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

Abstract. Silesiaite (Ca 2 Fe +3 Sn(Si 2 O 7) (Si 2 O 6 OH)), the Fe 3+ analogue of kristiansenite (Ca 2 ScSn(Si 2 O 7) (Si 2 O 6 OH)), has been found in the calcic Cu–Au skarn of El Valle-Boinas, 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 Fe +3 and Fe +2 proportions calculated by stoichiometry suggest a couple substitution such as 2 ( Fe , Al ) + 3 ⇔ ( Sn , Ti ) + 4 + ( Fe , Mn , Mg ) + 2 . According to this, the formula of the silesiaite can be written as Ca 2 Fe 1 - 2 x + 3 Fe x + 2 Sn x Sn Si 2 O 7 Si 2 O 6 OH , where x is between 0 and 0.4.