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The Mines and Minerals of Lavrion - Mixite

Last Updated: 13th Nov 2018

By Branko Rieck

MIXITE, BiCu6(AsO4)3(OH)6 · 3H2O, hexagonal

General Appearance
Generally, mixite forms acicular crystals elongated along the c-axis. In open spaces very nice radial sprays are found. More common however are mats and fibrous aggregates filling spaces in the host rock. Its color is a distinctive blue-green. Mixite may be confused with other minerals of the mixite-group (especially the La-dominant agardite, which is very similar in color).

Paragenesis
Mixite is a secondary mineral formed as the result of the oxidation of bismuth- bearing sulfides and arsenides. It is commonly accompanied by other bismuth- bearing secondary minerals like beyerite, atelestite, preisingerite, rooseveltite, kettnerite, bismite and bismutite. As a mineral of the oxidized zone it is also usually accompanied by adamite, brochantite, malachite, azurite, georgeite, stibiconite, oxyplumboroméite (bindheimite), mimetite and gartrellite. The matrix mainly consists of quartz and red iron oxides typical for the “iron cap”. Chalcopyrite, pyrite, arsenopyrite, bismuthinite (and/or bismuth sulphosalts), native bismuth, maldonite and bournonite are common primary ore minerals sometimes still visible while chalcocite, digenite, covellite, native bismuth and native gold have also formed as secondary minerals.
At Lavrion there are three distinct paragenetical sequences in which mixite occurs. All three are found at the 4th Level of the Hilarion area, while only one is found at Jean Baptiste.
The first has mixite as the last mineral to form and is characterized by dark green cuprian adamite as the base for mixite sprays (see Fig. 1). Mixite from this paragenesis is relatively pure, containing neither calcium nor REE. Unfortunately this paragenesis suffers from a rather crumbly, unstable red gossan matrix. Some of the bigger specimens on the market have been soaked in various glues to stabilize them. The specimens from this type are among the best for this species found so far (see Fig. 5). Crystals reach 10 mm in length and are generally thicker than their counterparts in the other sequences leading to a much more intense color.
In the second paragenesis some other bismuth-bearing minerals formed after mixite. There are some specimens that show the transition from this paragenesis to the first quite clearly. While the specimens are still fragile due to the nature of mixite the matrix is usually quite stable and often quite hard. Mixite of this type contains noticeable amounts of calcium but no REE It is thus a REE-free solid solution between zálesíite and mixite. It is generally much paler in color (see Fig. 2).
The third paragenesis contains less arsenate minerals and mainly carbonates of bismuth. Mixite forms thin veins in host rock and sometimes includes grains of atelestite. In open spaces flattened sprays of crystals may form (see Fig. 3). This paragenesis is most abundant in the locality at the 2nd Level of the Jean Baptiste area.

Localities
Mixite has been identified from a locality on the 4th Level of the Hilarion area and from the 2nd Level of the Jean Baptiste area.
The locality in Hilarion has been productive for more than 20 years and has yielded the best specimens of this species on a worldwide scale. The most significant specimens date from one stunning find in 1999. At this time Christos Kapellas (after whom the mineral kapellasite is named) removed more than 1000 specimen from this site, about 40 of those ranking as “world class”. As of 2018 there is now very hard work required to find and collect more specimens and the exhaustion of this locality seems to be near.
The locality in Jean Baptiste (locally called "ασπρα χωματα" - "white soil") yielded only scientifically significant specimens of mixite and is now thoroughly exhausted.

The locality at the 2nd Level of the Jean Baptiste Mine locally called "ασπρα χωματα" - "white soil"

Lavrion District Mines, Lavreotiki, East Attica, Attica, Greece
The entrance to the locality at the 2nd Level of the Jean Baptiste mine. The reason why it is called "ασπρα χωματα" - "white soil" is clearly visible in this view.

This is the thoroughly mined out place where gold, bismuth and bismuth secondary minerals were found at the 2nd Level of the Jean Baptiste mine. This photo was stitched together from a dozen photos, hence the missing pieces. The hammer is 279 mm long.


It is possible that mixite has been overlooked as a secondary mineral in such places where there is a relative abundance of primary bismuth-bearing minerals.

Fig. 1: Mixite and cuprian adamite on gossan matrix from the 4th Level of the Hilarion Area. FOV: 5 mm.


Fig. 2: Ca-bearing mixite in paragenesis with pale yellow atelestite, green cuprian adamite and white beyerite on quartz and a hard limonitic matrix. Hilarion area, 4th Level. FOV: 11 mm.


Fig. 3: Pale, fibrous mixite on quartz associated with minor yellow-green, waxy atelestite from the 2nd Level of the Jean Baptiste area. FOV: 11 mm.


Fig. 4: The best mixite specimen I ever saw from the "ασπρα χωματα" locality on the 2nd level of Jean Baptiste. Typical fibrous crystals, together with abundant dark green cuprian adamite in a mainly quartz matrix. FOV: 4 mm. The entire specimen is about 15 cm long.


Fig. 5: A stunning, world class example of mixite from the 4th Level of the Hilarion Area. It was found in 1999 by Christos Kapellas. At that time the author wanted to acquire this piece, but did not have the money to do so - even after much haggling. FOV: 9.4 cm.




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




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