Vcu

Figure 1. Backscattered electron images of MMs with XRD charts of them. These MMs show four types of typical mineral assemblages in MMs. Abbreviations: Ol: olivine, Px: pyroxene, Mt: magnetite, Sul: Fe-sulfide.

contain Mg- and Fe-oxide (magnesiowtistite); the first discovery of magnesiowustite in MMs.

2.2 TEM observation of MMs

Only one phyllosilicate-bearing MM (type 1) was found among 83 MMs collected in 1996 [7]. It contains abundant saponite. Its mineralogy is similar to the phyllosilicate-bearing MM previously reported [4], However, this MM contains abundant aggregates of magnesiowustite (hereafter, MW) and Fe sulfide (Figure 2). The aggregates are surrounded by bundles of saponite. Most of the MW grains in such aggregates are < 20 nm across. During the preliminary study of MMs collected in 1998-1999, another saponite-rich MM was found. It also contains MW-bearing aggregates. As well as the absence of serpentine, MW-bearing aggregates may be also an unique feature of saponite-rich MMs. Although whether such aggregates are primary constituents or not, it is more plausible that they were formed from minerals that decomposed during weak atmospheric entry heating. MW in the recently found MM contains Mn as well as Mg and Fe. Its composition suggests that MW in this MM was formed from Mg, Fe, Mn-bearing minerals such as Mg, Fe, Mn-bearing carbonates found in CI chondrites [10].

As described in 2.1, MW is often found in MMs that experienced relatively weak heating (type 3). TEM observation revealed that MW coexists with low-Ca pyroxene intimately (Figure 3). These minerals have typical morphology of recrystallization. Because these minerals are absent in the peripheries of MMs, where abundant anhydrous minerals are olivine and magnetite, mixture of MW and low-Ca pyroxene may have been formed from phyllosilicates under slightly reducing conditions during atmospheric entry.

Figure 3. A TEM photomicrograph of MW

Figure 2. (A) A TEM photograph of a MW in a moderately heated type 3 MM.. MW

and Fe-sulfide aggregate in a smectite-rich in them coexists with low-Ca pyroxene, type 1 MM. (B) MW contains only Mg and Grain boundaries of these minerals show

Fe peaks. (Cu peaks from the supporting typical texture of recrystallization. Their grid) (C) SAED pattern showing EDS spectrum and SAED pattern indicate diffraction rings from MW. that the crystals are MW.

Figure 3. A TEM photomicrograph of MW

Figure 2. (A) A TEM photograph of a MW in a moderately heated type 3 MM.. MW

and Fe-sulfide aggregate in a smectite-rich in them coexists with low-Ca pyroxene, type 1 MM. (B) MW contains only Mg and Grain boundaries of these minerals show

Fe peaks. (Cu peaks from the supporting typical texture of recrystallization. Their grid) (C) SAED pattern showing EDS spectrum and SAED pattern indicate diffraction rings from MW. that the crystals are MW.

2.3 Noble gas analysis

Noble gas isotopic compositions in fine particles including individual MMs were analyzed by stepped pyrolysis [10]. Figure 4 shows a three-isotope diagram of Ne in MMs collected in 1996. The high ratio of solar/cosmogenic gas in MMs suggests that they had been small particles in space to have wide surface areas exposed to solar winds, being consistent with the results of previous works [9-14], These MMs are not particles produced by breaking-up of hydrous carbonaceous chondrites during the atmospheric entry; otherwise a set of such particles must have Ne isotopic ratio close to an average value of hydrous carbonaceous chondrites enriched in cosmogenic gases. Based on these noble-gas compositions, they emerged into interplanetary space less than a few million years ago as individual particles and came to the earth in 1950-1980.

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