Copper-Containing Magnesioferrite in Vesicular Trachyandesite in a Lava Tube from the 2012–2013 Eruption of the Tolbachik Volcano, Kamchatka, Russia

Abstract

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 Mg2+ ↔ Cu2+. Compositions with maximal CuO content nominally belong to Mg-rich cuprospinel: (Cu0.48Mg0.41Mn0.09Zn0.02Ca0.02) (Fe3+1.94Al0.03Ti0.02)O4. 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−1 and the appearance of an additional band at 596 cm−1. This supports the main isomorphic scheme and may indicate a degree of inversion in the spinel structure.

Keywords

• magnesioferrite
• cuprospinel
• hematite
• tenorite
• trachyandesite
• lava tube
• Tolbachik volcano
• Kamchatka