Silica-rich coatings of up to 660 μm thickness were found on the juvenile volcanic lapilli and blocks of the 1783 CE pyroclastic flow deposits 1.9–3.6 km northeast of the Maekake crater of Asama volcano in Central Japan. These coatings are composed of fine-grained volcanic ash particles with silica-rich cement. Raman spectroscopic measurements revealed that the interstitial silica-rich phase is hydrous amorphous silica and is considered to be a reaction product of ash particles with acidic volcanic rain and fog like volcanic smog (vog) as it has been recorded in Hawaii. The formation of amorphous silica without jarosite and the preferential alteration of plagioclase compared with pyroxene indicate that the reaction occurred with a sulfuric acid solution of pH < ~3. The porous structure and large surface area of the ash layer would have been favorable for the dissolution of sintered volcanic ash and the precipitation of amorphous silica. The thickness of the coating and degree of alteration in the plagioclase phenocrysts adjacent to the coating tend to decrease with increasing distance from the volcanic crater, which is consistent with the absence of fumarolic areas in the flank. The Asama-Maekake volcano has repeated Vulcanian and phreatic eruptions since 1783 CE, especially in the first half of the 20th century. The amorphous silica coating recognized in this study is assumed to have formed mainly during this period, which can thus be an indicator of the acidity and distribution of past acidic volcanic rain and fog.
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