Glass With Embedded Metal And Sulfides Research Articles

Discovery of ancient silicate stardust in a meteorite.

We have discovered nine presolar silicate grains from the carbonaceous chondrite Acfer 094. Their anomalous oxygen isotopic compositions indicate formation in the atmospheres of evolved stars. Two grains are identified as pyroxene, two as olivine, one as a glass with embedded metal and sulfides (GEMS), and one as an Al-rich silicate. One grain is enriched in 26Mg, which is attributed to the radioactive decay of 26Al and provides information about mixing processes in the parent star. This discovery opens new means for studying stellar processes and conditions in various solar system environments.

Iron-nickel sulfides in anhydrous interplanetary dust particles

Fe,Ni-sulfide grains in nine anhydrous chondritic porous (CP) interplanetary dust particles (IDPs) and one hydrated chondritic smooth (CS) IDP were examined using 200 and 400 keV transmission electron microscopy.Crystal structures of the grains were investigated using selected area electron diffraction, electron microdiffraction, and high-resolution lattice fringe imaging. Grain compositions were measured using quantitative energy-dispersive x-ray spectroscopy. Three types of sulfide grains were examined: 10 to 100 nm diameter nanocrystals within and on the surfaces of GEMS (glass with embedded metal and sulfides), small 100 to 500 nm diameter isolated grains, and large 0.5 to 5 μm diameter grains dispersed throughout the fine-grained matrices of IDPs.Most sulfide nanocrystals within and on the surfaces of GEMS are low-Ni pyrrhotite (0 ≤ Ni ≤ 5.5 atomic %) with a hexagonal unit cell where ao = 0.34 nm and co = 0.57 nm. Small 100 to 500 nm mafic grains dispersed throughout the matrices of the IDPs include hexagonal pyrrhotite (0 ≤ Ni ≥ 20 atomic %), ordered hexagonal pyrrhotite exhibiting prominent superlattice reflections, and a cubic sulfide that appears to have a sulfur-deficient “spinel-like” (Fd3m) structure. A large (∼2 × 5 μm) grain within one anhydrous IDP is a polycrystalline mixture of hexagonal pyrrhotite and cubic sulfide. Electron diffraction and lattice fringe imaging show that hexagonal and cubic sulfide are coherently intergrown on a unit cell scale.When heated in the electron beam the cubic sulfide transforms into hexagonal pyrrhotite. Therefore, it is possible that most of the pyrrhotite, the dominant sulfide in anhydrous chondritic IDPs, is a secondary thermal alteration product of frictional heating and partial sulfur loss during atmospheric entry. Neither troilite (FeS) nor pentlandite was identified in any of the nine anhydrous IDPs. Pentlandite was identified in the single hydrated IDP, in accordance with a previous study of sulfides in chondritic IDPs.

Analysis of a deuterium‐rich interplanetary dust particle (IDP) and implications for presolar material in IDPs

Deuterium (D)‐rich interplanetary dust particles (IDPs) are the most primitive extraterrestrial materials available for laboratory studies in terms of their mineralogy, chemistry, and isotopic compositions. Transmission electron microscopy analysis of one such D‐rich IDP shows it to be a highly porous object that consists of crystalline grains (Mg‐rich pyroxene and olivine, and FeNi sulfides) and glass with embedded metal and sulfides (GEMS) enclosed within a matrix of amorphous carbonaceous material. The nonsolar H isotopic anomaly measured in this IDP provides direct evidence for the incorporation of partially preserved cold molecular cloud material that predates the solar system. These results indicate that organic molecules associated with the carbonaceous matrix of the IDP are the most likely D carrier phase in the particle. GEMS are a major constituent of this D‐rich IDP. The physical and chemical characteristics of GEMS show many similarities to the properties of interstellar amorphous silicates inferred from astronomical observations including: size (diameters of 0.1–0.5 um), solar abundances for heavy elements, presence of superparamagnetic FeNi metal, and an amorphous silicate matrix. Infrared transmission spectra from GEMS‐rich IDPs show marked similarities to astronomical data for interstellar silicates. The close association of GEMS and mineral grains to D‐rich matter of probable interstellar origin suggests that these inorganic materials are themselves interstellar grains.

A Point in Favor of the Superparamagnetic Grain Hypothesis

If the GEMS (glass with embedded metal and sulfides) particles recently identified by meteoriticists are indeed examples of interstellar silicate grains, then the spacing of the embedded (SPM) inclusions in GEMS particles appears consistent with the theory of superparamagnetic grain alignment first proposed by Jones & Spitzer. Using an SPM hypothesis, Mathis has shown that the observed wavelength dependence of the polarization of background starlight is readily explained if grains smaller than about 0.09 μm in size have no SPM inclusions. Martin has already noted that the bulk properties of GEMS particles are consistent with the SPM hypothesis, as well as with the notion that GEMS particles are interstellar grains. In this Letter we make the single point that the spatial frequency (~0.1 μm) of the SPM inclusions implied by the GEMS particles for interstellar grains is tantalizingly close to the Mathis value needed to explain the observed wavelength dependence of polarization.

Chemically Anomalous, Preaccretionally Irradiated Grains in Interplanetary Dust from Comets

Nonstoichiometric grains with depletions of magnesium and silicon (relative to oxygen) and inclusions of iron-nickel metal and iron-rich sulfides have been identified in interplanetary dust particles from comets. These chemical anomalies accumulate in grains exposed to ionizing radiation. The grains, known as GEMS (glass with embedded metal and sulfides), were irradiated before the accretion of comets, and their inferred exposure ages, submicrometer sizes, and "amorphous" silicate structures are consistent with those of interstellar silicate grains. The measured compositional trends suggest that chemical (as well as isotopic) anomalies can be used to identify presolar interstellar components in primitive meteoritic materials.