Abstract
In the experiments on volborthite in situ and ex situ heating, analogues of all known natural anhydrous copper vanadates have been obtained: ziesite, pseudolyonsite, mcbirneyite, fingerite, stoiberite and blossite, with the exception of borisenkoite, which requires the presence of As in the V site. The evolution of Cu-V minerals during in situ heating is as follows: volborthite Cu3(V2O7)(OH)2·2H2O (30–230 °C) → X-ray amorphous phase (230–290 °C) → ziesite β-Cu2(V2O7) (290–430 °C) → ziesite + pseudolyonsite α-Cu3(VO4)2 + mcbirneyite β-Cu3(VO4)2 (430–510 °C) → mcbirneyite (510–750 °C). This trend of mineral evolution agrees with the thermal analytical data. These phases also dominate in all experiments with an ex situ annealing. However, the phase compositions of the samples annealed ex situ are more complex: fingerite Cu11(VO4)6O2 occurs in the samples annealed at ~250 and ~480 °C and quickly or slowly cooled to room temperature, and in the sample annealed at ~850 °C with fast cooling. At the same time, blossite and stoiberite have been found in the samples annealed at ~480–780 and ~780–850 °C, respectively, and slowly cooled to room temperature. There is a trend of decreasing crystal structure complexity in the raw phases obtained by the in situ heating with the increasing temperature: volborthite → ziesite → mcbirneyite (except of pseudolyonsite). Another tendency is that the longer the sample is cooled, the more complex the crystal structure that is formed, with the exception of blossite, most probably because blossite and ziesite are polymorphs with identical crystal structure complexities. The high complexity of fingerite and stoiberite, as well as their distinction by Cu:V ratio, may explain the uncertain conditions of their formation.
Highlights
Volborthite, Cu3(V2O7)(OH)2·2H2O, is commonly considered as a mineral formed in the oxidation zones of Cu-V deposits [1,2] (Table 1)
Seven copper vanadates have been found in nature: blossite [8] and ziesite [9], which are orthorhombic and monoclinic polymorphs of Cu2(V2O7), respectively; fingerite [10], Cu11(VO4)6O2; mcbirneyite [11] and pseudolyonsite [12], which are triclinic and monoclinic polymorphs of Cu3(VO4)2, respectively; borisenkoite [13], Cu3[(V,As)O4]2; stoiberite, Cu5(VO4)2O2 [14]
Ziesite, fingerite, mcbirneyite and stoiberite have been found in fumaroles at the Izalco volcano (Sonsonate Department, El Salvador), while pseudolyonsite and borisenkoite come from the fumaroles of Tolbachik volcano (Kamchatka, Russia)
Summary
Volborthite, Cu3(V2O7)(OH)2·2H2O, is commonly considered as a mineral formed in the oxidation zones of Cu-V deposits [1,2] (Table 1). Seven copper vanadates have been found in nature: blossite [8] and ziesite [9], which are orthorhombic and monoclinic polymorphs of Cu2(V2O7), respectively; fingerite [10], Cu11(VO4)6O2; mcbirneyite [11] and pseudolyonsite [12], which are triclinic and monoclinic polymorphs of Cu3(VO4), respectively; borisenkoite [13], Cu3[(V,As)O4]2; stoiberite, Cu5(VO4)2O2 [14]. These anhydrous copper vanadates are endemic or found in a couple of localities worldwide. Ziesite, fingerite, mcbirneyite and stoiberite have been found in fumaroles at the Izalco volcano (Sonsonate Department, El Salvador), while pseudolyonsite and borisenkoite come from the fumaroles of Tolbachik volcano (Kamchatka, Russia)
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