Evaporation experiments of metallic iron in vacuum

Evaporation experiments were carried out to examine evaporation kinetics of metallic iron, one of the important materials forming terrestrial planets and meteorites. Platelets of pure metallic iron were heated at temperatures ranging from 1075 to 1312℃ under vacuum (10^ to 10^ Torr) for 0.5 to 96 hrs. The evaporation proceeds by forming evaporation steps, although small wustite crystals were formed on the surfaces by partial oxidation of iron under vacuum. Amounts of evaporated iron estimated from mass loss of experimental charges increased with time at constant temperatures, showing a linear rate law. The evaporation rates, j, can be represented by ln j=22.21±2.29[mol m^s^]-390.6±29.2[kJ mol^]/RT. The evaporation coefficients, α_v, were obtained by comparing the experimental results with calculated rates using the Hertz-Knudsen equation. The value of α_v is close to unity if effects of partial oxidation are taken into consideration. The present results give basic information for discussing chemical evolution of the primordial solar nebula

Fe-Mg heteorogeneity in the low-Ca pyroxenes during metamorphism of the ordinary chondrites

Pyroxenes in nine ordinary chondrites, ALH-764 (LL3), ALH-77214 (L3.4), ALH-77015 (L3.5), Yamato-74191 (L3.6), Hedjaz (L3.7), ALH-77304 (LL3.8), ALH-78084 (H3.9), Yamato-75097 (L4) and ALH-77230 (L4), were examined by an optical microscope, a scanning electron microscope with a back-scattered electron image technique, and an X-ray microprobe analyzer. Characteristic textures due to alternating lamellae of Fe-rich and Fe-poor compositions have been found in the low-Ca pyroxenes in the chondrites irrespective of their chemical groups, H, L and LL. As far as the author knows, this is the first observation of such lamellae textures in the pyroxenes. These textures are common and remarkable in the higher subtypes of type 3 chondrites (L3.6,L3.7,LL3.8 and H3.9), while they are rare in lower subtypes (<3.5) and type 4 chondrites. These textures are considered to have been formed in the Fe-Mg homogenization process of the ordinary chondrites during metamorphism

Petrological and geochemical study of the Yamato-74359 and Yamato-74360 achondrites

Two Antarctic achondrites, Yamato (Y)-74359 and Y-74360,are very similar in bulk chemical compositions to the silicate portions of H chondrites, but the siderophile elements are extremely depleted in comparison to those of H chondrites. They consist of olivine, pyroxene and cryptocrystalline feldspar with minor amounts of chromite, kamacite, and troilite, and the chemical compositions of the constituent minerals are similar to those in H chondrites. Taking into consideration that the two achondrites have oxygen isotopic compositions typical of H chondrites, they were produced from H chondrites or the precursors of H chondrites. Chondrules are not observed, and the texture of the two achondrites is similar to one another, although Y-74360 is coarser-grained than Y-74359. Olivine occurs as euhedral or subhedral grains, mostly smaller than 100 microns in diameter, and they show slight chemical zoning, from magnesian cores (Fo_) to ferroan rims (Fo_). Orthopyroxene occurs as euhedral grains, larger than olivine grains, and also shows slight chemical zoning, from Ca-poorer magnesian cores to Ca-richer ferroan rims. Clinopyroxene occurs as rims of orthopyroxenes and shows remarkable chemical zoning continuously from pigeonitic interiors to augitic rims. Cryptocrystalline feldspar occurs in interstitial spaces between olivine and/or pyroxene, and seems to have crystallized in interstitial liquids in a rapid cooling condition. The cryptocrystalline feldspars in Y-74359 are chemically heterogeneous, classified into three groups, albite (An_Ab_Or_), anorthoclase (An_Ab_Or_), and intermediate alkali feldspar (An_Ab_Or_), and occur in different portions of the thin section. The two achondrites seem to have formed by partial melting from heterogeneous silicate precursors of unequilibrated H chondrites

Evaporation of Fe and FeS dust in the active stage of the primordial solar nebula, and Fe/S fractionation

The evaporation kinetics of troilite and metallic iron was applied to evaporation of dust particles moving toward the protosun in the turbulent solar nebula. In the calculations, it was assumed that dust particles do not grow by collision, evaporated gas and residual dust are not separated, and dust particles move only radially along the midplane or the surface of the nebula. It was found that evaporation of metallic iron would occur almost in equilibrium both along the midplane and the surface. Troilite could survive to higher temperature than the equilibrium evaporation temperature due to its evaporation kinetics. However, the kinetic effects are not so large, and the incongruent evaporation of troilite is also regarded to occur roughly in equilibrium. The timescales for evaporation of metallic iron and troilite were compared with the timescales for drifts along r-and z-directions and that for coagulation to understand general aspects of the effect of evaporation kinetics. Since the temperature of the surface is lower than that of the midplane, dust particle at the surface can get closer to the sun than those at the midplane. This can cause Fe/S fractionation in a wide range of the nebula if effective solid-gas separation occurred

Antarctic primitive achondrites Yamato-74025, -75300, and -75305:Their mineralogy, thermal history and the relevance to winonaite

Three Antarctic primitive achondrites, Yamato (Y)-74025,-75300,and -75305 were mineralogically and chemically studied. They consist of anhedral to subhedral silicate and opaque minerals. The major constituent minerals are typical of equilibrated ordinary chondrites. However, they do not have any relic of chondrule, and the presence of various accessory minerals, such as K-feldspar, schreibersite, daubreelite, phosphate, Nb-bearing rutile, and magnesiochromite, characterizes these meteorites. Y-75305 has a composite grain containing Cu, Mn, and S, probably consisting of alabandite, an unknown Mn-bearing Cu-sulfide, and digenite. Y-74025 has a REE pattern typical of chondrite. Siderophile elements in Y-74025 are depleted relative to Cl chondrites, which is consistent with poor abundance of Fe-Ni metal in Y-74025. Holocrystalline texture, homogeneous mineral compositions, and high equilibration temperatures for pyroxenes, suggest that these primitive achondrites experienced high-temperature metamorphism. Mineralogical and chemical characteristics suggest that they resemble Winona-like meteorites (winonaites). The compositions of pyroxene and olivine, and accessory minerals suggest that winonaites formed under an intermediate redox condition between E-chondrites and Acapulco-like primitive achondrites. The abundance of troilite and Fe-Ni metal varies widely. The metal-sulfide fractions of winonaites probably melted and fractionated, although silicate fractions of winonaites do not have any evidence for melting

High resolution X-ray computed tomographic (CT) images of chondrites and a chondrule

In order to study internal textures of meteorites, images were obtained by X-ray computer tomography (CT). This combined high resolution X-ray radiography and computer tomography system belongs to the so-called third generation type with a micro-focus X-ray source and a linear CCD detector with 2048 elements. This allows a spacial resolution of a few μm in the images. Samples examined include the Moorabie meteorite (L3), Allende meteorite (CV3), and a chondrule removed from Allende meteorite. In images, Fe-Ni alloy, troilite, and silicates can be distinguished clearly, and chondrules can be resolved from their Fe-rich rims in Moorabie meteorite. In Allende Fe-Ni alloy, pentlandite, and silicates can be distinguished, and chondrules, CAI\u27s, and matrix can be recognized. Many euhedral crystals, probably olivine and/or pyroxene, were identified in a chondrule, suggesting that the chondrule has a porphyritic texture. In addition to minerals or their assemblages, holes can be identified by the X-ray CT method and were found in chondrules in Allende

3D Observation of GEMS by Electron Tomography

Amorphous silicates in chondritic porous interplanetary dust particles (CP-IDPs) coming from comets are dominated by glass with embedded metal and sulfides (GEMS). GEMS grains are submicron-sized rounded objects (typically 100-500) nm in diameter) with anaometer-sized (10-50 nm) Fe-Ni metal and sulfide grains embedded in an amorphous silicate matrix. Several formation processes for GEMS grains have been proposed so far, but these models are still being debated [2-5]. Bradley et al. proposed that GEMS grains are interstellar silicate dust that survived various metamorphism or alteration processes in the protoplanetary disk and that they are amorphiation products of crystalline silicates in the interstellar medium by sputter-deposition of cosmic ray irradiation, similar to space weathering [2,4]. This consideration is based on the observation of nano-sized crystals (approximately 10 nm) called relict grains in GEMS grains and their shapes are pseudomorphs to the host GEMS grains. On the other hand, Keller and Messenger proposed that most GEMS formed in the protoplanetary disk as condensates from high temperature gas [3,5]. This model is based on the fact that most GEMS grains have solar isotopic compositions and have extremely heterogeneous and non-solar elemental compositions. Keller and Messenger (2011) also reported that amorphous silicates in GEMS grains are surrounded by sulfide grains, which formed as sulfidization of metallic iron grains located on the GEMS surface. The previous studies were performed with 2D observation by using transmission electron microscopy (TEM) or scanning TEM (STEM). In order to understand the structure of GEMS grains described above more clearly, we observed 3D structure of GEMS grains by electron tomography using a TEM/STEM (JEM-2100F, JEOL) at Kyoto University. Electron tomography gives not only 3D structures but also gives higher spatial resolution (approximately a few nm) than that in conventional 2D image, which is restricted by sample thickness ) approx. or greater than 50 nm). Three cluster IDPs (L2036AA5 cluster4, L2009L8 cluster 13 and W726A2) were used for the observations. ID W726A2 was collected without silicon oil, which is ordinary used to collect IDPs, so this sample has no possibility of contaminations caused by silicon oil or solvent to rinse it [6]. The samples were embedded in epoxy risin and sliced into ultrathin sections (50-300 nm) using an ultramicotome. The sections were observed by BF-TEM and HAADF-STEM (high angle annular dark field-scanning TEM) modes. Images were obtained by rotating the sample tilt angle over a range of +/- 65 deg in 1 deg steps. The obtained images were reconstructed to slice images. Mineral phases in the slice images were estimated by comparing with a 2D elemental map obtained by an EDS (energy dispersive X-ray spectroscopy) system equipped in the TEM/STEM. Careful examination of the slice images confirmed that iron grains are embedded in the amorphous silicate matrix of the GEMS grains, but sulfide grains were mainly present on the surface of the amorphous silicate. These results are consistent with the model that GEMS grains formed as condensates [3,5], although more data are needed to conclude the origin of GEMS grains. The present study is the first successful example adapting the electron tomography to the IDPs. This type of analysis will be important for planetary material sciences in the future

An electron microscopic study of gas condensates in the system Mg-Si-O-H

Condensates of MgSiO_3 and SiO_2 from a gas formed by evaporation of enstatite at an H_2 pressure of 4.4×10^ bar and a temperature of 1525℃ by B. O. MYSEN and I. KUSHIRO (Am. Mineral. (in press), 1988) and I. KUSHIRO and B. O. MYSEN (Advances in Physical Geochemistry, New York, Springer (in press), 1988) were investigated with an analytical transmission electron microscope (ATEM), a scanning electron microscope (SEM) and an electron probe microanalyzer (EPMA). With decreasing temperature at an approximately constant total pressure the Mg/(Mg+Si) atomic ratio of the condensate (mixture of MgSiO_3 and SiO_2 polymorphs) decreases first, then increases, and finally reaches a constant value. This compositional change of the condensate is inconsistent with the equilibrium condensation model. The TEM studies suggest that metastable condensation of coesite and probably of protoenstatite and cristobalite took place. Coesite probably condensed by heterogeneous nucleation on protoenstatite. Fibrous quartz was also formed by heterogeneous nucleation on molybdenum fibers which condensed from a molybdenum vapor by a partial evaporation of a Knudsen cell used in the experiment. Heterogeneous nucleation might have played an important role in condensation process in the solar nebula. The texture of the experimental clinopyroxene condensate is different from that in interplanetary dust particles (J. P. BRADLEY et al., Nature, 301,473,1983)

Fragment shapes in impact experiments ranging from cratering to catastrophic disruption

Laboratory impact experiments have found that impact fragments tend to be elongated. Their shapes, as defined by axes a, b and c, these being the maximum dimensions of the fragment in three mutually orthogonal planes (a&ge; b&ge; c), are distributed around mean values of the axial ratios b/. a~. 0.7 and c/. a~. 0.5. This corresponds to a:. b:. c in the simple proportion 2:2:1 The shape distributions of some boulders on Asteroid Eros, the small- and fast-rotating asteroids (diameter &lt;200. m and rotation period &lt;1 h), and asteroids in young families, are similar to those of laboratory fragments created in catastrophic disruptions. Catastrophic disruption is, however, a process that is different from impact cratering. In order to systematically investigate the shapes of fragments in the range from impact cratering to catastrophic disruption, impact experiments for basalt targets 5-15. cm in size were performed. A total of 28 impact experiments were carried out by firing a spherical nylon projectile (diameter 7.14. mm) perpendicularly into the target surface at velocities of 1.60-7.13. km/s. More than 12,700 fragments with b&ge; 4 mm generated in the impact experiments were measured. We found that the mean value of c/ a in each impact decreases with decreasing impact energy per unit target mass. For instance, the mean value of c/ a in an impact cratering event is nearly 0.2, which is considerably smaller than c/ a in a catastrophic disruption (~0.5). The data presented here can provide important evidence to interpret the shapes of asteroids and boulders on asteroid surfaces, and can constrain current interpretations of asteroid formation. As an example, by applying our experimental results to the boulder shapes on Asteroid Itokawa's surface, we can infer that Itokawa's parent body must have experienced a catastrophic disruption. &copy; 2015 The Authors