Nanometer-scale mineralogy and petrography of fine-grained aggregates in anhydrous interplanetary dust particles

Abstract

Fine-grained matrices in seven (solar flare) track-rich anhydrous interplanetary dust particles (IDPs) were studied using (200 keV) analytical electron microscopy. The IDPs are U222B42, U220A19, U222B28, U230A3, W7027A11, W7027H14, U219-C2, (and U219-C11). The matrices are composed of aggregates of glass, disordered carbonaceous material, and crystalline mineral grains with diameters between 1 and 150 nm. The mineral grains were characterized using lattice fringe imaging, electron microdiffraction, and quantitative energy-dispersive X-ray spectroscopy (EDS). The glasses were studied using EDS and electron energy-loss near edge structure analysis (ELNES). Three types of aggregates were observed, unequilibrated, equilibrated, and reduced aggregates. Unequilibrated aggregates (UAs) are 0.1–0.3 μm diameter objects with bulk major element compositions that are approximately chondritic (solar) except for carbon. They contain FeNi metal and Fe-rich sulfide crystals, between 1 and 20 nm diameter, embedded in silicate glass. Equilibrated aggregates (EAs) are 0.1 to 1 μm diameter objects that exhibit melt textures and mineralogy. They contain Fe-bearing olivine and pyroxene grains, with equilibrated Fe Mg ratios, and Fe-sulfides embedded in feldspathic (alumino-silicate) glass. Reduced aggregates (RAs) are 0.1 to 1.0 μm objects with FeNi metal, FeNi carbides, and Fe-rich sulfides embedded in a carbonaceous matrix. The aggregates in the seven anhydrous IDPs appear to have formed in diverse physiochemical environments.