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