| Summary: | Cu-rich magnesioferrite was found in vesicular basaltic trachyandesite in one of lava tubes (Duplex) that formed during the 2012⁻2013 eruption of the Tolbachik volcano, Kamchatka. This mineral is commonly associated with hematite, tenorite, halite, sylvite, and Ca-rich silicates (mainly, esseneite and Na-rich melilite) in high-temperature (800⁻1000 °C) reactionary zones (up to 100 µm) covering vesicular rocks and lava stalactites in the Duplex tube. The mineral relationships of this assemblage indicate the following crystallization sequence: Ca-rich silicates + hematite → Cu-rich magnesioferrite → tenorite → chlorides. This formed due to the reaction of hot gases containing Cu, alkalis, and Cl with solidified lava rock. The composition of magnesioferrite varies strongly in CuO (5.8⁻17.3 wt %; cuprospinel end-member—15⁻47 mol %), whereas the contents of other oxides are minor, indicating the main isomorphic substitution is Mg<sup>2+</sup> ↔ Cu<sup>2+</sup>. Compositions with maximal CuO content nominally belong to Mg-rich cuprospinel: (Cu<sub>0.48</sub>Mg<sub>0.41</sub>Mn<sub>0.09</sub>Zn<sub>0.02</sub>Ca<sub>0.02</sub>) (Fe<sup>3+</sup><sub>1.94</sub>Al<sub>0.03</sub>Ti<sub>0.02</sub>)O<sub>4</sub>. Increasing CuO content of the Duplex Cu-rich magnesioferrite is reflected in Raman spectra by moderate right shifting bands at ≈700⁻710 and 200⁻210 cm<sup>−1</sup> and the appearance of an additional band at 596 cm<sup>−1</sup>. This supports the main isomorphic scheme and may indicate a degree of inversion in the spinel structure.
|
|---|
