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Uranium minerals from the Lavrion Mining District

Last Updated: 3rd Oct 2017

By Branko Rieck

A recent German publication (Simon & Kapellas, 2017) reported the occurrence of sklodowskite in the famous Lavrion Mining District (Greece), of which the users of MinDat have been aware since March 2017. Unfortunately, that article did not give the full picture of this occurrence, gave incorrect or misleading information and neglected to cite relevant publications.
It is the aim of the present article to correct the publication where necessary and to fill in the gaps where possible.
Uranium minerals have been reported from Lavrion previously by the author in the Lapis special issue on Lavrion in 1999 [johannite and heinrichite (Rieck, 1999); senaite (Rieck & Rieck, 1999)]. The uranium-titanium oxide brannerite was reported recently (Kolitsch et al., 2015).
In an area like the Lavrion Mining District it always pays off to visit places that have not seen any attention for a long time. Our improved knowledge of the mineralogy, geology, ore-forming processes and petrography of the area helps us to identify places of interest and thus leads to new discoveries.
The Paelokamariza #18 Mine became famous in the 1970s when a cavity containing red baryte was found. After most of this find was recovered, the interest in this mine waned. Most visits to this mine were triggered by scientific interest rather than by motivated collectors. Especially the late Christos Solomos (cf. Solomos et al., 2004; Katerinopoulos et al., 2005; Voudouris et al., 2008) and his close companion Alkiviadis Tsolakos were active in this mine. Alkis uploaded a phototo Mindat which shows the now famous cavity with the uranium minerals. This photo dates to April 2005 and shows the massive gypsum layers partly colored red by iron oxide/hydroxide inclusions. At the time when the photo was taken the small yellow tufts of sklodowskite that occur in the deeper levels of this pocket were not recognized.
When the uranium minerals were finally unearthed they were sent to Michalis Fitros at the University of Patras who analyzed the samples late in 2016 by means of SEM-EDS and reported the discovery of sklodowskite not only to the finder, but also to the Mindat community in March 2017.
SEM micrograph showing acicular Sklodowskite crystals on gypsum matrix. The SEM-EDS analysis had been carried out at University of Patras. FOV: 1mm

Late in January 2017, I obtained a large number of specimens from this find from Konstantinos Kapellas and was not only able to confirm the analytical work by Michalis Fitros by means of single-crystal X-ray diffraction analysis (in collaboration with Prof. Dr. Gerald Giester at the University of Vienna), but could also identify the uranium minerals andersonite, bayleyite, ianthinite, novacekite-II and uranophane, all previously unknown from Lavrion.
The locality itself is a partly collapsed karst cavity (“Hydrothermal Solution Carst” after Marinos & Petraschek, 1956) with minimum dimensions of 13 x 7 x 3 m3. This cavity is one of a series of similar cavities in the closer vicinity but the only one (so far) yielding uranium minerals.

Fissures in the host rock allowed the metal-bearing solutions to enter the cavity. This can be seen very well in the photo below. The layers of gypsum are much thicker in places, delineating the fissures and thinner on massive rock.

At least for some time the cavity was completely filled with metal-bearing solutions as shown by the crystallization of gypsum on all surfaces. Also, the detritus within the cavity is cemented by gypsum. This needs to be observed when collecting. As can also be seen in the photo above, there are slabs of host rock weighting several hundreds of kilograms cemented only by one of the softest minerals known. There are indications for at least four cycles of gypsum deposition in the cavity, with the uranium minerals occurring between the third and fourth cycle.

Andersonite, Na2Ca(UO2)(CO3)3 · 6H2O
The andersonite from the Paleokamariza is not to be compared to the better specimens from the sedimentary deposit in the southwestern United States and elsewhere. So far, no individual crystals have been found. Andersonite rather forms thin, yellow, smudgy crusts on and below gypsum that are not particularly attractive. They are however important indicators of the uranium mineralization underground, as they show a very bright light green fluorescence also with long-wave UV light. This allows cheap and easily transportable UV LED torches to be used (see also this thread) which are available on Ebay and from other sources for minimal costs.

Bayleyite, Mg2(UO2)(CO3)3 · 18H2O
Bayleyite and andersonite are very similar in appearance unfortunately, but can be distinguished by their different reaction to short-wave UV light. That of bayleyite is markedly weaker than that of andersonite. It is also much rarer than andersonite with only a handful of confirmed specimens recovered at the time of writing.

Ianthinite, U4+(UO2)5O7 · 10H2O
On two specimens purple to dark blue, opaque, acicular crystals could be seen included in gypsum. These were identified by PXRD as ianthinite by matching the 5 strongest peaks, starting with the second, in the correct ratio of intensity as published in ICDD card number 12-272 (7.63(100), 3.81(80), 3.59(60), 3.35(60), 3.24(80), 2.95(20); note: the strongest peak at 7.63Å of ianthinite is at the same d-spacing as the strongest peak of gypsum.). Due to their occurrence solely as inclusions and their scarcity, a definite statement to their possible origin as pseudomorph after an unknown other uranium-bearing mineral cannot be made.

Nováčekite-II, Mg(UO2)2(AsO4)2 · 10H2O
Upon discussing the find with my fellow collector Karl Heinz Fabritz (St. Pölten, Austria) he noticed platy crystals of what looked like a uranium mica on one of the specimens. At the University of Vienna these were then identified as nováčekite-II by single-crystal X-ray diffraction. The diffracting quality was so good that we even tried a redetermination of the crystal structure, but unfortunately the sample self-destructed under the X-ray beam during measurement of a single-crystal intensity dataset. Well-formed crystals rarely exceed one millimeter in size, but one large boulder yielded platy aggregates up to 8 mm. There is some evidence that the occurrence is restricted to a small area within the locality. Nováčekite-II has the brightest fluorescence yellow of all the uranium minerals found at that locality. There is both a reaction in SW- and LW-UV.
The above-mentioned article by Simon & Kapellas (2017) reports the presence of a “greenish” uranium mica, but unfortunately engaged in guesswork instead of a scientific approach, and assumed the presence of “zeunerite?” or “heinrichite?”.

Sklodowskite, Mg(UO2)2(SiO3OH)2 · 6H2O
Sklodowskite was the first uranium-bearing mineral found at this locality. Initially considered to be “yellow agardite” by Dimitrios Syrigos they were first analyzed at the University of Patras by SEM-EDS. Confirmation of this analysis was achieved by single-crystal X-ray diffraction analysis at the University of Vienna.The bright to pale yellow crystals of sklodowskite are usually elongated after [010] and appear as single or radiating clusters up to 20 mm in length. Most are, however, in the range between 1 and 5 mm. Two different habits are evident. Stocky crystals with a length-to-width ration of 1:10-100 and thin needles where the ratio is 1:>1000. The stocky crystals are multifaceted and highly interesting from a crystallographic point of view. Sklodowskite from this locality has a weak but very interesting greenish to yellowish reaction in both SW- and LW-UV light. Some, but not all crystals exhibit a zoning of the emitted light with some areas reacting light green and others yellow. The individual zones do not show noticeable differences in chemistry as proven by electron microprobe work. Thus, the reason for this effect is unclear.

Uranophane, Ca(UO2)2(SiO3OH)2 · 5H2O
The above-mentioned reaction of sklodowskite to UV light lead to the discovery of another, visually very similar uranium-bearing silicate by its absence of any reaction. On the few specimens with confirmed uranophane there is not even one enclosed in gypsum. It is therefore likely the youngest uranium species in this paragenesis. The lacking UV reaction is the only “easy” clue as to the ID, because apart from this it is virtually indistinguishable from thin sklodowskite needles.

Apart from the uranium-bearing minerals – without doubt the great surprise from Lavrion – there are also some uranium-free species worth mentioning.

Celestine, SrSO4
The occurrence of celestine is a first for the Lavrion Mining District. The crystals are either included in or grown on gypsum. They reach sizes up to 2 mm and come in a wide variety of habits. Some are thin platy and were first considered to be baryte due to their typical habit. There were, however, crystals found that are almost multifaceted spheres. They range in color from completely colorless to yellow to orange to dark brown. Both anglesite and baryte are well known from Lavrion and the initial analysis was actually made to distinguish between these two. To my big surprise the unit-cell parameters and later the refinement of the atomic positions from single-crystal X-ray diffraction data clearly revealed celestine with more than 98% Sr and less than 2% Ba and Pb. This result was later also confirmed by EDX.

Gypsum, CaSO4 · 2H2O
There are three different types of gypsum in this locality.
The clefts in the host rock and vesicles in the collapse debris are filled with fibrous gypsum where the individual fibers can reach 7 cm in length.
The open surfaces in the cavity are almost completely covered with a crust formed by water-clear single crystals up to 2 cm in length. These crusts are usually 1 to 2 cm thick but may reach 10 cm in places where the initial solutions entered through cracks in the host rock. The clear gypsum crystals were deposited in several stages, some earlier than the crystallization of the uranium minerals, some later. Therefore e.g. sklodowskite can be found included in gypsum as well as grown upon it. Also quite often included are red iron oxides/hydroxides that give the gypsum a cherry-red colour. Some iron-stained gypsum from here has been brought to the market as “cinnabar-bearing”.
The most sought-after gypsum variety is the rams horn formation. Such curved aggregates can reach a height of 20 cm.

Cinnabar, HgS
In close spatial proximity to the uranium-bearing minerals a vein of cinnabar-bearing calcite was found. The vein had (it has been completely removed by collectors) a maximum thickness of 5 cm. At one place the vein was lain bare over an area of 20 x 50 cm2. It is currently not clear if the uranium mineralization and the cinnabar mineralization are related in any way. There is, however, at least one common denominator in the structural control of the veins.
Careful etching of the calcite matrix reveals millimeter-sized crystals of cinnabar that sometimes show twinning.
Unfortunately, quite a lot of specimens from this find consisting of iron-stained gypsum were also thought to contain cinnabar and subsequently sold as such. As of writing this article not a single crystal of cinnabar has been found included in gypsum.

Acknowledgements
I want to thank Prof. Dr. Gerald Giester (University of Vienna) for the help with the single-crystal X-ray diffraction studies and Ing. Petra Rieck for SEM-EDX and microprobe work. Thanks go also to Dr. Uwe Kolitsch for constructive comments and diligent proofreading to improve this article.

References
Katerinopoulos, A., Solomos, C. & Voudouris, P. (2005): Lavrion smithsonites: a mineralogical and mineral chemical study of their coloration. In: Mao, J. & Bierlein, F.P. (eds.), Mineral deposit research: Meeting the global challenge. Springer Verlag, 983-986.
Kolitsch, U., Rieck, B. & Voudouris, P. (2015): Mineralogy and genesis of the Lavrion ore deposit: new insights from the study of ore and accessory minerals. MinPet 2015, Leoben, Austria, September 10-13; abstract in Mitteilungen der Österreichischen Mineralogischen Gesellschaft, 161, 66.
Marinos, G. & Petrascheck, W.E. (1956): Geology and ore deposits of Laurium. Inst. geol. geophys. Meletai, 4 (No. 1), 1-247; English 223-236.
Rieck, B. (1999): Seltene Arsenate aus der Kamariza und weitere Neufunde. Lapis, 24 (7-8),. 68-76 (in German).
Rieck, B. & Rieck, P. (1999): Silber, Arsen und Antimon: Vererzungen im Revier Plaka (Teil II). Lapis, 24 (7-8), 59-63 (in German)
Simon, P. & Kapellas, K. (2017): Uranmineralien und rote Barytkristalle: Erstfund von Sklodowskit in Lavrion, Griechenland. Lapis, 42 (7-8), 58-65 (in German).
Solomos, C., Voudouris, P. & Katerinopoulos, A. (2004): Mineralogical studies of bismuth-gold-antimony mineralization at the area of Kamariza, Lavrion. Bulletin of the Geological Society of Greece 34, Proceedings of the 10th International Congress, Thessaloniki, Greece, 387-396 (in Greek with English summary).
Voudouris, P., Melfos, V., Spry, P.G., Bonsall, T.A., Tarkian, M. & Solomos, C. (2008) Carbonate-replacement Pb-Zn-Ag±Au mineralization in the Kamariza area, Lavrion, Greece: Mineralogy and thermochemical conditions of formation. Mineralogy and Petrology, 94, 85-106.




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Comments

Thank you Branko for sharing! Those in situ images are really amazing!!! The colors are unexpected!!

Scott Rider
3rd Oct 2017 9:52pm

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