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.
Highlights
Copper-rich spinels are well-documented in synthetic compounds, but rarely occur as minerals in the natural environment
Identification of minerals was based on energy-dispersive spectra (EDS), back-scattered electron (BSE) images, and elemental mapping (EDS system), using a TESCAN MIRA 3MLU scanning electron microscope equipped with an INCA Energy 450 XMax 80 microanalysis system (Oxford Instruments Ltd., Abingdon, UK) at the V.S
The groundmass in this naturally solidified sample is sometimes finely crystallized with dendritic crystals, which consists of clinopyroxene, Ti-rich magnetite, fluorapatite, K-feldspar (Or74–90 ), and locally Si-rich glass (SiO2 —65–71 wt %)
Summary
Copper-rich spinels are well-documented in synthetic compounds, but rarely occur as minerals in the natural environment Their origin is largely attributed to combustion processes or fumarole activity in young volcanoes. Cuprospinel, CuFe3+ 2 O4 , was first described as a new mineral species from burnt dump from the Consolidated Rambler Mines Limited near Baie Verte, Newfoundland, Canada [1] In this semi-anthropogenic system, cuprospinel is associated with Cu-rich magnesioferrite Minerals 2018, 8, x FOR PEER REVIEW were found as(CuFeO products of oxidative combustion of copper sulfides and related phases CuO: gahnite, spinel, and magnesioferrite) were recently (CuAl2O4; IMA 2018-021), cuprospinel, and other Cu-bearing spinel-group minerals Provide new insights into a new Cu-dominant clan within the spinel supergroup [6]
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