Bulk mineralogy of individual micrometeorites determined by X-ray diffraction analysis and transmission electron microscopy

Bulk mineralogy of individual fine-grained micrometeorites from 50 to 200 microns in diameter was determined on the basis of the powder XRD patterns and the observation of internal textures by TEM. XRD analysis of 56 micrometeorites indicated that 42, 11, and 3 samples are olivine-rich, pyroxene-rich, and phyllosilicate-rich micrometeorites, respectively. Among the phyllosilicate-rich micrometeorites, one contains saponite and other two contain serpentine. No samples contain both saponite and serpentine. We found that saponite-rich micrometeorite was weakly heated, which results in shrinkage of 001 basal spacing of saponite down to 9.7 A, and that cronstedtite, which is commonly contained in CM chondrites, occurs in serpentine-rich micrometeorites. Micrometeorites that consist entirely of anhydrous minerals and amorphous phases are predominant in the samples studied. The major phases of such micrometeorites are olivine, low-Ca pyroxene, magnetite, and Fe-sulfide and the average abundances are 65, 17, 11, and 7 wt pct, respectively, when the total abundance of the four minerals are normalized to 100 wt pct. The relative mineral abundance varies greatly between samples: low-Ca pyroxene/olivine ratios range from 0 to 3.5, with a mean of 0.3. TEM observations of inner portions of some micrometeorites revealed that they are aggregates of very small equigranular grains of olivine + magnetite, or low-Ca pyroxene + olivine + magnesiowuestite. (Author)