Occurrence of silesiaite, a new calcium–iron–tin sorosilicate in the calcic skarn of El Valle-Boinás, Asturias, Spain

Cepedal, Antonia; Fuertes-Fuente, Mercedes; Martin-Izard, Agustín

Silesiaite (Cainline-formula2Feinline-formula+3Sn(Siinline-formula2Oinline-formula7)(Siinline-formula2Oinline-formula6OH)), the Feinline-formula3+ analogue of kristiansenite (Cainline-formula2ScSn(Siinline-formula2Oinline-formula7)(Siinline-formula2Oinline-formula6OH)), has been found in the calcic Cu–Au skarn of El Valle-Boinás, in the north of Spain, which is the second occurrence of this mineral in the world. The study under optical microscopy shows crystals with a distinct pleochroism, from uncoloured to yellowish, high relief and imperfect cleavage under plain polarized light. Under polarized and analysed light, the mineral shows anomalous colours of interference and hourglass and sector optical zoning. Backscattered electron images reveal compositional zoning mimicking optical zoning with light grey (Sn-rich) and dark grey (Fe-rich) zones. The electron microprobe analyses showed that Fe-rich zones are also the richest in Al and Ti, whereas the Sn-rich zones are richest in Mn. The Feinline-formula+3 and Feinline-formula+2 proportions calculated by stoichiometry suggest a couple substitution such as inline-formula M15inlinescrollmathml normal 2 ( normal Fe , normal Al ) + normal 3 0.125emnobreak ( normal Sn , normal Ti ) + normal 4 + ( normal Fe , normal Mn , normal Mg ) + normal 2 194pt15ptsvg-formulamathimg3d0efe3bc76cdaaf2633d86b0507aed3 ejm-33-165-2021-ie00001.svg194pt15ptejm-33-165-2021-ie00001.png . According to this, the formula of the silesiaite can be written as inline-formula M16inlinescrollmathml normal Ca normal 2 () normal Fe normal 1 - normal 2 x + normal 3 normal Fe x + normal 2 normal Sn x normal Sn )( normal Si normal 2 normal O normal 7 )( chem normal Si normal 2 normal O normal 6 normal OH 213pt22ptsvg-formulamathimge12b5725e5987469ca5a933313869a04 ejm-33-165-2021-ie00002.svg213pt22ptejm-33-165-2021-ie00002.png , where inline-formulax is between 0 and 0.4.

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Cepedal, Antonia / Fuertes-Fuente, Mercedes / Martin-Izard, Agustín: Occurrence of silesiaite, a new calcium–iron–tin sorosilicate in the calcic skarn of El Valle-Boinás, Asturias, Spain. 2021. Copernicus Publications.

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