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The Mines and Minerals of Lavrion - Acanthite

Last Updated: 23rd Nov 2018

By Branko Rieck

Acanthite, Ag2S, monoclinic



General Appearance
Acanthite generally occurs as spiky crystals on wires and plates of native silver. The name of the mineral derives from Greek άκανθα ("akantha") meaning "thorn", in allusion to its crystal shape and nothing could be more fitting. It is in this form that it is highly sought-after, but it also occurs as earthy, dull masses and as such it is mostly not recognized.

Paragenesis
It is unlikely that acanthite occurs as primary mineral in the Lavrion Mining District in any noticeable amounts as the deposition of the ore minerals occurred at temperatures above 177°C. The paragenesis is therefore mostly native silver, silver minerals and silver-bearing minerals (like argentiferous galena) from which acanthite has formed as a secondary product. The most common is referred to as the “arsenic-paragenesis” that is dominated by native arsenic, galena and sphalerite, with several sulfosalts – silver-bearing and not – and late-stage gangue minerals. Another source of acanthite is a paragenesis that consists primarily of weathered galena, plenty of cerussite and silver sulfosalts. This was the most sought-after silver ore in ancient times. All other occurrences of acanthite do not play a role from the collectors’ point of view.

Localities
Acanthite is found throughout the Lavrion Mining District. There are however a few places that have yielded noteworthy specimens.
The first and foremost is the locality called “Silver-Place” by collectors. This locality can be reached through the Plaka #80 mine and up a steep incline. Here, vugs lined by white calcite crystals were found to have wire silver partially included and partially free-standing. These wires reached 3 cm in length. Those collectors lucky enough to find one such piece were able to watch a mineral grow. The acanthite forms within hours of opening the vugs containing the native silver. Usually the formation of acanthite is complete within three weeks when the specimen reaches a stable state. Sealing the specimen in an air-tight container immediately after opening the vugs does not protect the specimen but it slows the deposition of acanthite that it takes up to six months to reach a stable state. Cleaning the native silver from the acanthite only results in its re-deposition. It is interesting to observe that some parts of the native silver seem to be immune to the superficial development of acanthite with areas remaining bright and shiny over a long period of time, becoming only slightly tarnished over time. It is possible (although not having been checked on the few specimens that are available) that the chemical composition of the silver varies slightly leading to this phenomenon. As could be expected, this locality has been thoroughly mined out – so exhaustive that it can now be easily identified by its “vacuum-cleaned” look.Fig. 1: The “Silver-Place” as it looked like in May 2015.


Not too far away, in an area belonging to the Plaka #145 mine a vein with plenty of native arsenic can be followed over several meters. This place yielded quite a lot of acanthite of the aesthetically less attractive, but still interesting variety. Here it was possible to find earthy or spongy masses of acanthite that only occasionally showed that they were in fact pseudomorphs after a preexisting – probably pyrargyrite – silver mineral. The largest lump of acanthite that I know of which was found there is about fist-sized.Fig. 2: Status of May 2018. There is still plenty of native arsenic visible.



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