BULK MINERALOGY OF INDIVIDUAL MICROMETEORITES DETERMINED BY X-RAY DIFFRACTION ANALYSIS AND TRANSMISSION ELECTRON MICROSCOPY

Abstract

Bulk mineralogy of individual fine-grained micrometeorites from 50 to 200 μm in diameter was determined on the basis of the powder X-ray diffraction patterns and the observation of internal textures by a transmission electron microscope (TEM). X-ray diffraction 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 Α, 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%, respectively, when the total abundance of the four minerals are normalized to 100 wt%. 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 (~100 nm) of olivine + magnetite, or low-Ca pyroxene + olivine + magnesiowustite. The textures are very similar to those of hydrous carbonaceous chondrite that was experimentally heated to temperature below melting point, thus suggesting that the micrometeorites had been hydrous particles but were decomposed by the brief heating upon atmospheric entry. It is newly found that magnesiowustite was formed in micrometeorites instead of magnetite as a product of phyllosilicate decomposition under low oxygen fugacity. The decomposed hydrous micrometeorites gave two types of characteristic X-ray diffraction patterns: (1) broad olivine and magnetite reflections or (2) variable intensities of magnesiowustite reflections together with magnetite, low-Ca pyroxene, and olivine reflections. Twenty-nine olivine- or pyroxene-rich micrometeorites showed such diffraction patterns, thus suggesting that more than half of micrometeorites investigated must be decomposed hydrous particles. The results confirmed that hydrous dust particles are much more abundant in the interplanetary space than in the micrometeorites recovered on the Earth.