Mineralogical evidence for the origin of diamond in ureilites

Abstract— The x‐ray powder diffraction patterns of 50–100 μm C‐rich grains from five ureilitic meteorites—Kenna, Allan Hills (ALH) 78019, Yamato (Y)‐82100, Y‐791538, and ALH 77257—were obtained by using a Gandolfi camera. The results reveal that the basal spacing of part of the graphite coexisting with diamond is slightly smaller compared to the normal spacing. Compressed graphite is experimentally known to occur at the initial stage of the direct transformation from graphite to diamond structures at high pressures and temperatures. The presence of the compressed graphite in ureilites, therefore, gives clear evidence that the diamond formed by high‐pressure conversion of graphite. The modes of occurrence of C minerals observed with reflected light through an optical microscope reveal that graphite coexisted with olivine and pyroxene during igneous or metamorphic processes and, furthermore, that part of the graphite was converted to diamond by impact. The relative x‐ray intensity of diamond to graphite increases in the following order: ALH 78019 and Y‐82100 < Y‐791538 < Kenna < ALH 77257. This correlates with the shock level that is estimated mainly on the basis of the shock features of silicates. Therefore, the relative amounts of diamond to graphite suggested by x‐ray intensities may be useful as a measure of the degree of shock.     

Formation of diamond and lonsdaleite in ureilites by impact shock processing of graphite

The origin of diamond in ureilites has been frequently debated. We investigated carbon phase assemblages (CPAs) in five ureilitic samples of the brecciated asteroid 2008 TC₃, found within the Almahata Sitta (AHS) strewn field, by transmission electron microscopy, Raman spectroscopy, synchrotron X-ray diffraction, and cathodoluminescence. Samples MS-MU 006, MS-187, and MS-170, are of low to moderate shock degree (U-S2 and U-S3), and samples MS-MU 027 (U-S4) and MS-MU 045 (U-S5) have a higher shock degree. In MS-MU 006 and MS-187, we did not find any diamond grains. MS-170 contains disordered and distorted graphite with diamond grains up to 12 μm in size and containing inclusions of Fe,Ni-metal, FeS, Fe-phosphide, and Cr,Fe-oxide. These diamond grains formed under relatively low (5–15 GPa) shock pressures through a catalytic process in the presence of a Fe,Ni,Cr,S,P-rich melt. The highly shocked and fine-grained ureilites MS-MU 027 and MS-MU 045 have three different types of CPAs, namely a nanopolycrystalline assemblage of diamond and defect-rich diamond/lonsdaleite, disordered and distorted graphite, and polycrystalline diamond with abundant Fe-rich mineral inclusions. The CPAs that have only diamond and planar defect-rich diamond (e.g., MS-MU 027) most likely formed through martensitic transformation of graphite to diamond and lonsdaleite at >15 GPa and >2000 K. The assemblage of diamond, defect-rich diamond, and disordered and distorted graphite (e.g., MS-MU 045) formed by martensitic transformation of graphite to diamond and lonsdaleite, followed by back-transformation to disordered graphite. We did not find any conclusive evidence to support the formation of diamond grains under high static pressure.     

MicroRaman spectroscopy of diamond and graphite in Almahata Sitta and comparison with other ureilites

This work is the first detailed study of carbon phases in the ureilite Almahata Sitta (sample #7). We present microRaman data for diamond and graphite in Almahata Sitta, seven unbrecciated ureilites, and two brecciated ureilites. Diamond in Almahata Sitta was found to be distinct from that in unbrecciated and brecciated ureilites, although diamond in unbrecciated and brecciated ureilites is indistinguishable. Almahata Sitta diamond shows a peak center range of 1318.5–1330.2 cm^?1 and a full width at half maximum (FWHM) range of 6.6–17.4 cm^?1, representing a shock pressure of at least 60 kbar. The actual peak shock pressure may be higher than this due to postshock annealing, if shock synthesis is the source of ureilite diamonds. Diamond in unbrecciated and brecciated ureilites have peak center wave numbers closer to terrestrial kimberlite diamond, but show a wider range of FWHM than Almahata Sitta. The larger peak shift observed in Almahata Sitta may indicate the presence of lonsdaleite. Alternatively, the lower values in brecciated ureilites may be evidence of an annealing step either following the initial diamond‐generating shock or as a consequence of heating during reconsolidation of the breccia. Graphite in Almahata Sitta shows a G‐band peak center range of 1569.1–1577.1 cm^?1 and a G‐band FWHM range of 24.3–41.6 cm^?1 representing a formation temperature of 990 ± 120 °C. Amorphous carbon was also found. We examine the different theories for diamond formation in ureilites, such as chemical vapor deposition and shock origin from graphite, and explore explanations for the differences between Almahata Sitta and other ureilites.     

Metamorphism of four desert ureilites and luminescence spectroscopy of defects in ureilitic diamonds

Four ureilites subjected to impact metamorphism in a pressure range of ~15–100 GPa were investigated for mineralogical and petrological features and optical luminescence of their diamonds with the aim to understand how properties of ureilitic diamonds are correlated with shock and thermal histories of the host meteorite. Petrological data show that all the investigated ureilites experienced multistage metamorphic histories. Some of them were shocked at least twice or/and underwent high‐temperature thermal metamorphism and fluid metasomatism in the parent body interior. Photoluminescence spectra of individual diamond grains reveal the presence of neutral and negatively charged nitrogen‐vacancy (NV⁰ and NV^?, respectively) and H3 (two nitrogens and a vacancy) defects, indicating relatively high nitrogen contents of the diamonds and some degree of thermal annealing of the grains. The diamond grain size and morphology, a texture of graphite‐diamond aggregates, and spectroscopic properties of the diamond phase vary widely both within an individual meteorite and between the ureilites. Shock‐driven transformation of sp2‐C into diamond provides the most natural explanation of the observed spectroscopic diversity of the diamond grains if one takes into account strong dependence of the PT parameters and efficiency of the transformation on structure of the carbonaceous precursor.     

Nitrogen in diamond‐free ureilite Allan Hills 78019: Clues to the origin of diamond in ureilites

Abstract— Nitrogen and noble gases were measured in a bulk sample and in acid‐resistant carbon‐rich residues of the ureilite Allan Hills (ALH) 78019 which has experienced low shock and is free of diamond. A small amount of amorphous carbon combusting at ≤500 °C carries most of the noble gases, while the major carbon phase consisting of large crystals of graphite combusts at ≥800 °C, and is almost noble‐gas free. Nitrogen on the other hand is present in both amorphous carbon and graphite, with different δ¹⁵N signatures of ?21%_o and +19%_o, respectively, distinctly different from the very light nitrogen (about ?100%_o) of ureilite diamond. Amorphous carbon in ALH 78019 behaves similar to phase Q of chondrites with respect to noble gas release pattern, behavior towards oxidizing acids as well as nitrogen isotopic composition. In situ conversion of amorphous carbon or graphite to diamond through shock would require an isotopic fractionation of 8 to 12% for nitrogen favoring the light isotope, an unlikely proposition, posing a severe problem for the widely accepted shock origin of ureilite diamond.     

Mineralogy of augite-bearing ureilites and the origin of their chemical trends

Abstract— Three augite-bearing ureilites from Antarctica, Y74130, MET78008 and ALH82106, have been studied by electron microprobe, scanning electron microscope (SEM), and analytical transmission electron microscope (TEM). The first two belong to the low-¹⁶O subgroup of Clayton and Mayeda (1988) and are closely related; ALH82106 belongs to the high-¹⁶O subgroup. MET78008 is an augite-olivine ureilite, similar to the augite-bearing part of the Y74130. Augites poikilitically include ellipsoidal pigeonites, and low-Ca pyroxene poikilitically encloses ellipsoidal olivine and augite. The temperature of last equilibration deduced from an orthopyroxene-pigeonite-augite assemblage is above 1200 °C. The ALH82106 pigeonite contains irregular augite inclusions produced by decomposition. Augites in Y74130, MET78008 and ALH82106 (decomposed one) all show similar spinodal decomposition textures on the TEM scale. Cooling rates estimated from an experimentally calibrated diagram for the wavelengths of spinodal decomposition versus cooling rates are about 20 °C/hr. The calcic trend for the low-¹⁶O subgroup including Y74130 and MET78008 shows large Ca variations in pyroxene with similar Mg/Fe ratio; differentiation involving reduction was not an important process. Augite in ALH82106 is a minor phase and contains lower Na₂O contents than Y74130 and MET78008, confirming that ALH82106 is not directly related to them.     

Experimental shock synthesis of diamonds in a graphite gneiss

Abstract— The occurrence of diamonds in terrestrial impact craters and meteorites is related to dynamic shock loading during hypervelocity impacts. To understand the mechanism of impact diamond formation in natural rocks, shock‐recovery experiments with graphite gneiss were carried out at shock pressures between 35 and 79 GPa. This is the first report on the successful shock synthesis of microdiamonds in a natural rock. Micrometer‐size diamonds and a wide range of intermediate, presently unclassified, amorphous, and disordered carbon phases were observed within vesiculated biotite melts in the vicinity of relic graphite grains using microRaman spectrometry. We explain these findings by jetting mechanisms of carbon and graphite clusters, originating at the edges of graphite grains, into the very hot and volatile rich biotitic melt veins during shock loading. This environment enabled the thermally activated crystallization of diamonds during shock compression in a period of less than 0.5 μsec. Regraphitization of diamonds during pressure release was widespread and caused the formation of the amorphous to disordered carbon phases recorded frequently with microRaman spectroscopy. The surviving diamonds must have cooled down to 2000 K during the compression phase at local thermal sinks and cooler interfaces to avoid regraphitization.     

Mineralogy of interstitial rim materials of the Y74123 and Y790981 ureilites and their origin

Abstract— Y74123 is an olivine-rich, relatively unshocked ureilite and contains more interstitial pigeonitic materials than do ureilites which have been reported previously. Thus, Y74123 is especially suited for detailed study of the interstitial materials. We have studied these materials by optical microscope, electron microprobe, scanning electron microscope, high resolution transmission electron microscope (TEM) and analytical TEM to gain a better understanding of their nature and origin. Y790981, with shock partial melts, has also been examined by the same techniques. Bulk chemical compositions of the interstitial materials in Y74123 are pyroxene-like and have higher CaO and Al₂O₃ contents than the large pigeonite and olivine core. Interstitial materials at olivine-pigeonite grain boundaries are richer in CaO and Al₂O₃ than those at olivine-olivine grain boundaries. TEM observations of the interstitial material of Y74123 show that it consists of alternating pigeonite-augite lamellae more than 3.5 μm thick on (001). This texture suggests that the rim material had already crystallized before the parent body breakup. The shock-produced glassy veins in Y790981 cut through the rim materials. These observations are consistent with the idea that the interstitial materials in this ureilite are a mixture of residual liquids of high Ca melts and shock-produced partial melts of olivine and pigeonite. This mixture accumulated along the grain boundaries and some of it is trapped within grains.     

Carbon isotope fractionation between graphite and diamond during shock experiments

Abstract— Carbon isotopic compositions were measured for shock‐produced diamond and shocked graphite formed at peak pressures ranging from 37 to 52 GPa. The δ¹³C values of diamonds produced in a sealed container were generally lower than that of the initial graphite. The differences in the carbon isotopic composition between initial graphite and shocked graphite/diamond may reflect kinetic isotopic fractionation during the oxidation of the graphite/diamond and/or analytical artifacts possibly induced by impurities in the samples. The pressure effect on the isotopic fractionations between graphite and diamond can be estimated from the δ¹³C values of impurity‐free diamonds produced using a vented container from which gases, including oxygen, in pore spaces escaped during or after the diamond formation (e.g., 0.039 ± 0.085‰ at a peak pressure of 52 GPa). Any isotopic fractionation induced by shock conversion of graphite to diamond is too small to be detected in natural shock‐induced diamond‐graphite systems related to terrestrial impact cratering processes.     

Noble gases in presolar diamonds I: Three distinct components and their implications for diamond origins

Abstract— High-purity separates of presolar diamond were prepared from 14 primitive chondrites from 7 compositional groups. Their noble gases were measured using stepped pyrolysis. Three distinct noble gas components are present in diamonds, HL, P3, and P6, each of which is found to consist of five noble gases. P3 , released between 200 °C and 900 °C, has a “planetary” elemental abundance pattern and roughly “normal” isotopic ratios. HL , consisting of isotopically anomalous Xe-HL and Kr-H, Ar with high ³⁸Ar/³⁶Ar, and most of the gas making up Ne-A2 and He-A, is released between 1100 °C and 1600 °C. HL has “planetary” elemental ratios, except that it has much more He and Ne than other known “planetary” components. HL gases are carried in the bulk diamonds, not in some trace phase. P6 has a slightly higher median release temperature than HL and is not cleanly separated from HL by stepped pyrolysis. Our data suggest that P6 has roughly “normal” isotopic compositions and “planetary” elemental ratios. Both P3 and P6 seem to be isotopically distinct from P1, the dominant “planetary” noble-gas component in primitive chondrites. Release characteristics suggest that HL and P6 are sited in different carriers within the diamond fractions, while P3 may be sited near the surfaces of the diamonds. We find no evidence of separability of Xe-H and Xe-L or other isotopic variations in the HL component. However, because ～10¹⁰ diamonds are required to measure a Xe composition, a lack of isotopic variability does not constrain diamonds to come from a single source. In fact, the high abundance of diamonds in primitive chondrites and the presence of at least three distinct noble-gas components strongly suggest that diamonds originated in many sources. Relative abundances of noble-gas components in diamonds correlate with degree of thermal processing (see companion paper), indicating that all meteorites sampled essentially the same mixture of diamonds. That mixture was probably inherited from the Sun's parent molecular cloud.     

A Raman spectroscopic study of organic matter in interplanetary dust particles and meteorites using multiple wavelength laser excitation

Raman spectroscopy was used to investigate insoluble organic matter (IOM) from a range of chondritic meteorites, and a suite of interplanetary dust particles (IDPs). Three monochromatic excitation wavelengths (473 nm, 514 nm, 632 nm) were applied sequentially to assess variations in meteorite and IDP Raman peak parameters (carbon D and G bands) as a function of excitation wavelength (i.e., dispersion). Greatest dispersion occurs in CVs > OCs > CMs > CRs with type 3 chondrites compared at different excitation wavelengths displaying conformable relationships, in contrast to type 2 chondrites. These findings indicate homogeneity in the structural nature of type 3 chondrite IOM, while organic matter (OM) in type 2 chondrites appears to be inherently more heterogeneous. If type 2 and type 3 chondrite IOM shares a common source, then thermal metamorphism may have a homogenizing effect on the originally more heterogeneous OM. IDP Raman G bands fall on an extension of the trend displayed by chondrite IOM, with all IDPs having Raman parameters indicative of very disordered carbon, with almost no overlap with IOM. The dispersion effect displayed by IDPs is most similar to CMs for the G band, but intermediate between CMs and CRs for the D band. The existence of some overlapping Raman features in the IDPs and IOM indicates that their OM may share a common origin, but the IDPs preserve more pristine OM that may have been further disordered by ion irradiation. H, C, and N isotopic data for the IDPs reveal that the disordered carbon in IDPs corresponds with higher δ¹⁵N and lower δ¹³C.     

In situ micro‐Raman and X‐ray diffraction study of diamonds and petrology of the new ureilite UAE 001 from the United Arab Emirates

Abstract— A new olivine‐pigeonite ureilite containing abundant diamonds and graphite was found in the United Arab Emirates. This is the first report of a meteorite in this country. The sample is heavily altered, of medium shock level, and has a total weight of 155 g. Bulk rock, olivine (Fo_79.8–81.8) and pyroxene (En_73.9–75.2, Fs_15.5–16.9, Wo_8.8–9.5) compositions are typical of ureilites. Olivine rims are reduced with Fo increasing up to Fo_96.1–96.8. Metal in these rims is completely altered to Fehydroxide during terrestrial weathering. We studied diamond and graphite using micro‐Raman and in situ synchrotron X‐ray diffraction. The main diamond Raman band (LO = TO mode at ?1332 cm^?1) is broadened when compared to well‐ordered diamond single crystals. Full widths at half maximum (FWHM) values scatter around 7 cm^?1. These values resemble FWHM values obtained from chemical vapor deposition (CVD) diamond. In situ XRD measurements show that diamonds have large grain sizes, up to >5 μm. Some of the graphite measured is compressed graphite. We explore the possibilities of CVD versus impact shock origin of diamonds and conclude that a shock origin is much more plausible. The broadening of the Raman bands might be explained by prolonged shock pressure resulting in a transitional Raman signal between experimentally shock‐produced and natural diamonds.     

Light rare earth element enrichments in ureilites: A detailed ion microprobe study

Abstract— This paper explores the possible origin of the light rare earth element (LREE) enrichments observed in some ureilites, a question that has both petrogenetic and chronologic implications for this group of achondritic meteorites. Rare earth element and other selected elemental abundances were measured in situ in 14 thin sections representing 11 different ureilites. The spatial microdistributions of REEs in C‐rich matrix areas of the three ureilites with the most striking V‐shaped whole‐rock REE patterns (Kenna, Goalpara, and Novo Urei) were investigated using the ion imaging capability of the ion microprobe. All olivines and clinopyroxenes measured have LREE‐depleted patterns with little variation in REE abundances, despite large differences in their major element compositions from ureilite to ureilite. Furthermore, we searched for but did not find any minor mineral phases that carry LREEs. The only exception is one Ti‐rich area (～20μm) in Lewis Cliff (LEW) 85400 with a major element composition similar to that of titanite; REE abundances in this area are high, ranging from La ? 400 × CI to Lu ? 40 × CI. In contrast, all ion microprobe analyses of C‐rich matrix in Kenna, Goalpara, and Novo Urei revealed large LREE enrichments. In addition, C‐rich matrix areas in the three polymict ureilites, Elephant Moraine (EET) 83309, EET 87720, and North Haig, which have less pronounced V‐shaped whole‐rock REE patterns, show smaller but distinct LREE‐enrichments. The C‐rich matrix in Antarctic ureilites tends to have much lower LREE concentrations than the matrix in non‐Antarctic ureilites. There is no obvious association of the LREEs with other major or minor elements in the C‐rich areas. Ion images further show that the LREE enrichments are homogeneously distributed on a microscale in most C‐rich matrix areas of Kenna, Goalpara, and Novo Urei. These observations suggest that the LREEs in ureilites most probably are absorbed on the surface of fine‐grained amorphous graphite in the C‐rich matrix. It is unlikely that the LREE enrichments are due to shock melts or are the products of metasomatism on the ureilite parent body. We favor LREE introduction by terrestrial contamination.     

Noble gases in oxidized residue prepared from the Saratov L4 chondrite and Raman spectroscopic study of residues to characterize phase Q

We analyzed noble gases in an oxidized residue prepared from a HF‐HCl residue of the Saratov L4 chondrite. The Ar, Kr, and Xe concentrations in the oxidized residue are two orders of magnitude lower than those in the HF‐HCl residue, and they are close to concentrations in the bulk. The He and Ne concentrations are similar in the three samples. The Ne isotopic ratios are almost purely cosmogenic, indicating absence of presolar diamonds (the carrier of the HL component). Thus, Saratov contains phase Q without presolar diamond. A study of the Raman spectroscopic parameters for the HF‐HCl residue and the oxidized residue shows large changes due to oxidation. The directions of these changes are the same as observed in Allende, except oxidation increased the I_D/I_G (intensity ratio of the D band to the G band) in Saratov but decreased in Allende. This difference may be attributed to the different crystalline stages of carbon in both meteorites. The shifts in the Raman parameters to a discrete and/or more expanded region suggest that (1) oxidation changes the crystalline condition of graphitic carbon, (2) phase Q is not a dissolved site, and (3) the release of Q‐gas is simply related to the rearrangement of the carbon structure during oxidation.     

Microdistributions and petrogenetic implications of rare earth elements in polymict ureilites

Abstract— Polymict ureilites contain various mineral and lithic clasts not observed in monomict ureilites, including plagioclase, enstatite, feldspathic melt clasts and dark inclusions. This paper investigates the microdistributions and petrogenetic implications of rare earth elements (REEs) in three polymict ureilites (Elephant Moraine (EET) 83309, EET 87720 and North Haig), focusing particularly on the mineral and lithic clasts not found in monomict ureilites. As in monomict ureilites, olivine and pyroxene are the major heavy (H)REE carriers in polymict ureilites. They have light (L)REE‐depleted patterns with little variation in REE abundances, despite large differences in major element compositions. The textural and REE characteristics of feldspathic melt clasts in the three polymict ureilites indicate that they are most likely shocked melt that sampled the basaltic components associated with ureilites on their parent body. Simple REE modeling shows that the most common melt clasts in polymict ureilites can be produced by 20–30% partial melting of chondritic material, leaving behind a ureilitic residue. The plagioclase clasts, as well as some of the high‐Ca pyroxene grains, probably represent plagioclase‐pyroxene rock types on the ureilite parent body. However, the variety of REE patterns in both plagioclase and melt clasts cannot be the result of a single igneous differentiation event. Multiple processes, probably including shock melting and different sources, are required to account for all the REE characteristics observed in lithic and mineral clasts. The C‐rich matrix in polymict ureilites is LREE‐enriched, like that in monomict ureilites. The occurrence of Ce anomalies in C‐rich matrix, dark inclusions and the presence of the hydration product, iddingsite, imply significant terrestrial weathering. A search for ²⁶Mg excesses, from the radioactive decay of ²⁶Al, in the polymict ureilite EET 83309 was negative.     

Igneous petrology of the new ureilites Nova 001 and Nullarbor 010

Abstract— The Nova 001 [= Nuevo Mercurio (b)] and Nullarbor 010 meteorites are ureilites, both of which contain euhedral graphite crystals. The bulk of the meteorites are olivine (Fo₇₉) and pyroxenes (Wo₉En₇₃Fs₁₈, Wo₃En₇₇Fs₂₀), with a few percent graphite and minor amounts of troilite, Ni-Fe metal, and possibly diamond. The rims of olivine grains are reduced (to Fo₉₁) and contain abundant blebs of Fe metal. Silicate mineral grains are equant, anhedral, up to 2 mm across, and lack obvious preferred orientations. Euhedral graphite crystals (to 1 mm x 0.3 mm) are present at silicate grain boundaries, along boundaries and protruding into the silicates, and entirely within silicate mineral grains. Graphite euhedra are also present as radiating clusters and groups of parallel plates grains embedded in olivine; no other ureilite has comparable graphite textures. Minute lumps within graphite grains are possibly diamond, inferred to be a result of shock. Other shock effects are limited to undulatory extinction and fracturing. Both ureilites have been weathered significantly. Considering their similar mineralogies, identical mineral compositions, and identical unusual textures, Nova 001 and Nullarbor 010 are probably paired. Based on olivine compositions, Nova 001 and Nullarbor 010 are in Group 1 (FeO-rich) of Berkley et al. (1980). Silicate mineral compositions are consistent with those of other known ureilites. The presence of euhedral graphite crystals within the silicate minerals is consistent with an igneous origin, and suggests that large proportions of silicate magma were present locally and crystallized in situ.     

Photometric and spectroscopic study of R CMa

UBV light curves and spectrograms of R CMa obtained with the 48-inch telescope of Japal-Rangapur Observatory during 1980–82 have been used for deriving the eclipse and orbital elements as well as the absolute dimensions of the components. The primary is found to be a Main-Sequence F2V star of mass 1.52M and the secondary a subgiant star of spectral type G8 and mass 0.20M which fills its Roche lobe, in agreement with Kopal and Shapley (1956) results, Kopal (1959), or Sahade's (1963) results. From a consideration of the possible evolution of this system it is concluded that a large fraction of the original mass of the secondary is lost from the system. A study of the period changes indicates the possible presence of a third component of mass of about 0.5M which is most likely to be anM dwarf.Paper presented at the Lembang-Bamberg IAU Colloquium No. 80 on Double Stars: Physical Properties and Generic Relations, held at Bandung, Indonesia, 3–7 June, 1983.     

Infrared,ultraviolet, and electron paramagnetic resonance measurements on presolar diamonds: Implications for optical features and origin

Abstract— Infrared (IR) and ultraviolet (UV) absorption spectra were obtained for diamonds from the Allende and Murchison meteorites. In addition, and for the first time, electron paramagnetic resonance spectra were measured. The IR and UV data confirm the suspicion of Russell et al. (1996) that N in presolar diamonds predominantly appears in the form of dispersed N atoms, as is the case for terrestrial type Ib diamonds. In accordance with other observations, our electron paramagnetic resonance measurements suggest a high H content in presolar diamonds. The presolar diamonds most likely originated in a H‐rich region, an environment in which nanometer‐sized diamonds may be more stable than graphite (Badziag et al., 1990). This adds to the evidence—previously based mainly on the twin microstructures of presolar diamonds (Daulton et al., 1996) and the absence of graphite with the same isotopic composition as presolar diamonds (Anders and Zinner, 1993)—for a homogeneous nucleation of presolar diamonds from a gas phase. Based on our results for detection of diamonds in space, we suggest searching for the N‐induced IR and UV absorption features of type Ib diamonds. Other characteristic diamond features that could also be used to detect diamonds in space are the (‐CH_n) IR absorption features due to H‐coated diamonds, as they are described by Allamandola et al. (1993) and the IR multiphonon absorption features of the diamond lattice. The multiphonon features are very weak (Edwards, 1985), but their intensity increases somewhat with increasing temperature (Collins and Fan, 1954), so perhaps a search for them is not totally hopeless.