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The Mines and Minerals of Lavrion - Sideronatrite and Metasideronatrite

Last Updated: 13th Nov 2018

By Branko Rieck

Sideronatrite, Na2Fe(SO4)2(OH) · 3H2O, orthorhombic and Metasideronatrite, Na2Fe(SO4)2(OH) · H2O, orthorhombic



General Appearance
The two minerals are dealt with here in one article because sideronatrite reversibly dehydrates to metasideronatrite depending on relative humidity and exposure to sunlight. Most specimens found are in fact sideronatrite due to the humid conditions in the mines but may turn to metasideronatrite if kept in a sunlit showcase or very dry and otherwise hot conditions. It forms sprays of yellow needles in open spaces or vein-fillings with the fibers perpendicular to the face of the vein. It may occur as the only mineral on the matrix or in the company of numerous other sulfates.

Paragenesis
Sideronatrite is a secondary mineral formed as the result of the oxidation of iron sulfides (pyrite, marcasite, and pyrrhotite) or arsenopyrite. As it contains sodium in its chemical formula – which is not overly abundant in the Lavrion mining district – it is only found in places where sodium is either available through the decomposition of sodium feldspar or where sea water seeps in at the deepest levels. Sideronatrite is almost always accompanied by gypsum and natrojarosite. Other minerals also found in the paragenesis are the numerous iron sulfate minerals like copiapite, coquimbite, metavoltine and melanterite. Also other sulfate minerals like chalcanthite, anhydrite, polyhalite and hilarionite may be found alongside sideronatrite.

Localities
As a secondary mineral sideronatrite can be found throughout the deposits but the occurrences are usually not very noteworthy. There are however two exceptions where it has been found in very nice specimens, some of them actually being important for the species.
In the Sounion #19 mine a small number of very rich specimens were found on a marl matrix. These specimens were characterized by a very thin layer of natrojarosite covering the matrix on which the sideronatrite formed areas of several cm2. It is noteworthy that this is so far the only occurrence where both, sideronatrite and metasideronatrite were found to have formed naturally and in close proximity. These two minerals are rarely accompanied by ferrinatrite, but are otherwise the only minerals on the matrix.
On the 4th Level of the Hilarion area there are some areas where the primary sulfides have almost entirely been replaced by secondary sulfates. In many cases this process has gone so far that it threatens the stability of the adits. At one such place sideronatrite was discovered in paragenesis with rich metavoltine and some of the best hilarionite specimens to be recovered to date. Also on the 4th level an occurrence of sideronatrite was discovered, that let the other finds pale by comparison. About 1/10th of a square meter were covered by a crust of sideronatrite up to 3 cm thick and containing sideronatrite crystals up to 1 cm in length.


Fig. 1: Well terminated, clear sideronatrite crystals. Sounion #19 mine. FOV: 2 mm.



Fig. 2: Pale lath-like metasideronatrite grown on yellow transparent sideronatrite crystals on light brown natrojarosite with extremely thin, white needles of ferrinatrite. Sounion #19 mine. FOV: 5 mm.



Fig. 3: Sprays of sideronatrite needles with worm-like aggregates of bright yellow metavoltine, light grey polyhalite (along the top of the view) and white gypsum (along the bottom of the view) from the 4th Level of the Hilarion area. FOV: 7 mm.


Fig. 4: FOV: approx. 1 m. Crusts of sideronatrite/metasideronatrite at the 4th Level of the Hilarion Mine, partially already removed. All together about 1/10th of a square meter were collected.




Acknowledgements
Thanks go to Dr. Uwe Kolitsch for constructive comments and diligent proofreading to improve this article.




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