Quantitative Analysis of Mt. St. Helens Ash by X-Ray Diffraction and X-Ray Fluorescence Spectrometry

Abstract

A quantitative study by x-ray diffraction, optical polarizing microscopy, and x-ray fluorescence spectrometry of fallout and ambient ash from three Mt. St. Helens eruptions has revealed a consistent picture of the mineralogical and elemental composition. The major components observed are amorphous glass, plagioclase of about An50 composition, minor amounts of quartz and other SiO2 polymorphs, as well as ferromagnesian constituents such as hypersthene, magnetite, etc. Free SiO2 exists in all samples, but in no case (even considering large uncertainties in the cristobalite analyses) does the free SiO2 exceed 8% of the total aerosol mass. The variation in the glass/plagioclase ratio with particle size observed in four samples suggests that much of the crystalline material exists in the finer particle size fraction of the ash. Reduction of the mineralogical constituents to component oxides has made it possible to compare the diffraction results with direct elemental determination by x-ray fluorescence; the two data sets are generally in good agreement. Exceptions to this are in the Fe-Mg content and Al2O3/SiO2 ratio. In the former case, the differences are explained by the lack of ferromagnesian accessory minerals detectable by the x-ray diffraction method. All data support an andesite classification for the ash. Record precipitation observed in the northern Rocky Mountains during May coincides roughly with the volcanic plume trajectory, which raises the question of a possible seeding influence on storms over the area. The mineralogical makeup of the plume, however, is not particularly suited to precipitation enhancement through nucleation of the ice phase.