Kazumasa Ohsumi

Mineralogy and Petrology of Comet 81P/Wild 2 Nucleus Samples

The bulk of the comet 81P/Wild 2 (hereafter Wild 2) samples returned to Earth by the Stardust spacecraft appear to be weakly constructed mixtures of nanometer-scale grains, with occasional much larger (over 1 micrometer) ferromagnesian silicates, Fe-Ni sulfides, Fe-Ni metal, and accessory phases. The very wide range of olivine and low-Ca pyroxene compositions in comet Wild 2 requires a wide range of formation conditions, probably reflecting very different formation locations in the protoplanetary disk. The restricted compositional ranges of Fe-Ni sulfides, the wide range for silicates, and the absence of hydrous phases indicate that comet Wild 2 experienced little or no aqueous alteration. Less abundant Wild 2 materials include a refractory particle, whose presence appears to require radial transport in the early protoplanetary disk.

Radar-Enabled Recovery of the Sutter’s Mill Meteorite, a Carbonaceous Chondrite Regolith Breccia

The Meteor That Fell to Earth In April 2012, a meteor was witnessed over the Sierra Nevada Mountains in California. Jenniskens et al. (p. 1583) used a combination of photographic and video images of the fireball coupled with Doppler weather radar images to facilitate the rapid recovery of meteorite fragments. A comprehensive analysis of some of these fragments shows that the Sutters Mill meteorite represents a new type of carbonaceous chondrite, a rare and primitive class of meteorites that contain clues to the origin and evolution of primitive materials in the solar system. The unexpected and complex nature of the fragments suggests that the surfaces of C-class asteroids, the presumed parent bodies of carbonaceous chondrites, are more complex than previously assumed. Analysis of this rare meteorite implies that the surfaces of C-class asteroids can be more complex than previously assumed. Doppler weather radar imaging enabled the rapid recovery of the Sutter’s Mill meteorite after a rare 4-kiloton of TNT–equivalent asteroid impact over the foothills of the Sierra Nevada in northern California. The recovered meteorites survived a record high-speed entry of 28.6 kilometers per second from an orbit close to that of Jupiter-family comets (Tisserand’s parameter = 2.8 ± 0.3). Sutter’s Mill is a regolith breccia composed of CM (Mighei)–type carbonaceous chondrite and highly reduced xenolithic materials. It exhibits considerable diversity of mineralogy, petrography, and isotope and organic chemistry, resulting from a complex formation history of the parent body surface. That diversity is quickly masked by alteration once in the terrestrial environment but will need to be considered when samples returned by missions to C-class asteroids are interpreted.

Florenskyite, FeTiP, a new phosphide from the Kaidun meteorite

Abstract Florenskyite is a new phosphide species from the Kaidun chondritic meteorite, which fell in South Yemen in 1980. Kaidun is a unique chondritic breccia containing a huge variety of fragments of different chondritic types. Florenskyite was found as four dispersed grains with a maximum dimension of 14 μm within a single mass of Fe-rich serpentine within one Kaidun clast. Florenskyite is associated with submicrometer-sized grains of pentlandite and small (up to 1.5 μm in width) laths of a still uncharacterized Fe-Cr phosphide. Florenskyite is creamy white in reflected light, and its luster is metallic. The average of three electron microprobe analyses gave (wt%) Fe 40.52, Ti 30.08, Ni 5.47, Cr 0.93, V 0.91, Co 0.60, P 21.69, Si 0.59, sum 100.79, corresponding to Fe1.01(Ti0.87Ni0.13Cr0.03V0.02Co0.01)1.06(P0.97Si0.03). Single-crystal structure analysis was performed on florenskyite using a Laue pattern collected from a multiple crystal by in-situ synchrotron X-ray diffraction. Florenskyite crystallizes in the space group Pnma, and has the anti-PbCl2 structure. Previously determined cell constants of synthetic material [a = 6.007(1), b = 3.602(1), c = 6.897(1) Å] were used in the single-crystal data reduction. We used the POWD12 program to calculate a powder XRD pattern; the 5 most intense reflections are d = 2.301 (I = 100), 2.188 (88), 2.307 (47), 1.938 (45), and 1.801 Å (45). Florenskyite is only the fourth phosphide to be described from nature. Its paragenesis may be unique, and may be due to melting of a mineral assemblage including Fe-Ni metal, schreibersite, daubreelite, osbornite, or heideite and subsequent crystallization of phosphides from the melt.

Development of a System to Analyse the Structure of a Submicrometre-Sized Single Crystal by Synchrotron X-ray Diffraction

A system has been developed for the analysis of the structure of a submicrometer-sized single crystal using synchrotron radiation (SR) at the Photon Factory (PF), KEK. The Laue method combined with an imaging plate was employed for the collection of diffracted intensities. To reduce background, the experiment was carried out in a vacuum and with a very thinly collimated incident X-ray beam. The system has been shown to reach a level where a molybdenum sphere with diameter as small as 0.8 Ixm was found to be twinned and the volume ratio of the twin domains was determined, together with an isotropic temperature factor, which was comparable with the value determined in the same experiment on another single-crystal sphere of almost the same size. It was in good agreement with that determined by a powder diffraction study. The present study showed that any single crystal detectable under an optical microscope can be analysed and, further, that the diffraction intensities from a crystal with volume of 0.02 Ix m 3, which is composed of 10 9 atoms, can be detected.

Andreyivanovite: A second new phosphide from the Kaidun meteorite

Abstract Andreyivanovite (ideally FeCrP) is another new phosphide species from the Kaidun meteorite, which fell in South Yemen in 1980. Kaidun is a unique breccia containing an unprecedented variety of fragments of different chondritic as well as achondritic lithologies. Andreyivanovite was found as individual grains and linear arrays of grains with a maximum dimension of 8 μm within two masses of Fe-rich serpentine. In one sample, it is associated with Fe-Ni-Cr sulfides and florenskyite (FeTiP). Andreyivanovite is creamy white in reflected light, and its luster is metallic. The average of nine electron microprobe analyses yielded the formula Fe(Cr0.587Fe0.150V0.109Ti0.081Ni0.060Co0.002)P. Examination of single grains of andreyivanovite using Laue patterns collected by in situ synchrotron X-ray diffraction (XRD), and by electron-backscatter diffraction revealed it to be isostructural with florenskyite; we were unable to find single crystals of sufficient quality to perform a complete structure analysis. Andreyivanovite crystallizes in the space group Pnma, and has the anti-PbCl2 structure. Previously determined cell constants of synthetic material [a = 5.833(1), b = 3.569(1), and c = 6.658(1) Å] were consistent with our XRD work. We used the XPOW program to calculate a powder-XRD pattern; the 5 most intense reflections are d = 2.247 (I = 100), 2.074 (81), 2.258 (46), 1.785 (43), and 1.885 Å (34). Andreyivanovite is the second new phosphide to be described from the Kaidun meteorite. Andreyivanovite could have formed as a result of cooling and crystallization of a melted precursor consisting mainly of Fe-Ni metal enriched in P, Ti, and Cr. Serpentine associated with andreyivanovite would then have formed during aqueous alteration on the parent asteroid. It is also possible that the andreyivanovite could have formed during aqueous alteration; however, artificial FeTiP has been synthesized only during melting experiments, at low oxygen fugacity, and there is no evidence that a hydrothermal genesis is reasonable.

Identification of mineral in thin section by energy-scanning X-ray diffraction

The performance of an X-ray detector depends to a large extent on the signal to noise ratio (SNR). However, this parameter is rather difficult to determine experimentally and thus it is only roughly known (if at all) for many detectors. Here we present a simple experimental technique for determining the quantum gain and the noise of a CCD detector. In principle the simplest and most direct way to measure the quantum gain of a detector would be a measurement of the response of the detector to a single absorbed X-ray and repeat this measurement a number of times to determine the average gain. However, depending on the X-ray energy and the CCD noise it may be difficult to distinguish reliably a single X-ray “hit” above the detector noise floor, in particular at the rather low energies typically used for X-ray diffraction (e. g. 8-20 keV). Thus, the “classical” approach to measuring the gain is to use an absolutely calibrated X-ray source. However, such sources are often not easily available. Also, the use of an absolutely calibrated source requires that the measurement be corrected for the window transmission and the scintillator absorption efficiencies and these parameters are often not known with precision. Here we describe a simple method for determining the quantum gain without the use of an absolutely calibrated source and also without the need to compensate for absorption in the window and in the scintillator screen. We similarly describe the techniques for determining the total system noise so that the signal-to-noise ratio (SNR) of any given camera can be evaluated. Specific examples for a Bruker camera will be given, but the techniques can be applied to any detector.

Synchrotron X-ray diffraction studies of two olivines from the comet Wild 2

011 of Mg or Al between the tetrahedral and octahedral sites is almost constant against Ga content in the MgAl2-xGaxO4 solid solution. A compositional variable of the Ga/(Mg+Ga) ratio in the octahedral site is not influenced by the occupancy of Al. The occupancy of Al is independent of the occupancy of Ga, though it depends on the occupancy of Mg according to thermal history. The local Al-O bond length in the tetrahedral site is 0.15 Å longer than the expected bond length. The nature that Al in spinel structure occupies mainly the octahedral site arises from the character of Al itself.