Relaxation in the glass-former acetyl salicylic acid studied by deuteron magnetic resonance and dielectric spectroscopy

Supercooled liquid and glassy acetyl salicylic acid was studied using dielectric spectroscopy and deuteron relaxometry in a wide temperature range. The supercooled liquid is characterized by major deviations from thermally activated behavior. In the glass the secondary relaxation exhibits the typical features of a Johari-Goldstein process. Via measurements of spin-lattice relaxation times the selectively deuterated methyl group was used as a sensitive probe of its local environments. There is a large difference in the mean activation energy in the glass with respect to that in crystalline acetyl salicylic acid. This can be understood by taking into account the broad energy barrier distribution in the glass.Comment: 8 pages, 3 figures, Submitted to Phys. Rev.

Evidence for mechanical and chemical alteration of iron‐nickel meteorites on Mars: Process insights for Meridiani Planum

The weathering of meteorites found on Mars involves chemical and physical processes that can provide clues to climate conditions at the location of their discovery. Beginning on sol 1961, the Opportunity rover encountered three large iron meteorites within a few hundred meters of each other. In order of discovery, these rocks have been assigned the unofficial names Block Island, Shelter Island, and Mackinac Island. Each rock presents a unique but complimentary set of features that increase our understanding of weathering processes at Meridiani Planum. Significant morphologic characteristics interpretable as weathering features include (1) a large pit in Block Island, lined with delicate iron protrusions suggestive of inclusion removal by corrosive interaction; (2) differentially eroded kamacite and taenite lamellae in Block Island and Shelter Island, providing relative timing through crosscutting relationships with deposition of (3) an iron oxide–rich dark coating; (4) regmaglypted surfaces testifying to regions of minimal surface modification, with other regions in the same meteorites exhibiting (5) large‐scale, cavernous weathering (in Shelter Island and Mackinac Island). We conclude that the current size of the rocks is approximate to their original postfall contours. Their morphology thus likely results from a combination of atmospheric interaction and postfall weathering effects. Among our specific findings is evidence supporting (1) at least one possible episode of aqueous acidic exposure for Block Island; (2) ripple migration over portions of the meteorites; (3) a minimum of two separate episodes of wind abrasion; alternating with (4) at least one episode of coating‐forming chemical alteration, most likely at subzero temperatures

The first samples from Almahata Sitta showing contacts between ureilitic and chondritic lithologies: implications for the structure and composition of asteroid 2008 TC 3

Almahata Sitta (AhS), an anomalous polymict ureilite, is the first meteorite observed to originate from a spectrally classified asteroid (2008 TC3). However, correlating properties of the meteorite with those of the asteroid is not straightforward because the AhS stones are diverse types. Of those studied prior to this work, 70–80% are ureilites (achondrites) and 20–30% are various types of chondrites. Asteroid 2008 TC3 was a heterogeneous breccia that disintegrated in the atmosphere, with its clasts landing on Earth as individual stones and most of its mass lost. We describe AhS 91A and AhS 671, which are the first AhS stones to show contacts between ureilitic and chondritic materials and provide direct information about the structure and composition of asteroid 2008 TC3. AhS 91A and AhS 671 are friable breccias, consisting of a C1 lithology that encloses rounded to angular clasts (<10 μm to 3 mm) of olivine, pyroxenes, plagioclase, graphite, and metal‐sulfide, as well as chondrules (~130–600 μm) and chondrule fragments. The C1 material consists of fine‐grained phyllosilicates (serpentine and saponite) and amorphous material, magnetite, breunnerite, dolomite, fayalitic olivine (Fo 28‐42), an unidentified Ca‐rich silicate phase, Fe,Ni sulfides, and minor Ca‐phosphate and ilmenite. It has similarities to CI1 but shows evidence of heterogeneous thermal metamorphism. Its bulk oxygen isotope composition (δ18O = 13.53‰, δ17O = 8.93‰) is unlike that of any known chondrite, but similar to compositions of several CC‐like clasts in typical polymict ureilites. Its Cr isotope composition is unlike that of any known meteorite. The enclosed clasts and chondrules do not belong to the C1 lithology. The olivine (Fo 75‐88), pyroxenes (pigeonite of Wo ~10 and orthopyroxene of Wo ~4.6), plagioclase, graphite, and some metal‐sulfide are ureilitic, based on mineral compositions, textures, and oxygen isotope compositions, and represent at least six distinct ureilitic lithologies. The chondrules are probably derived from type 3 OC and/or CC, based on mineral and oxygen isotope compositions. Some of the metal‐sulfide clasts are derived from EC. AhS 91A and AhS 671 are plausible representatives of the bulk of the asteroid that was lost. Reflectance spectra of AhS 91A are dark (reflectance ~0.04–0.05) and relatively featureless in VNIR, and have an ~2.7 μm absorption band due to OH− in phyllosilicates. Spectral modeling, using mixtures of laboratory VNIR reflectance spectra of AhS stones to fit the F‐type spectrum of the asteroid, suggests that 2008 TC3 consisted mainly of ureilitic and AhS 91A‐like materials, with as much as 40–70% of the latter, and <10% of OC, EC, and other meteorite types. The bulk density of AhS 91A (2.35 ± 0.05 g cm−3) is lower than bulk densities of other AhS stones, and closer to estimates for the asteroid (~1.7–2.2 g cm−3). Its porosity (36%) is near the low end of estimates for the asteroid (33–50%), suggesting significant macroporosity. The textures of AhS 91A and AhS 671 (finely comminuted clasts of disparate materials intimately mixed) support formation of 2008 TC3 in a regolith environment. AhS 91A and AhS 671 could represent a volume of regolith formed when a CC‐like body impacted into already well‐gardened ureilitic + impactor‐derived debris. AhS 91A bulk samples do not show a solar wind component, so they represent subsurface layers. AhS 91A has a lower cosmic ray exposure (CRE) age (~5–9 Ma) than previously studied AhS stones (11–22 Ma). The spread in CRE ages argues for irradiation in a regolith environment. AhS 91A and AhS 671 show that ureilitic asteroids could have detectable ~2.7 μm absorption bands

Anatomy of an Allende coarse-grained inclusion

Inclusion 5241 is a petrographically complex, coarse-grained CAI from the Allende meteorite, containing a core with two petrographically and chemically distinct units. The core is surrounded by a melilite mantle similar to that described for Allende Type B1 inclusions, which consists of large, millimetersized crystals. The two different units of the core are (1) spinel-free islands consisting of melilite, fassaite, and minor anorthite (2) spinel-rich areas with major fassaite and reversely zoned melilite. In this unit, spinel occurs in fassaite, melilite, and in densely packed belts surrounding the spinel-free islands. Mineral chemistries of melilite and fassaite in the spinel-free islands are different from those in the spinel-rich areas: melilite in the spinel-free islands is unzoned and displays a very narrow chemical variation (<10 mol %) within individual crystals, ranging between Åk_(52) and Åk_(68) in all islands. They also contain considerable amounts of Na-melilite molecule (as much as 2.7 mol %). In contrast, melilite in the spinel-rich areas is reversely zoned and barren of Na-melilite: the composition varies between Åk_(43) and Åk_(62), with the lowest Åk-content within a zone decorated with fassaite and spinel inclusions. Zoning in individual melilite crystals in the mantle is not continuous (as in Allende Type B1 inclusions), but is rhythmic outwards, from Åk_(30) to Åk_(35) near the core/mantle boundary followed by a continuous decrease with the lowest value near the center of the melilite crystals (Åk_(25)). The Åk-content rises again and reaches a value of Åk_(28) at the rim of the inclusion. Fassaite in the spinel-free islands is typically concentrically zoned, with Ti-Al-rich cores and Ti-Al-poor rims. The concentration of Ti^(3+) in some fassaite crystals in the islands appears to be lower than in the spinelrich areas. Zoning of fassaite in the spinel-rich areas is nonsystematic and the Tio_2 variations are considerable (2–12wt%). Bulk compositions of the spinel-free islands plot inside the Type B CAI field but considerably below the Fo-An-Ge plane because they are spinel deficient. In contrast, compositions of the spinel-rich areas plot outside the Type B CAI field close to pyroxene-saturated compositions (STOLPER, 1982). Textural relations and mineral chemistries in the spinel-free islands, spinel-rich areas, and melilite mantle cannot be explained by gas solid condensation, simple stage crystallization from a refractory silicate liquid, or evaporative loss by distillation, but suggest a series of complex events including the capture of solid spinel-free bodies by a refractory liquid; the melilite mantle may have formed by crystallization from a liquid which enveloped the complex core