cu/Zn 1) was also clearly identified by SEM/EDS and Raman in the same piece bearing the guanacoite.
It is possible that due to the big resemblance between this mineral and aurichalcite others could have found this mineral previously in las Minicas. In fact, after seeing a picture of very similar bluish needles in a piece of Adrian Pesudo from this place, only the Raman analysis of a sample kindly provided by Adrian revealed that it was aurichalcite indeed.
At this point if it is considered that the small content of Zn is due to substitution Zn=>Cu and that this element is not essential in the composition of the mineral, a search in mindat by the elements gives only ahrbarite (physically very different), erikapohlite (contains Ca, which is not the case) and guanacoite (which looks very similar).
The Raman spectrum of the sample is weak but agrees well with the one of guanacoite from the RRUFF database as shown in the following spectra. Therefore, from the available data and thanks to the effort of the APGA group, it seems clear that the mineral is a Zn-bearing guanacoite.
For those interested in the paragenesis, zincolivenite (ratio Cu/Zn 1) was also clearly identified by SEM/EDS and Raman in the same piece bearing the guanacoite.
It is possible that due to the big resemblance between this mineral and aurichalcite others could have found this mineral previously in las Minicas. In fact, after seeing a picture of very similar bluish needles in a piece of Adrian Pesudo from this place, only the Raman analysis of a sample kindly provided by Adrian revealed that it was aurichalcite indeed.
