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The Mines and Minerals of Lavrion - Alunogen

Last Updated: 21st Mar 2020

By Branko Rieck

ALUNOGEN, Al2(SO4)3 · 17H2O, triclinic



General Appearance
It is rare that a mineral occurs in two so different habits as alunogen (Hanke, 1994; Schnorrer and Breitenbach, 1996). As can be seen in Figure 1 in some cases it forms pincushion-like hemispheres of needle-like crystals. This type is widespread in Lavrion, occurring in all major sub-districts. The second type occurs at the outside of the Plaka Sulfate Localities at overhanging cliffs or near the entrances. In summer the surface temperature of the dark, schistose matrix may reach 70°C, having an effect on the mineralogy (e.g. the occurrence of millosevichite). During the wet and cool periods in winter, different minerals precipitate and alunogen is one of them. These crystals show a completely different habit as can be seen in Figure 2: colorless, transparent platelets, generally with well-developed crystallographic terminations, grow either on remnants of millosevichite crusts or directly on iron-stained schist matrix. The crystals have a vitreous luster that in some cases is hidden due to the ductility and the resulting pearlescent sheen on the crystal surfaces. Most prominently the form {010} is exhibited with occasional striations in the direction [131]. The outline is often pseudohexagonal.

Paragenesis
Both types of alunogen are usually the only mineral grown on limonitic or schist matrix. Schnorrer and Breitenbach (1996) mention metavoltine as a rare constituent of this paragenesis. As mentioned above, remnants of millosevichite crusts are common in the type 2 paragenesis. It is unclear however, if millosevichite is a precursor mineral in this case.

Localities
The Plaka Sulfate Locality, the Plaka Sulfate Locality II and the Sounion Sulfate Locality are the main areas, where alunogen can be found. A careful examination of sulfate-rich areas in different sub-districts has yielded several – if minor – occurrences of alunogen.


01382680015833447856703.jpg
Fig. 1: White needles of alunogen (PXRD) with a small aggregate of yellow metavoltine. FOV: 6 mm.



01880330015832665336703.jpg
Fig. 2: Transparent platelets of alunogen on iron-stained schist matrix from a spot directly at the southern entrance of the Plaka Sulfate Locality II. Also on this specimen, but not visible in this view are crusts of millosevichite. FOV: 5 mm.





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

References
Hanke, H. (1994) Der Bergbau und die Mineralien von Lavrion, Griechenland. Emser Hefte, 15(2), 1-80 (in German).
Schnorrer, G. and Breitenbach, H. (1996): Lavrion: Neufunde aus dem antiken Bergbau und in historischen Schlacken. Lapis, 21 (1), 45-48; 62 (in German).




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