morphology and defects of diamond grains in carbonado: clues to carbonado genesis

Morphology and defects of diamond grains in carbonado:

clues to carbonado genesis

VITALY A. PETROVSKY1, ANDREI A. SHIRYAEV2,*, VLADIMIR P. LYUTOEV1,

ALEXANDER E. SUKHAREV1 and MAXIMILIANO MARTINS3

1 Institute of Geology, Komi Science Centre RAS, Pervomayskaya Str. 54, Syktyvkar 167982, Russia2 A.V. Shubnikov Institute of Crystallography RAS, Leninsky Pr. 59, Moscow 119333, Russia

*Corresponding author, e-mail: [email protected] Federal University of Minas Gerais State, Av.Antonio Carlos 6672, Belo Horizonte 30270-901, Brazil

Abstract: The morphology and defects in diamond grains comprising Brazilian carbonado have been investigated using X-raydiffraction, photo- and X-ray luminescence, electron paramagnetic resonance and Raman spectroscopy. Paramagnetic and non-paramagnetic defects in the diamond structure indicate that many of the studied carbonado samples were annealed under mantleconditions, although for a relatively short period of time. Diamond grains show various growth morphological forms with low degreesof dissolution. Individual diamond grains are characterised by a number of important features, such as reentrant apices and incompletegrowth layers on faces. We suggest that micron-sized diamond single crystals of predominantly octahedral and cubooctahedral shapegrew under conditions of decreasing carbon supersaturation. The temperature decrease serves as a plausible driving force forcrystallisation. During the second stage of carbonado formation, mass crystallisation of diamond has occurred. The necessary Csupersaturation was likely caused by crystallisation of other minerals, leading to decrease in the volume and/or structure of the parentsolution.

Key-words: carbonado, diamond morphology, point defects, mass crystallisation.

1. Introduction

Diamondiferous placers of different genesis (e.g., glacial,alluvial) and variable age are widespread on many cratonsand play a considerable role in diamond production.Diamonds in placers originate either directly from erodedparent diamondiferous pipes or from ancient sediments.Discussions of the origin of diamonds in Brazilian placersbegan in the 19th century, although no consensus has beenreached yet (Svizero, 1995). Brazilian placers are wellknown for the occurrence of a peculiar type of polycrystal-line diamond, the so-called ‘‘carbonado’’. Carbonado dia-mond is characterised by several important, sometimesmutually controversial features making modelling of theirformation non-trivial. Hypotheses about the origin of thecarbonado formation range from an extraterrestrial originto growth in the Earth’s mantle. However, many of thesemodels lack any solid proof (for review see Heaney et al.,2005).

Carbonado is usually defined as porous microcrystallinediamond aggregate with very strong direct bondingbetween the grains and a light (d13C��25 to �30%)isotopic composition of carbon (Vinogradov et al., 1966;Galimov et al., 1985; Kaminskiy, 1991). Spectroscopic

investigation of defects (Kagi et al., 1994, 2007) in dia-mond grains comprising carbonado suggested existence ofat least two generations of diamond in a single carbonadospecimen. SIMS study of C isotopic composition and Nconcentration (De et al., 2001) pointed to possible coex-istence of isotopically different grains in a single carbo-nado specimen. Electron microscopy showed that in somecarbonado specimens the diamond grains are heavilydeformed indicating considerable stresses during forma-tion (De et al., 1998; Chen & van Tendeloo, 1999).However, in other cases the stresses are largely relieved(Fukura et al., 2005) or distributed very heterogeneously indifferent parts of the carbonado specimen (Kagi & Fukura,2008). Some additional observations are of interest, but itis not yet clear how general they are: high abundance ofhydrocarbons (Galimov et al., 1985; Kaminskiy et al.,1985), presence of water and OH-groups (Garai et al.,2006; Kagi & Fukura, 2008) and of H-related structuraldefects (Nadolinny et al., 2003).

A very well known feature of this diamond variety is thepresence of large amounts of non-diamond material inintergrain space and pores. X-ray diffraction (Trueb &DeWys, 1969) and electron microscopy (Gorshkov et al.,1995, 1996a, 1996b) were used to identify inclusions

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Eur. J. Mineral.

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Published online October 2009

typical for mantle conditions and/or requiring extremelylow fO2 (e.g. native metals, SiC) as well as minerals corre-sponding to low-temperature processes (e.g. clays, REE-alumophosphates). Many of these mineral phases are veryrare or not present at all in monocrystalline or coarse-grained diamond polycrystals. However, the presence ofalumophosphate florencite (CeAl3(PO4)2(OH)6), which iscommon in carbonado and is usually ascribed to secondaryalteration, was recently reported as a crack-filling materialin lower mantle diamond from Kankan, Guinea (Brenkeret al., 2008). It was suggested that this compound wasformed from volatiles expelled from entrapped Ca–Si–P–O phases on decompression.

It is usually believed that many of the pores in carbo-nado are hermetically sealed, since treatment of carbo-nado with hot acids for 1–2 days usually fails to removenon-diamond phases. However, in the only systematicstudy of demineralization of carbonado (Dismukeset al., 1988) it was shown that only very special treatmentusing sequential leaching with several carefully selectedacids leads to almost complete removal of non-diamondphases. Moreover, it was convincingly demonstrated thatleaching for up to 200 hours is required to completedemineralisation a 100 mg (�3 mm in size) sample.Difficulties with sample demineralisation are due to thevery small diameter (1–2 mm) of channels connecting thepores, and diffusion-limited rates of the leaching. Afterthorough demineralisation only Ca and Fe were observed,indicating the presence of carbides inside diamond grains(Dismukes et al., 1988). Based on recent knowledge wemight also add native metals, alloys and, perhaps, carbo-nates to this list. To the best of our knowledge, none ofgeological studies of carbonado employed such thoroughcleaning. Therefore, one should be extremely careful ininterpretation of the carbonado major and trace elementchemistry. However, studies of mineral phases even ininsufficiently etched carbonado are valuable, since theymay shed some light on conditions of post-genetic historyof this diamond variety.

It is important to note that it is often not possible tostrictly define which polycrystalline diamond is carbonadoand which one is fine-grained boart. Usually this distinc-tion is made on basis of grain size, with carbonado havingthe smallest ones (,100 mm). In reality the majority ofpolycrystalline diamonds comprise grains of variable sizeand the distinction between polycrystalline varieties is notalways objective. Probably, the unusual smooth surface ofthe carbonado specimens can serve as a good discriminat-ing criterion. Based on comprehensive isotopic analyses itwas suggested (Burgess et al., 1998) that carbonado mightbe ‘‘an old framesite’’, i.e. ‘‘common’’ polycrystallinediamond which underwent a long history of secondaryprocesses. Since the ages of inclusions in carbonado areconsiderable (Sano et al., 2002), such scenario looks fea-sible. However, low N aggregation state (Kagi & Fukura,2008) is a strong argument against extensive recrystallisa-tion of diamond material.

One of the most popular hypotheses about the carbonadoorigin calls for a meteorite impact (Smith & Dawson,

1985). Polycrystalline diamonds found in meteorite cratersoften preserve features of initial graphite and may reach1 cm in size. However, microstructures of impact-related diamond and carbonado differ in many aspects.It is not clear how carbonado samples as big as 3167carats (633 g) could have been formed during meteoriteimpact and related processes, where combination ofsufficiently high pressures and temperatures persistsfor fraction of a second at most. Moreover, there is noindication for a big astroblem in the CAR or Brazil. ThePopigai impact structure in Northern Siberia is famousfor being extremely rich in impact diamonds (Masaitiset al., 1972). High abundance of the impact diamonds isdue to the carbonaceous target rocks. Nevertheless,despite intensive study there is no single line of evidencefor the occurrence of stricto sensu carbonado at thePopigai. Therefore, although in some rare cases carbo-nado-like diamond (not mentioning yakutite) could havebeen formed during impacts, the application of thishypothesis to all carbonado is not justified.

Isotopically light carbon and trace elements patternswere used to suggest a crustal origin for the carbonado(Shibata et al., 1993). However, considering the presum-ably insufficient demineralization of the carbonado speci-mens (compare the procedure by Dismukes et al. (1988)with other papers on the geochemistry of carbonado), itappears more likely that the radioactive and many REE-rich minerals are of secondary origin. Secondary altera-tions might also be responsible for isotopically-light car-bon. More importantly, origin of isotopically lightdiamond carbon is still debatable and involvement oforganic (e.g. subducted) C is not the only possibility. Infact, both isotopic fractionation (Galimov, 1991) andprimordial mantle heterogeneity (Deines, 1980) couldbe also responsible for enrichment in 12C. Remarkably,recent study of microdiamonds in 3–4 Ga old zirconsshow compositions with median as low as �31% andan organic origin of this carbon is unlikely (Nemchinet al., 2008).

Some authors suggest that radiation from radioactiveminerals induced diamond nucleation from carbonaceousmaterial and that radiation-induced defects promoted sin-tering of diamond grains (Kagi et al., 1994; Kaminskiy,1987). Nano-sized diamond grains created by natural irra-diation have been found in rare U- and organics richschales (Dubinchuk et al. 1976; Daulton & Ozima,1996), but it is difficult to understand the formation oflarge natural carbonado samples from these nanodia-monds. It was suggested (Kaminskiy, 1987) that the nano-diamond might have served as nuclei for subsequentgrowth of larger crystals. However, whereas numeroustheoretical calculations do suggest that at sizes below�10 nm (exact value depends on PT conditions) diamondis a stable form of carbon, at larger scales preservation ofthe diamond phase requires significant external pressures(Chaikovskii et al., 1985; Tauson & Abramovich, 1986;Davydov et al., 2007). Without significant external pres-sure, carbon onions and/or graphite become stable.Moreover, numerous studies of radiation-induced defects

36 V.A. Petrovsky, A.A. Shiryaev, V.P. Lyutoev, A.E. Sukharev, M. Martins

show no evidence for sintering of irradiated diamond pow-ders, even though total fluences of ionizing particles wereorders of magnitude higher than possible in natural condi-tions (e.g., Primak et al., 1956).

There have been several reports of diamond-like carbongrowth under hydrothermal conditions (e.g. Szymanskiet al., 1995). The mechanism of formation of sp3–hybri-dised carbon under these conditions is not yet clear. Sizeeffects and relative stability of the sp2– and sp3– carbonsagainst etching by the fluid may play an important role. Inview of these experiments one can, in principle, explaingrowth of natural carbonado diamond in hydrothermalcrustal conditions, e.g., on radiation-formed nanodiamondnuclei. The contrast between the generally large sizes ofnatural carbonado and micrometer-sized synthetic hydro-thermal diamonds might be due to widely different time-scales of growth in the laboratory and nature. However,creation of direct diamond-diamond bonding, i.e. existenceof sp3-hybridisation across intergrain boundaries, usuallyrequires high pressures and temperatures. Possibly, theonly exception is ultra-nano crystalline diamond (UNCD)grown by chemical-vapour deposition (CVD) process.Whereas some of the UNCD properties do indicate theexistence of direct intergrain bonding (e.g., absence ofsize-dependent confinement of Raman phonons), allUNCD samples always contain a significant fraction ofsp2–bonded carbon, observable in TEM and in conductiv-ity measurements (Ralchenko et al., 2007). Another obser-vation supports the necessity of high stresses during certainstages of the carbonado history: individual diamond grainsin carbonado often show high degrees of deformation(De et al., 1998; Kagi et al., 2007). Therefore, though itis difficult to completely rule out the formation of carbo-nado in low-pressures environments, it seems that manyfactors indicate that its formation took place at HP-HTconditions.

Polycrystalline diamonds which are in many aspects(appearance, composition of inclusions, isotopically lightcarbon) similar to African or Brazilian carbonado arefound among technical-grade diamonds in Yakutian kim-berlites and in placers in different regions (Kaminskiyet al., 1978; Titkov et al., 2001; Shcheka et al., 2006).This cast doubts on the axiom that carbonado arerestricted to Prephanerozoic Brazilian and African pla-cers of unknown origin, though the relative abundance ofcarbonado in these regions is apparently higher than any-where else.

It is evident from this discussion that there are difficul-ties in proposing a consistent model of carbonado forma-tion. Although in some cases formation by exotic processessuch as meteoritic impact is possible, we believe that manyproperties of carbonado could be explained as combinationof growth in mantle and subsequent alteration in the crust.In this paper we report results of a study of defects andmorphology of diamond grains, comprising carbonadodiamond from several Brazilian placers. Based on mor-phology of the composing diamond grains we suggest anew model of carbonado formation in one- or two-stageprocess in the upper mantle.

2. Samples, geological settings, characterizationmethods

Polycrystalline diamonds studied in this work (more than60 specimens) are typical examples of carbonado and largeaggregates of irregular shape with dull smooth surfaces andcolours ranging from light- or dark grey to almost black,sometimes having a brownish or greenish tint (Fig. 1). Thesizes of the samples varied from 5.4 � 4.7 � 5 mm to 5 �10� 15 mm. No signs of serious mechanical abrasion weredetected, although corrosion caverns are sometimesobserved (Fig. 2a).

All samples were collected in Brazil. The majority of thesamples are from alluvial placers in the Macaubas Riverbasin (Minas Gerais state). Some specimens came fromChapada-Diamantina National Park (Bahia state) and theGarsas and Diamantina regions (Matu-Grosso state).Spatially, and possibly genetically, the diamondiferous pla-cers are related to exposed early-mid-Proterozoic metaterri-genic rocks. Diamondiferous metaconglomerates of seamargin origin interstratified with phyllites and quartzitesof the Espinaco series are the source of the Minas Geraissamples. These conglomerates contain three generations ofclastic zircons. According to Pb–Pb isochrones (Sano et al.,2002) their ages are: 3599 � 12; 2151–2931 � (17–22);and 1750 � 58 Ma. The first generation is the oldest oneon the South American platform and is not related to cur-rently existing geological formations. The second genera-tion is correlated with Archean basement and the earlyProterozoic Minas series. The third generation representsthe formation of the Espinaco series. The Chapada-Diamantina samples originate from metaconglomeratesand psammites of the Tombador series; the correspondingrocks are �1400 Ma old. These metaconglomerates are,presumably, temporary collectors of diamonds. The onlyexceptions could be sills and dike-like bodies of thediamondiferous fillites from the Itakolomi series.

Samples characterization was performed at the Instituteof Geology of the Komi branch of the RAS. Morphology ofdiamond grains and chemical composition of some mineral

Fig. 1. General view of the sample MGC0, carbonado from Matu-Grosso state.

Morphology and defects of diamond grains in carbonado 37

inclusions were studied using a scanning electron micro-scope (SEM) JSM-6400 equipped with Link energy-dispersive and Microspec wavelength-dispersive (WDS)detectors. LiF, PET, TAP and LSM80 analyser crystalswere employed for the WDS study. Operating conditionsof the SEM were 20 kV, 1 nA, measuring time 50 s. Prior tothe analyses the samples were ultrasonically cleaned inacetone, mechanically crushed and carbon-coated.

Structural perfection and defects in the diamond latticewere studied for ten selected samples using X-ray diffrac-tion (XRD), photo- and X-ray luminescence (PL and XL),electron paramagnetic resonance (EPR) and Raman spec-troscopy. Photoluminescence was excited at 77 K using the365 nm line from a mercury lamp DRSh-250 and spectrawere registered using a SPM-2 monochromator withoutcorrection for spectral sensitivity. X-ray luminescencewas excited at room temperature by Fe Ka-radiation froma sealed tube operating at 50 keV–14 mA. Spectra wererecorded using an AAS-1 (Carl Zeiss) monochromator andcorrected for device sensitivity.

Raman and some of the photoluminescence spectra wereobtained using a Jobin-Yvon T64000 Raman spectrometerequipped with an Olympus microscope and a CCD camera.An argon laser (514.5 nm) was used for the excitation.Strong luminescence observed in many samples obscureddiamond Raman features and, therefore, some Ramanspectra were obtained using a LabRam (Jobin Yvon) spec-trometer with excitation by He–Ne laser (632.8 nm). Thespectra were recorded in quasi-backscattering mode forseveral spots on each sample using various objectiveswith the majority of spectra acquired using various objec-tives (10�, N.A. 0.25; 50�, N.A. 0.75 and 100�, N.A.0.9). The smallest slits (0.5 cm�1 at both spectrometers)were used to increase spectral resolution at both devices.Low power of the incident laser beam (laser output atT64000 was 0.17 mW and at LabRam � 0.25 mW) wasused to avoid significant heating of the analysed spots.Calibration of the wavelength was made using first orderRaman peak from a piece of high quality silicon wafer andthe accuracy of the line position was �0.1 cm�1. Size ofthe beam on the sample surface was 1–10 mm and penetra-tion depth was is order of 1 mm.

Paramagnetic defects were studied by EPR using acommercial RadioPAN SE/X-2547 spectrometer at roomtemperature. The X-band with 100 kHz frequency modula-tion was used. The spectra were modelled using the pro-gram described in Netar & Villafranca (1985).

XRD patterns on bulk and broken carbonado wereobtained using Fe and Mo radiation from sealed tubesusing various geometries. X-ray film was used in investi-gation of bulk samples with a Gandolfi camera and a CCD-camera for detailed studies of small chips.

3. Results

X-ray diffraction showed that the majority of studiedcarbonado specimens are polycrystalline aggregates

Fig. 2. SEM images of carbonado. (a) Part of the outer surface with acavern due to mechanical damage or dissolution. (b) Polishedsurface of the carbonado. Diamond grains of widely different sizesare clearly visible. (c) Internal part of the carbonado exposed bymechanical breaking. Well-formed diamond grains (1) areembedded into fine-crystalline and amorphous diamond matrix (2).


consisting of chaotically oriented diamond grains 1–10 mmin size. Diamond reflections (111), (220), (311) and(400) were clearly observed. Debye patterns of somesamples indicate the presence of texture. There havebeen few studies of carbonado texture (Kharlashina &Naletov, 1990, for yakutite) and it is not yet clearwhether textured carbonado specimens are commonor not.

Halfwidth and position of diamond Raman line are use-ful for assessment of stress and strain in crystalline lattice.In the studied samples the full width at half maximum(FWHM) of this line varied between 2.8 and 6.5 cm�1,indicating moderate-to-high stresses. The upper value isclose to that found in coarse-grained sintered diamond(Evans et al., 1984). On some of the grains brownishspots were observed. The Raman peak from these spots isdownshifted to 1327 cm�1 and is broadened (Fig. 3a) (seealso chapter ‘‘Unusual morphological features’’ fordetailed discussion of downshifted Raman peak). Thesezones are assigned to radiation damage from radioactivemineral inclusions.

4. Defects in diamond matrix: spectroscopicdata

During excitation by filtered light from the a mercury lampthe luminescence of the carbonado is typically red-orangeand rarely yellow-green. The red-orange colour is mostlydue to the 575 nm system, i.e. a neutral nitrogen-vacancy(NV0) complex. This defect is very common in Braziliancarbonado (Kaminskiy et al., 1979; Kagi et al., 1994). Oneof the samples differs from other studied specimens andhas a PL spectrum with peaks at 503 and 496 nm. Theselines may correspond to the H3 (vacancy trapped at thenitrogen pair) and the H4 defects (lines at 503 and 496 nm,correspondingly). The H3 defect in carbonado wasreported by Kaminskiy et al. (1979), Mineeva et al.(2000), Kagi et al. (1994, 2007). A PL feature resemblingthe H4 defects in carbonado was also described byKaminskiy et al. (1979). However, these authors wereuncertain about assignment of this line to the H4 defect.It is plausible that the observed spectrum is not due to theH4 defect, a vacancy trapped at the nitrogen-relatedB-defect, since its formation requires prolonged annealing.In this case, the observed spectrum would be due to yetunidentified defect. In one of the samples the N3 defectwas found together with the NV0 (Fig. 3b, curve 1). X-rayluminescence spectra (Fig. 3c) of four samples showed the575 nm line with its long wavelength wing. The XL spec-trum of one sample is characterised by yellow–greenbroad band.

EPR spectra reveal the presence of the P1-defect (singlesubstitutional N) in majority of the carbonado samples(Fig. 3d). The width and other parameters of the P1 tripletsare sample-dependent. Most of the samples show a well-resolved triplet with narrow lines. Spectra from severalother specimens are strongly broadened. In the EPR

spectrum of one of the samples (MGC6/2) only a veryweak line with g ¼ 2.0024 (�B ¼ 0.5 mT) was observed.Similar lines are present in spectra from other carbonadowhere they overlap with the strong central feature of the P1defect. The defect responsible for the g¼ 2.0024 line is notknown, but it may consist of nitrogen-vacancy complex.Support for this suggestion comes from the fine structureof the P1 central line in spectra of MGC0, MGC1 andMGC3, which is possibly related to a non-resolved hyper-fine contribution from N atoms. However, this feature mayoriginate from mineral inclusions and/or dislocations. OurEPR results are fully consistent with an earlier study byKlyuev et al. (1978).

5. Structure of the polycrystalline aggregates,morphology and surface features

The surfaces of whole carbonado samples are rather fea-tureless, though occasionally corrosion/dissolution fea-tures were observed (Fig. 2a).

The majority of SEM morphological investigations wereconducted on polished or freshly broken carbonado speci-mens. In line with previous studies (Orlov, 1963) weobserve that a single carbonado specimen could containdiamond grains of widely different sizes (Fig. 2b). Theinterstitial space is often filled by nondiamond, possiblysecondary material.

The carbonado samples consist of a dense aggregate ofcrystallomorphic diamond grains surrounded by a crypto-crystalline diamond mass. The grains are characterised byvariying degrees of morphological perfection (from well-formed to skeletal and rounded crystals); in many cases thegrains form aggregates and interpenetrations in variousorientations.

The euhedral diamond grains (Fig. 2c) are representedby micron-sized (1–3 mm) octahedra {111}, and, morerarely, tetragontrioctahedra {221}. In rare cases theirsizes reach 35–40 mm. The majority of the octahedralgrains are elongated along L2 or is flattened along L3 axis(where Ln is the n-th order symmetry axe). In the latter casethe apices are often terminated by remnants of (001) faces.Moreover, apices of some octahedral grains show reentrantfeatures, which are commonly observed on monocrystal-line diamonds, showing transition from cube to octahedralmodes of growth (Orlov, 1963; Titkov et al., 2002) (Fig. 4).In some specimens groups of diamond crystallites formaggregates of sub-parallel octahedra, elongated along L4

axis. Whereas the octahedral morphology is a well-knowndiamond growth form, the origin of tetragontrioctahedra isstill debatable. They might be a dissolution or growth form,and without detailed investigation of their internal struc-ture it is impossible to reach a firm conclusion. A consider-able fraction of the crystallites show well-formed faces,sometimes with clear manifestations of incomplete layer-by-layer growth (Fig. 5a). Some faces are covered by roughgrowth steps (Fig. 5b). In many carbonado specimens thediamond octahedra form intergrowths and twinning,


Fig. 3. Spectra of carbonado diamonds. (a) Raman spectra of different parts of the carbonado specimens. The dashed curve shows broadenedand downshifted spectrum from domains characterised by structural disorder due to radiation damage and/or presence of stacking faults.(b) Photoluminescence (PL) spectra of several specimens. (c) Low-temperature PL and X-ray luminescence spectra of carbonado.(d) Electron Paramagnetic Resonance (EPR) spectra of carbonado. The P1 defect and a defect with g�2 are seen.


indicating a high density of growth nuclei. Several speci-mens contain skeletal diamond crystallites (Fig. 6), presum-ably grown at very high carbon supersaturation or as asurface film on some preexisting mineral.

The majority of diamond crystallites show only growthmorphological forms, but dissolution marks are sometimesobserved (Fig. 7). A similar shagreen-like (or mat) reliefwas observed on surfaces of monocrystals during the firststages of dissolution (Kaminskiy et al., 2000). In mostcases the dissolution marks are present only on smallparts of the diamond grains, suggesting that the grainswere partly protected from dissolution, probably by non-diamond material. Remarkably, no trigons were observedon the diamond crystallites comprising carbonado. This isinteresting, since trigons are often directly related to dis-location outcrops and TEM studies of carbonado usuallyshow very high densities of dislocations. Therefore,absence of trigons and relative rarity of obvious dissolu-tion-related features suggest that the well-formed diamondcrystallites were only slightly resorbed. This observationhas important consequences for modelling formation of

Fig. 4. SEM images of individual crystallites showing reentrantapices, manifesting partial transition from cubic to octahedralmorphologies on crystals with different morphologies (a and b).

Fig. 5. Growth steps and layers on faces of diamond crystallites.(a) Incomplete growth layers of an octahedral diamond crystallite.(b) Rough growth steps on diamond crystallite.

Fig. 6. Skeletal diamond crystallite.


carbonado. Some of the grains show mechanical damage.Healed cracks were also observed.

The anhedral diamond grains are surrounded by crypto-crystalline diamond mass. Cracks formed after mechanicalbreaking resemble cracks in glass (Fig. 8a), and are similarto those observed on polycrystalline diamond where clea-vage planes were not activated. Unfortunately, no reliableconclusions on microstructure of these domains (e.g., crys-tallite size, presence of amorphous fraction etc) could bemade. Interestingly, some of the pores in the cryptocrystal-line mass exposed by polishing show negative diamondcrystal shapes (Fig. 8b). These pores were most likelyfilled by solid and/or fluid inclusions which were destroyedduring polishing.

6. Unusual morphological features

In several carbonado specimens diamond whiskers or nee-dle-like crystallites were observed in interstitial pores afterleaching of mineral phases (Fig. 9). In some cases these

Fig. 7. Dissolution features of different types observed on diamondgrains comprising carbonado.

Fig. 8. SEM images of cryptocrystalline diamond matrix. (a) Cracksurface with glass-like crack morphology. (b) Cavity exposed bypolishing and showing negative diamond shape. The cavity waspossibly filled prior to polishing.

Fig. 9. Diamond whisker (1) observed in pores of several carbonadospecimens. In some cases the whiskers are overgrown by otherminerals (2).


whiskers are overgrown by other minerals. Investigation ofthese needles at higher magnification shows that they con-sist of slightly misoriented blocks. Similar morphologicalforms were observed in diamond spontaneously grownfrom carbonate solutions at very high carbon supersatura-tions (Litvin, 2007) and in CVD growth.

Very unusual objects resembling remnants of explodedbubbles were found using SEM on some of the diamondgrains in one of the carbonado specimens (Fig. 10). Ramanspectra of these objects consist of a single broad peakcentred at 1327 cm�1 similar to that shown in Fig. 3a. Ingeneral, downshift and broadening of the diamond Ramanline could be caused by several mechanisms: (1) localheating of diamond by a laser; (2) radiation damage; (3)nanometer-range size of individual diamond crystallites;(4) presence of hexagonal diamond polytypes. May et al.(2008) presented a comprehensive study of diamondRaman peak position as a function of crystallite size,wavelength and power of incident laser. In our work ared laser (632.8 nm) with very low laser power(0.25 mW) was employed. Comparison of the results byMay et al. (2008) to our work strongly suggests that theoverheating was not very likely. Moreover, we acquiredRaman spectra from various spots on the carbonado sur-faces (pristine and exposed by polishing) and the downshiftwas exclusively confined to the brown spots and to theunusual morphological features discussed here. This cor-relation suggests that instrumental artefacts played a minorrole, if any. The radiation damage is likely present inseveral spots as described above, but cannot be responsiblefor all observed spectra. Shift of the line due to nanosizedgrains is unlikely for the carbonado. For the bulk diamondconfinement effects become important at a grain size ofapprox. 1.5 nm or less (Watanabe et al., 2006) whereas themajority of the diamond grains in carbonado are larger.Theoretical calculations of the lattice dynamics of hexago-nal diamond – lonsdaleite (Vigasina, 1991) suggest that itsRaman line should be downshifted from the position for

the cubic diamond. Results from the experimental study ofheavily deformed monocrystalline diamond using coherentanti-Stockes Raman scattering (CARS) (Titkov et al.,1992) can be used to infer the presence of hexagonaldiamond in this sample. Detailed X-ray structural investi-gation of samples containing various fractions of hexago-nal diamond (Yoshiasa et al., 2003) showed high density ofrandomly distributed occurrences of cubic stacking in ahexagonal stacking sequence and of hexagonal stacking ina cubic stacking sequence. Therefore, the downshift of thecarbonado Raman line observed in the current study maybe explained by the presence of many types of point andextended defects, including lonsdaleite-like stackingfaults.

In addition to the three-dimensional individual diamondgrains, carbonaceous films on the surfaces of diamondcrystals were found. The areas of the films reach 250–300 mm2, with thickness not exceeding 500 nm. The originof these films is uncertain, they may represent some sec-ondary process.

7. Discussion

Several nitrogen-related defects have been observed inspectroscopic studies of carbonado. Luminescence spectra(Kaminskiy, 1991; Kagi et al., 1994, 2007, this work) showseveral common defects: NV (ZPL at 575 nm), H3, 3H.Very often the NV and 3H defects in carbonado are con-sidered to be produced during radiation damage frommineral inclusions. Although it is clear that carbonadounderwent some radiation damage, as evidenced by radia-tion haloes around inclusions (Kagi et al., 2007) not all ofthese point defects could be ascribed to such type ofdamage, since both NV and 3H could be readily producedby deformation (see Zaitsev, 2001, for review). Presence ofthe NV and 3H defects is sometimes used to constrain thethermal history of carbonado (Kagi et al., 1994; Kagi &Fukura, 2008). Unfortunately, such constraints are notnecessarily correct as both of these defects may survivetemperatures up to 1500 �C (Zaitsev, 2001, and refs.therein).

It is interesting to consider the absence of spectral man-ifestations of a neutral vacancy (GR-defect) in carbonadospectra. The GR defect appears in any type of diamondafter any type of irradiation. Presence of dislocations andother similar defects increases the GR production rate incomparison with ideal lattice (Zaitsev, 2001). Since carbo-nado or its parts indeed underwent some irradiation fromradioactive nuclei in inclusions, the absence of the GRdefect could be either due to post-irradiation annealing ofcarbonado at temperatures higher than 600 �C and/or to thepresence of high concentrations of nitrogen and/or boron inthe diamond lattice.

In summary, the luminescence data available clearlyshow that the diamond grains in carbonado are heavilydeformed, contain nitrogen and (in conjunction withother observations) were subjected to some irradiation.

Fig. 10. Unusual bubble-like objects consisting of diamond materialpresent in some pores exposed by breaking and subsequent leachingof carbonado.


They were also probably annealed, but reliable constraintson the annealing conditions cannot be made from theluminescence behaviour alone.

The thermal history of carbonado can be more reliablyaddressed using infra-red (IR) spectroscopy. Carbonadois a difficult sample for IR investigation due to sampleopacity. However, a recent study by Kagi & Fukura(2008) using transmission geometry and our ownwork in reflectance (Shiryaev et al., 1998) indicate thatthe majority of carbonado belong to the Ib þ A type,that is, they contain weakly aggregated nitrogenin concentrations up to 400 ppm. The presence of singlesubstitutional nitrogens is confirmed by EPR (Kaminskiyet al., 1978, this work). Kagi et al. (1994) reported the lineat 1384 cm�1 in IR spectra of carbonado and tentativelyascribed it to platelets, a type of defect formed afterprolonged annealing and which are very common fornatural nitrogen-rich monocrystalline diamond. Thisassignment is often discussed in carbonado literature.However, this line is obviously not related to plateletsand is instead often observed in low quality natural dia-mond with non-aggregated nitrogen (Woods & Collins,1983). The defect responsible for this absorption isunknown, and it is not improbable that this is an inclu-sion-related feature. Therefore, IR and EPR spectroscopystrongly suggest that carbonadoo were only slightlyannealed, i.e. the temperature was low and/or durationwas insufficient. At the same time, dislocations in carbo-nado show a rather high degree of polygonisation (Chen& van Tendeloo, 1999; De et al., 2001). The polygonisa-tion is often observed in studies of diamond sintering athigh temperatures (Shulzhenko et al., 2003; De et al.,2004; Shiryaev et al., 2007).

Which information about the carbonado growth can weobtain from the morphological study? Morphology of thediamond grains in carbonado is an important clue to gen-esis of this diamond variety. We suggest that many featuresof carbonado are readily explained by one- or, at most,two-stage evolution of a growth system.

First of all, existence of well-formed micron-sized dia-mond octahedra show that during the early stages of thecarbonado formation, physical and chemical conditionswere close to those required for growth of microdiamondsobserved in many kimberlites worldwide (e.g.,Zedgenizov et al., 1998). The remarkable observation ofremnants of the cubic growth sectors indicate that carbonsupersaturation was decreasing during growth, resultingin initial crystallization of the cuboid growth sectors andsubsequent change to the octahedral mode of growth.Furthermore, many of the octahedra show growth steps,again suggesting that the C supersaturation was insuffi-cient to complete the formation of ideal octahedra. Theexistence of intergrown aggregates of diamond crystal-lites points to a high density of nucleation sites, but thediamond elongation suggests low symmetry (e.g. cylind-rical) of the growth medium.

Whereas crystallisation of the euhedral diamondis relatively well-understood, formation of cryptocrystal-line diamond mass is more enigmatic. Theory and

practice of crystal growth show that cryptocrystallinemasses are usually formed when the density of nucleationsites is very high. Morphologically similar diamond canbe formed from plasma at low temperatures. However,growth of this variety of CVD diamond is very slow andis, therefore, difficult to realise in nature. Alternatively,carbonado-like diamond was synthesised at PT condi-tions far from the diamond-graphite equilibrium line(Yakovlev et al., 2001). This synthesis is usually madeby applying very high pressures to induce formation ofnumerous diamond nuclei. Sharp increase of pressuremay occur in nature, but the responsible process isunclear.

When solubility of a compound strongly depends ontemperature, the supersaturation needed for its crystallisa-tion is usually caused by drop of the temperature. In thesecases polycrystalline aggregates are formed and diamondvarieties such as boart or framesites are obtained in experi-ments and, probably, in nature. However, when solubilityis weakly temperature dependent, mass crystallisationcould be caused by evaporation such as in the case ofsome evaporites and speleothemes, or, more relevant todiamond, by chemically-induced crystallisation. In the lat-ter case, the crystallisation of a compound B from solutiontriggers precipitation of a given compound A via changesin the solubility of A due to changes in solution structureand/or decreasing volume of a solution (e.g., Kuznetsov,1979). Numerous examples of chemically-induced crystal-lisation are known in growth of organic (addition of NaClcauses protein crystallisation) and inorganic (variouschlorides) materials. Mass crystallisation may also beassisted by high concentrations of foreign particles/col-loids (i.e. dirty solutions).

Therefore, carbonado formation could be represented asfollows: growth of micronmeter-sized diamond singlecrystals under conditions of decreasing supersaturation,but with a high density of nucleation centres. The mostlikely driving force for crystallisation was decreasing tem-perature. During this stage micron-sized octahedral dia-monds, sometimes showing traces of cube-to-octahedratransition, were formed. Although it is not possible toindicate unambiguously the depth in the mantle at whichthese octahedral diamonds grew, it is reasonable to assumethat their formation was under similar conditions to ‘‘nor-mal’’ mantle diamond formation. The second stage mayeither follow directly or with some delay and is charac-terised by a markedly different cause for the creation ofsupersaturation: crystallisation of another mineral(s), lead-ing to decrease of the volume and/or structure of the parentsolution. The crystallising mineral grains as well as exist-ing diamond grains may also serve as a substrate for themassive diamond nucleation. Note that mineral(s) whosecrystallisation triggered precipitation of diamond mass donot necessarily contain carbon.

As shown above, the microdiamond grains in carbonadoare generally only weakly resorbed. This suggests ashort period of time during which the newly formedmicrocrystals were in contact with the surrounding fluid.The subsequent mass crystallisation effectively prevented


dissolution. De et al. (2001) showed that interpenetratingdiamond crystallites in carbonado could be representedby two generations of diamond with carbon isotopiccomposition differing by 2 % and with variations inthe N concentration. The larger crystals (the first genera-tion) are N-poor and have lighter carbon in comparisonwith the anhedral mass (the second generation).Equilibrium carbon isotope fractionation between carbo-nate and diamond at high pressures and 1100 �C is approx.2.7 % (diamond is lighter than carbonate, Shiryaev et al.,2005 and refs therein). Therefore, coexistence of twodiamond generations is easily explained by our model.The first generation of well-formed diamonds depleted in13C crystallised under conditions approaching equilibrium.Afterwards, the chemically-induced mass crystallisation ofresidual carbon produced isotopically heavier diamondmass (second generation).

It is, however, obvious that the evolution of the carbo-nado parental system may differ from the sequencedescribed above, and the precipitation of the cryptocrystal-line diamond due to the chemically-induced crystallisationcould be the first stage with the euhedral diamond crystalsgrowing upon C supersaturation decrease. However, thisscenario appears less plausible since it would imply extre-mely high C supersaturation, which seems difficult toachieve in natural systems.

At this time little can be said about possible syngeneticminerals. The only relatively reliable, though debatable,clue is given by ages of inclusions in carbonado reportedby Sano et al. (2002). According to the SHRIMP U-Pb andPb–Pb dating, zircons, rutile, quartz and clays trapped incarbonado show ages around 3 Ga. Zircons are rarelypresent as inclusions in monocrystalline diamond andquartz was found almost exclusively in fibrous diamonds,also grown under rather unusual conditions. We may ten-tatively suggest that these minerals might be syngeneticwith carbonado, though this list is not necessarilycomplete.

The low degree of nitrogen aggregation in carbonadoindicates that soon after its formation the temperaturedropped significantly to prevent nitrogen diffusion.According to our model the principal difference betweencarbonado and other diamond varieties (in particular,polycrystalline) is the existence of conditions favouringchemically-induced over temperature-induced crystallis-ation. The relative rarity of carbonado is then due torather unusual conditions in which such a situationmay occur. These conditions might have been morecommon in very ancient kimberlites, and subsequentlycompletely destroyed by formation of the moderngeological structure of South America or Central Africa.However, existence of carbonado-like diamond in kimber-lite pipes (see Introduction) may indicate that suchconditions are not limited to these unknown parentalrocks.

The important third stage of carbonado history includesextensive secondary alteration, removal of some of thecoexisting minerals, precipitation of new phases and thegradual accumulation of radiation damage.

Acknowledgements: We are grateful to Drs. Yu.V.Glukhov, S.I. Isaenko, V.N. Filippov and L. Mikhailytsynfor assistance in analytical work and discussions.Discussions with Drs. A.E. Voloshin and A.F.Khokhryakov are greatly appreciated. Reviews by FelixKaminskiy, Lutz Nasdala and an anonymous reviewerwere very useful. We thank Dr. G. Bromiley for a thoroughediting of English. The work was partly supported by theRussian President Fellowship MK-147.2007.5 and RFBRgrant 08-05-000745 to AAS, MK-4015.2008.5 to AES;HLLI-3266.2008.5.

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Received 14 January 2009

Modified version received 20 July 2009

Accepted 10 August 2009


morphology and defects of diamond grains in carbonado: clues to carbonado genesis

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