Natural Fluid-Phase Alloying of Basaltic Lavas and Volcanic Bombs
DOI:
https://doi.org/10.17308/geology/1609-0691/2026/1/73-83Keywords:
volcano, lava, basalt, chlorides, sulfides, arsenides, selenides, gold, platinum, rheniumAbstract
Introduction: volcanoes are natural reactors that carry out high-parameter physical and chemical processes from the magma chamber to the surface. During an eruption, gases and ash are released, followed by the eruption of lava, which is accompanied by continuous degassing. On the surface of lava flows and volcanic bombs, a thin colored film (steel gray, bronze, or blue-green) sometimes forms, the composition of which is still poorly understood.
Methodology: the composition of iridescent films was studied using X-ray phase analysis, scanning electron microscopy, and microanalysis. The qualitative interpretation of the diffractionometry results was performed using the «ICDD PDF-2» database (2022) and the «Oxford Crystallographica» software. The quantitative analysis of the phase composition was performed using the Rietveld method in the «Siroquant» software.
Results and Discussion: the mineral composition of most iridescent films is based on calcium feldspars (anorthite), pyroxenes, and olivines. All samples contain micro- and nanosized phases of noble metals and rhenium (chlorides, iodides, arsenides, sulfides, selenides, fluoroplatinum, molybdate, and oxide forms). The zero-valent state of the elements is represented by ferroplatinum, compounds of platinum-group metals with copper, aurostibite, and an intermetallic alloy of gold with copper and zinc. Rhenium is present in an atypical fluoride form (rhenium tetrafluoride) and as a natural analog of lithium-rhenium oxides (lithium-rhenium glasses and conductive alloys of Li and Re with amorphous SiO2). The film surfaces have a uniform morphology and are characterized by a rosette-like or block-like structure. The identification of rare elements and their compounds in the surface layer of fresh lava material from basaltoid eruptions that has undergone degassing confirms the assumption of a genetic connection between ore mineralization and the fluid system of basaltoid volcanism and the mantle source of ore elements.
Conclusions: the ultrathin films on basalts from the Tolbachik Fissure Eruption (TTI-50) consist of uniform micron-sized particles. Their formation is likely related to a rapid drop in temperature and pressure, causing quick condensation of volcanic gases at the rock-gas interface and the formation of a passivation film – a silicate layer enriched with noble (Au, Pt, Pd) and rare (Re, Ir) metals. The main research outcome is the discovery and description of the natural alloying of volcanic rock surfaces with an ultrathin film. This film contains carbon, significant amounts of Fe, Cu, Ag, Au, Pt, Pd, as well as traces of Re, Rh, Ir, Mo, Os, and Sb.
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