CURIOSITY GOT TO ME: THE COPPER MERCURY LEAD SULFOSALT ROUXELITE

Curiosity is the wick in the candle of learning.      W.A. Ward

The sulfosalt minerals, members of the sulfide group (sulfur is the major anion), are quite complex minerals, at least to an ole plugger like me. In a couple of previous posts, I “described” cylindrite (FePb₃Sn₄Sb₂S₁₄) on 1/23/17 and dufrènoysite [Pb₂As₂S₅] on 6/19/19. Sulfosalts contain a metal (mostly lead, copper, iron or silver although a few others, mercury, zinc, vanadium may be present), a semi-metal like arsenic, germanium, bismuth, antimony, or the metals tin or vanadium, and then sulfur but perhaps selenium or tellurium (Richards, 1999).

At 2022 Tucson I had several good conversations with a European dealer on some slow days and learned much. He also pulled out of his collection a specimen of rouxelite and noted that this mineral was quite rare and was only described in 2005. I looked at the micro through a loupe and it was a gorgeous mineral and so I brought it home.

Rouxelite crystals from the Sant’Olga tunnel, Monte Arsiccio Mine. The longer whiskers are ~ 2 mm in length.

Crystals of rouxelite [Cu₂HgPb₂₃Sb₂₇S_65.5 ] are acicular, elongated and striated, black in color but at times with a bluish violet iridescence, and have a metallic luster. They belong to the Monoclinic Crystal System, Prismatic Class. MinDat noted that the hardness of rouxelite could not be measured, and no cleavage was observed. The Handbook of Mineralogy.org also noted that some rouxelite from the TL contains thallium, silver, arsenic and bismuth.

The Type Specimen of rouxelite is from the Buca della Vena mine, Apuan Alps, Tuscany Italy, where it actually is quite rare. The mine is a small Fe–Ba deposit in the Apuan Alps  and represents a complex mineralogy (no mining past 1988). Many lead–antimony sulfosalts have been collected from the mine (79 valid minerals including 14 Types). Like the other lead sulfosalts, rouxelite was formed in the latest stage of hydrothermal activity, within small veinlets cross-cutting dolomitic lenses interstratified in the Ba–Fe ore (Orlandi and others, 2005). Distribution of rouxelite in Tuscany is limited to an area from the Buca della Vena Fe-Ba deposit, near the village of Ponte Stazzemese, then extending about 11 km southwest to the Sant’Olga tunnel, Monte Arsiccio mine, near Sant'Anna di Stazzema. The only other know specimens are from the Magurka antimony deposit, Slovakia. Indeed, it is a rare mineral (www.handbookofmineralogy.org/pdfs/rouxelite.pdf).

My specimen came from the St. Olga Tunnel which is, to the best of my knowledge, an opening into the Monte Arsiccio Mine. The mine was the economically the most important pyrite ± baryte ± iron oxide deposit in the Apuan Alps. The orebodies are hosted within a Paleozoic metavolcanic–metasedimentary sequence, locally tourmalinized, close to the contact with the overlying Triassic metadolostone (“Grezzoni” Formation), belonging to the Apuane Unit (D’Orazio and others, 2021).

The rouxelite from the Monte Arsiccio Mine (72 valid minerals including 13 types) was the most recent locality described (Biagioni and others, 2014). Perhaps the most interesting aspect of this locality is that electronic gizmos (Inductively Coupled Plasma Mass Spectrometry) show there is a widespread substitution of Hg (mercury) by Ag (silver) and some incorporation of Tl (thallium) with the silver and therefore is a new example of a thallium sulfosalt (D'Orazio and others, 2021). This seemingly obscure notation may be important since thallium is a highly toxic metal associated with low-temperature hydrothermal mineralization. Weathering and oxidation of sulfides (including sulfosalts) may generate acid drainage with a high concentration of thallium and may pose a threat to surrounding environments.

At the end of this little exercise on a rare and complex mineral the question is—why learn about rouxelite? Well, to quote B.B. King, The beautiful thing about learning is that no one can take it away from you.

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REFERENCES CITED

Biagioni, C., Y. Moelo, and P. Orlandi, Paolo, 2014. Lead-antimony sulfosalts from Tuscany (Italy). XV. (Tl-Ag)-bearing rouxelite from Monte Arsiccio mine: Occurrence and crystal chemistry. Mineralogical Magazine. Vol. 78.

D’Orazio, M.; Mauro, D.; Valerio, M.; Biagioni, C., 2021, Secondary Sulfates from the Monte Arsiccio Mine (Apuan Alps, Tuscany, Italy): Trace-Element Budget and Role in the Formation of Acid Mine Drainage: Minerals, vol. 11, no. 2.

Orlandi P, Meerschaut A, Moelo Y, Palvadeau P, Leone P, 2005, Lead-antimony sulfosalts from Tuscany (Italy). VIII. Rouxelite, Cu₂HgPb₂₂Sb₂₈S₆₄(O,S)₂, A new sulfosalt from Buca Della Vena mine, Apuan Alps: definition and crystal structure The Canadian Mineralogist 43 919-933.

Richards, J.P., 1999, Encyclopedia of Geochemistry in Encyclopedia of Earth Sciences Series, C.P Marshall and R.W. Fairbridge, eds.: Springer Netherlands.

Zhao, F. and S. Gu, 2021, Secondary sulfate minerals from thallium mineralized areas: Their formation and environmental significance: Minerals, vol. 11, no. 8.