Τρίτη, 24 Οκτωβρίου 2017

Uranium minerals from the Lavrion Mining District, By Branko Rieck

The bright to pale yellow crystals of Sklodowskite, etc. Mine No. 18, Paleokamariza Mines, Paleokamariza area, Lavrion District Mines,Attikí Prefecture, Greece
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 photo to 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.

Fractures delineated by gypsum growth, Paleokamariza mine No 18
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.

 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.
Partially collapsed cavity, Paleokamariza mine No 18
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.
Sklodowskite, etc.Mine No. 18, Paleokamariza Mines, Paleokamariza area, Lavrion District Mines, photo by Branko Rieck
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. 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.

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 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.

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.

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.

See full article here