Interstellar Diamond Research Articles

Laboratory Investigation of Hydrogenated Diamond Surfaces: Implications for the Formation and Size of Interstellar Nanodiamonds

The formation and size of interstellar diamonds have been investigated by infrared spectroscopy in the laboratory. Employing hot-filament chemical vapor deposition (CVD) to synthesize the interstellar analogs, we successfully reproduced the infrared emission bands of nanodiamonds around HD 97048 and Elias 1. Analysis of the infrared absorption spectra of synthetic diamond crystallites (25-700 nm) from a commercial source reveals a strong size-dependent effect. The 3.53 μm feature emerges only for particles larger than 25 nm. Our experiments suggest that the carriers of the anomalous infrared emission bands at 3.43 and 3.53 μm could be nanodiamonds that are larger than 25 nm and are formed by a CVD-like process.

Solid-state (1H and 13C) nuclear magnetic resonance spectroscopy of insoluble organic residue in the Murchison meteorite: a self-consistent quantitative analysis

Complementary, double- and single-resonance solid-state (1H and 13C) nuclear magnetic resonance (NMR) experiments were performed on a solvent extracted and demineralized sample of Murchison meteorite organic macromolecule. These NMR data provide a consistent picture of a complex organic solid composed of a wide range of organic (aromatic and aliphatic) functional groups, including numerous oxygen-containing functional groups. The fraction of aromatic carbon within the Murchison organic residue (constrained by three independent experiments) lies between 0.61 and 0.66. The close similarity in cross-polarized and single-pulse spectra suggests that both methods detect the same distribution of carbon. With the exception of interstellar diamond (readily detected in slow magic angle spinning single-pulse NMR experiments), there is no evidence in the solid-state NMR data for a significant abundance of large laterally condensed aromatic molecules in the Murchison organic insoluble residue. Given the most optimistic estimation, such carbon would not exceed 10% and more likely is a fraction of this maximum estimate. The fraction of aromatic carbon directly bonded to hydrogen is low (∼30%), indicating that the aromatic molecules in the Murchison organic residue are highly substituted. The bulk hydrogen content, H/C, derived from NMR data, ranges from a low of 0.53 ± 0.06 and a high of 0.63 ± 0.06. The hydrogen content (H/C) determined via elemental analysis is 0.53. The range of oxygen-containing organic functionality in the Murchison is substantial. Depending on whether various oxygen-containing organic functional groups exist as free acids and hydroxyls or are linked as esters and ethers results in a wide range in O/C (0.22 to 0.37). The lowest values are more consistent with elemental analyses, requiring that oxygen-containing functional groups in the Murchison macromolecule are highly linked. The combined 1H and 13C NMR data reveal a high proportion of methine carbon, which requires that carbon chains within the Murchison organic macromolecule are highly branched.

Low-pressure synthesis of diamond without hydrogen: approach by charged cluster model

Although hydrogen was firmly believed to be essential for the low-pressure synthesis, there have been a few reports, where diamond was synthesized in the absence of hydrogen. All the process of low-pressure diamond formation including the interstellar diamond dust have one common factor: presence of electric charge. The charged cluster model (J. Crystal Growth 62 (1996) 55) predicts that stabilization of diamond originates from charge rather than hydrogen. The diamond synthesis without hydrogen could be approached successfully by the charged cluster model.

Microdistribution of primordial noble gases in CM chondrites determined by in situ laser microprobe analysis: decipherment of nebular processes

In situ noble gas analyses using a laser microprobe with a beam diameter of 50–100 μm were performed on thin slices of Murchison and Yamato (Y-) 791198 CM carbonaceous chondrites in order to see microdistribution of primordial noble gases. Petrographic observations prior to noble-gas analyses showed that the two meteorite slices are entirely composed of chondrules, PCPs (poorly characterized phases) and other chondritic components that are rimmed by layers of fine-grained dust. Based on an existing classification scheme for texture of CM chondrites, the two samples are classified as primary accretionary rocks (PARs). The noble gas analyses showed that primordial noble gases are rich in the fine-grained rims around chondrules in the two meteorite slices. This suggests that large amounts of carrier phases of the primordial noble gases in CM chondrites are located in these rims which seem to have accreted on the surfaces of chondrules prior to the formation of the meteorite parent bodies. Rims on different chondrules within a given meteorite showed similar concentrations of heavy primordial noble gases, suggesting that phase Q is distributed homogeneously in the rims and possibly in the nebular region where the chondrules had acquired their dust rims. ( 20Ne) diamond/( 20Ne) Q ratios are relatively constant among rims in a given meteorite, indicative of homogeneous mixing of interstellar diamonds and phase Q on a 10 μg scale. One location in a rim around a chondrule in the Murchison sample showed an enrichment of 22Ne, which suggests presence of carrier phases of Ne-E in the rim. A stepped heating analysis of a chip of Y-791198 was carried out to characterize the noble gases of bulk PAR. Abundances of primordial noble gases are only ∼50% of those of rims around chondrules, consistent with the relative abundances of gas-rich fine-grained material and gas-poor chondrules and inclusions in PAR. Solar gases were not detected, which implies that PARs, the primitive CM bodies, had grown to at least centimeter size in a dense solar nebula which shielded these objects from direct exposure to solar wind.

Facts and Artifacts in Interstellar Diamond Spectra

Absorption spectra of presolar diamonds extracted from the Murchison meteorite have been measured in the extended wavelength range 0.2-500 μm in order to make available optical properties of this supposed component of interstellar carbon dust. In contrast to terrestrial natural and synthetic diamonds, spectra of the meteoritic diamonds show prominent bands in the middle-IR. In this Letter, experimental evidence is presented that the OH band at 3200 cm-1 and the CH bands in the 2800-3000 cm-1 range are not intrinsic features of the diamonds and that the band at 1100 cm-1 contains an artificial component due to the extraction procedure. In addition, in our spectra a conspicuous band at 120 cm-1 was found. If the intrinsic character of this band, which, up to now, is unidentified, is confirmed, it would offer a chance to observe interstellar diamonds, e.g., by the ISO satellite. We encourage laboratory astrophysicists and observers to study this promising possibility.

Superkimberlites: A geodynamic diamond window to the Earth's core

Carbon is the fourth most abundant element in the solar system. In the Earth carbon is in atmospheric CO 2, limestone, other organic products, graphite and trace diamond; interstellar diamond, however, is ubiquitous. Diamond is well known for some unique physical and chemical properties, but it is perhaps less well known that the mineral is geologically ancient (3.3 Ga), that its origins are deep in the mantle (> 180 km), and that diamonds are among the deepest solid objects to reach the surface of the Earth; rare diamonds are from the transition zone (400–670 km), and other diamonds possibly nucleated in the lower mantle (> 670 km). Transport to the surface is in volatile (C O H N S)-charged highly explosive kimberlite and lamproite volcanoes. These volcanoes are sited exclusively in the oldest (> 1.7 Ga), tectonically most stable, and thickest (∼ 200 km) regions of crust and upper mantle lithosphere. The energetics required for volcanism are so exceptional and the sources so deep that possible connections between and among the core, geomagnetism, plumes and diamonds are explored. Some correlations are established and others are implied. The results are sufficiently enticing to propose that kimberlites and geographically and temporally associated carbonatites are continental recorders of plumes dating back to at least 2.8 Ga, and that some diamonds may have recorded core events dating back to 3.3 Ga, or possibly earlier. Peaks in kimberlite magmatic activity correlate, on average, with normal and reverse superchron and subchron behavior of the geomagnetic field. The time lag between magnetohydrodynamic activity in the core and kimberlite eruptive cycles at the Earth's surface is of the order of 25–50 Ma, consistent with the travel times modeled for the passage of plumes from D′' layer to the subcontinental lithosphere. Although the existence of plumes and the nature of D′' are debated, the correlations established for the past 500 Ma between and among superchrons, subchrons, kimberlites and entrained diamonds weigh heavily in favor of the following scenario: solid core growth, the consequent release of Si, O, C, H, S, K and possibly N and B to D′', disruption of D′' at some critically unstable threshold thickness (200–300 km), enhanced core convection and the stabilization of a constant non-reversing magnetic dipole field, rising plumes and subsequent volcanism. If protokimberlitic magma and entrainment begin at the core-mantle boundary, a number of geochemical and mineralogical anomalies in diamonds are at present best satisfied if D′' is invoked. These include but are not limited to intensely reduced (i.e., oxygen deficient) SiC, metallic Fe, an abundance of sulfides, silicate perovskite and wüstite-periclase mineral inclusions in diamonds. The most abundant source of diamonds is unequivocally from cratonic root zones with C possibly implanted by ancient plumes; eclogitic suite diamonds are equivocal, and diamonds transported from the transition zone and the lower mantle are best explained by entrainment in highly reduced plumes. Carbon in the overwhelming majority of diamonds appears to be primordial. By analogy with a chondritic Earth and chondrites, carbon was acquired during accretion in gaseous complexes, in the form of nanometer-size amorphous C, and as hydrocarbon particles; but carbon was possibly also added as crystalline nanodiamonds that served as seeds for subsequent diamond growth.

Interstellar grains in meteorites: I. Isolation of SiC, graphite and diamond; size distributions of SiC and graphite

A procedure has been developed for isolating three types of interstellar grains from primitive meteorites, in >90% purity and yields of generally ≥70%, and is here applied to the Murchison C2M meteorite. Silicates are dissolved in HF-HCl, kerogen (macromolecular organic matter) is destroyed by Cr 2O 7 =, KOH, and H 2O 2, and microdiamonds (~400 ppm) are recovered as a colloid. Graphite (<1 ppm) is isolated by density and size separations. Spinel in the residue is dissolved in H 2SO 4, leaving SiC (~6 ppm), hibonite, and corundum. The size distribution of SiC has been measured in the range 0.2 to 6 μm. Over part of this range, it can be fit either to a power-law or a log-normal distribution, but the deficiency of small grains strongly favors the latter. Statistics are more limited for graphite spherules, but they, too, follow a log-normal distribution, and so does interstellar diamond ( Lewis et al., 1989). Apparently the primary condensation process in stellar atmospheres consistently yields a log-normal distribution. The power-law distribution commonly inferred for interstellar grains may have been produced by secondary processes such as fragmentation.

Terrestrial Carbon and Nitrogen Isotopic Ratios from Cretaceous-Tertiary Boundary Nanodiamonds

One hypothesis for the origin of the nanometer-size diamonds found at the Cretaceous-Tertiary (K-T) boundary is that they are relict interstellar diamond grains carried by a postulated asteroid. The (13)C/(12)C and (15)N/(14)N ratios of the diamonds from two sites in North America, however, show that the diamonds are two component mixtures differing in carbon and nitrogen isotopic composition and nitrogen abundance. Samples from a site from Italy show no evidence for either diamond component. All the isotopic signatures obtained from the K-T boundary are material well distinguished from known meteoritic diamonds, particularly the fine-grain interstellar diamonds that are abundant in primitive chondrites. The K-T diamonds were most likely produced during the impact of the asteroid with Earth or in a plasma resulting from the associated fireball.

Supernovae as sources of interstellar diamonds

Small hydrocarbon grains in the vicinity of a supernova could be annealed by the absorption of several far-ultraviolet photons to produce the tiny diamonds found in meteorites. These freshly-synthesized diamond grains would be bombarded by the heavy ions and neutrals in the supernovae outflow and would thereby acquire the distinctive noble-gas isotopic signature by which they were first isolated. Only diamonds formed relatively close to supernovae would acquire such a signature, since grains formed farther out would be subjected to a much diluted and less energetic plasma environment.

A New Type of Meteoritic Diamond in the Enstatite Chondrite Abee

Diamonds with delta(13)C values of -2 per mil and less than 50 parts per million (by mass) nitrogen have been isolated from the Abee enstatite chondrite by the same procedure used for concentrating Cdelta, the putative interstellar diamond found ubiquitously in primitive meteorites and characterized by delta(13)C values of -32 to -38 per mil, nitrogen concentrations of 2,000 to 12,500 parts per million, and delta(15)N values of -340 per mil. Because the Abee diamonds have typical solar system isotopic compositions for carbon, nitrogen, and xenon, they are presumably nebular in origin rather than presolar. Their discovery in an unshocked meteorite eliminates the possibility of origins normally invoked to account for diamonds in ureilites and iron meteorites and suggests a low-pressure synthesis. The diamond crystals are approximately 100 nanometers in size, are of an unusual lath shape, and represent approximately 100 parts per million of Abee by mass.

Ubiquitous interstellar diamond and SiC in primitive chondrites: abundances reflect metamorphism

THE nature and distribution of interstellar grains in meteorites reflect both the material inherited from the Sun's parent molecular cloud and processes that operated in the early Solar System. The discovery of interstellar diamond and silicon carbide (SiC) in carbonaceous chondrites1,2 demonstrated that presolar grains were incorporated into at least some meteorites. Here I show that interstellar diamond and SiC were incorporated into all chondrite groups. Abundances rapidly go to zero with increasing metamor-phic grade (with diamond typically surviving better than SiC), suggesting that metamorphic destruction is responsible for the apparent absence of these grains in most chondrites. In unmetamor-phosed chondrites, abundances normalized to matrix content are similar for different classes (500–1,000 p.p.m. for diamond; 10–18 p.p.m. for SiC). Diamond samples from chondrites of different classes have remarkably similar noble-gas contents and isotopic compositions, although constituent diamonds may have come from many sources. SiC seems to be more diverse, in part because grains are large enough to measure individually, but average characteristics seem to be similar from meteorite to meteorite. These observations suggest that various classes of chondritic meteorites sampled the same Solar-System-wide reservoir of interstellar grains.

Electron energy loss spectrometry of interstellar diamonds

The results are reported of electron energy loss spectra (EELS) measurements on diamond residues from carbonaceous meteorites designed to elucidate the structure and composition of interstellar diamonds. Dynamic effective medium theory is used to model the dielectric properties of the diamonds and in particular to synthesize the observed spectra as mixtures of diamond and various pi-bonded carbons. The results are shown to be quantitatively consistent with the idea that diamonds and their surfaces are the only contributors to the electron energy loss spectra of the diamond residues and that these peculiar spectra are the result of the exceptionally small grain size and large specific surface area of the interstellar diamonds.

Properties, detectability and origin of interstellar diamonds in meteorites

Laboratory studies suggest that microdiamonds will be quite elusive in interstellar space, being detectable only if >10% of the carbon is present as diamond. The diamonds apparently formed by condensation in cool stellar atmospheres, and were then impregnated with isotopically anomalous noble gases, such as Xe, from a supernova.

The nature and origin of interstellar diamond.

Microscopic diamond was recently discovered in oxidized acid residues from several carbonaceous chondrite meteorites (for example, the C delta component of the Allende meteorite). Some of the reported properties of C delta seem in conflict with those expected of diamond. Here we present high spatial resolution analytical data which may help to explain such results. The C delta diamond is an extremely fine-grained (0.5-10 nm) single-phase material, but surface and interfacial carbon atoms, which may comprise as much as 25% of the total, impart an 'amorphous' character to some spectral data. These data support the proposed high-pressure conversion of amorphous carbon and graphite into diamonds due to grain-grain collisions in the interstellar medium although a low-pressure mechanism of formation cannot be ruled out.

Enhancement of structure of interstellar diamond microcrystals by image processing

Since the discovery of 0.5-7.5 nm diamond crystals in oxidized acid residues of carbonaceous chondrites much speculation has centered on the mechanism of their origin. Indeed; there is even some difference of opinion regarding the presence of “amorphous low-atomic number phases” intimately associated with the diamond crystallites. While the diamond-containing residue from the meteorites comprises only 50-200 ppm of the total meteorite mass, theories regarding the genesis of the diamonds have far-reaching consequences since noble gas isotopic data indicate that they predate the solar system and are from an interstellar source. Lewis et al. propose that the diamonds formed under low pressure conditions by processes similar to those used in recent low-pressure CVD laboratory syntheses. Blake et al. propose a second mechanism of formation, within the stability field of diamond, due to particle-particle collisions behind supernova shock waves. At the present time, no data exist which unequivocally support one model over the other.