Octahedral Sectors Research Articles

Mixed-Habit Type Ib-IaA Diamond from an Udachnaya Eclogite

The variety of morphology and properties of natural diamonds reflects variations in the conditions of their formation in different mantle environments. This study presents new data on the distribution of impurity centers in diamond type Ib-IaA from xenolith of bimineral eclogite from the Udachnaya kimberlite pipe. The high content of non-aggregated nitrogen C defects in the studied diamonds indicates their formation shortly before the stage of transportation to the surface by the kimberlite melt. The observed sectorial heterogeneity of the distribution of C- and A-defects indicates that aggregation of nitrogen in the octahedral sectors occurs faster than in the cuboid sectors.

Epitaxial Mechanism of Diamond Crystal Nucleation on a Mo {110} Substrate during Chemical Vapor Deposition

The epitaxial mechanism of diamond crystal nucleation by the (110) face on the Mo(110) face during chemical vapor deposition on a molybdenum plate has been established. It is shown that, under the synthesis conditions in use, only cubic sectors of diamond crystals continue to grow according to the tangential mechanism, whereas octahedral sectors are characterized by some complex nonequilibrium mechanism of normal growth. In particular, a situation where the {111} faces are absent and octahedral sectors grow by {100} layers of three orientations, which make a reentrant cube angle and are twins of the initial crystal on one of its {111} faces, has been observed. Geometric analysis of five-fold aggregates of multiply twinned diamond crystals with a shared 〈110〉 axis shows that at least one narrow truncating lateral {111} face oriented parallel to the shared 〈110〉 axis should occur.

Experimental and Theoretical Evidence for Surface-Induced Carbon and Nitrogen Fractionation during Diamond Crystallization at High Temperatures and High Pressures

Isotopic and trace element variations within single diamond crystals are widely known from both natural stones and synthetic crystals. A number of processes can produce variations in carbon isotope composition and nitrogen abundance in the course of diamond crystallization. Here, we present evidence of carbon and nitrogen fractionation related to the growing surfaces of a diamond. We document that difference in the carbon isotope composition between cubic and octahedral growth sectors is solvent-dependent and varies from 0.7‰ in a carbonate system to 0.4‰ in a metal-carbon system. Ab initio calculations suggest up to 4‰ instantaneous 13C depletion of cubic faces in comparison to octahedral faces when grown simultaneously. Cubic growth sectors always have lower nitrogen abundance in comparison to octahedral sectors within synthetic diamond crystals in both carbonate and metal-carbon systems. The stability of any particular growth faces of a diamond crystal depends upon the degree of carbon association in the solution. Octahedron is the dominant form in a high-associated solution while the cube is the dominant form in a low-associated solution. Fine-scale data from natural crystals potentially can provide information on the form of carbon, which was present in the growth media.

Diamond growth from C–H–N–O recycled fluids in the lithosphere: Evidence from CH4 micro-inclusions and δ13C–δ15N–N content in Marange mixed-habit diamonds

Mixed-habit (octahedral+cuboid) diamonds from the Marange alluvial deposits in the eastern Zimbabwe craton have high nitrogen and hydrogen contents that provide an opportunity to evaluate diamond growth mechanisms and C–N–H–O bearing fluids in the lithospheric keel. Light grey cuboid sectors with hydrogen-containing defects trap abundant dispersed CH4 inclusions (Raman peaks at 2917cm−1) associated with graphite (Raman peaks at 1580cm−1). Clear octahedral sectors are richer in nitrogen and free of any such inclusions. Core to rim co-variations of δ13C–δ15N and N content can be explained by a mixing trend between earlier fluids that are CH4-rich and later fluids that are more CO3- or CO2-rich. Marange diamonds have limited overall δ13C variation, but do show fractionation during growth towards higher δ13C values. This trend can be explained by diamond precipitation from mixed CH4and CO2fluids, where isotopic fractionation occurs as the amount of fluid wanes. Calculated δ15N values for diamond source fluids evolving in this manner are between +2.3 and +6.4‰. These N isotopic compositions require CH4-rich and CO3-/CO2-rich ‘end-member’ fluids to have a recycled metasedimentary component perhaps introduced with subduction of eclogite.

Sector Zoning In Uraninite

A uraninite crystal from the Roode pegmatite in southern Norway displays clear sector zoning mostly defined by the main U-Th substitution, with sharp changes in chemical compositions between the cubic and the octahedral sectors; a weaker concentric zoning defined by the same chemical variations is also present. The {100} sectors incorporate slightly more U whereas the {111} sectors incorporate slightly more Th, Y, and REEs. The cubic sectors contain on average ca. 77.0 wt.% UO2, ca. 6.6 wt.% ThO2, ca. 0.6 wt.% ΣREE2O3, and ca. 0.45 wt.% Y2O3; the octahedral sectors contain on average ca. 73.8 wt.% UO2, ca. 9.1 wt.% ThO2, ca. 0.8 wt.% ΣREE2O3, and ca. 0.55 wt.% Y2O3. The development of sector zoning in uraninite is possibly due to the interplay between the difference in effective ionic radius between U and Th and the difference in protosite density between the {100} and {111} sectors.

Nitrogen isotope systematics and origins of mixed-habit diamonds

Nitrogen isotope values from mantle diamonds are a commonly used tracer in the quest to track volatiles within the Earth’s mantle through deep time. Interpretations of this isotope data are valid so long as stable isotope fractionation processes in the mantle are understood. The fractionation of nitrogen isotopes between {111} and {100} growth sectors is well documented for high-pressure high-temperature (HPHT) synthetic diamonds, but there is little data on whether it also occurs in natural mixed-habit diamonds. We present 91 in-situ nitrogen isotope (δ15N) measurements, along with carbon isotope (δ13C) values and nitrogen abundances [N], obtained from three mixed-habit diamonds by secondary ion mass spectrometry (SIMS). While the well-documented enrichment of nitrogen concentrations in octahedral sectors compared to contemporaneous cuboid sectors is observed, a similarly clear disparity is not obvious in the δ15N data. Whereas HPHT synthetic diamonds exhibit 15N enrichment in the {100} sectors by ∼+30‰, the mixed-habit diamonds studied here show enrichment of the octahedral sectors in 15N by only 0.4–1‰. This major difference between HPHT synthetic and natural mixed-habit diamonds is proposed to be the result of different physical properties of the growth interfaces. The smooth interfaces of the octahedral sectors are the same in both types of crystal, but the outermost atoms on the smooth cube interfaces of an HPHT synthetic diamond behave differently to those on the rough cuboid interfaces of the natural mixed-habit diamonds, resulting in different δ15N values. Both the δ13C (average of ∼−8.7‰) and δ15N (average of ∼0‰) data show only minor offsets from the typical mantle values (δ13C=−5±3‰, δ15N=−5±4‰). This may indicate diamond formation from a mantle derived fluid/melt containing a minor subducted component (lowering δ13C values and elevating δ15N) or relate to moderate degrees of isotopic fractionation of a pure mantle fluid/melt by prior diamond precipitation. The homogeneous nature of both the carbon and nitrogen isotopic compositions of all three diamonds, however, documents continuous and unlimited supply of diamond forming fluid/melt, with a constant composition. Such homogenous isotopic compositions exclude fluid mixing or isotopic fractionation close to the site of diamond formation and preclude distinguishing between these two processes based on diamond analyses alone.

Trace element partitioning in mixed-habit diamonds

Mixed-habit diamonds are a special type of crystal that form by simultaneous growth by two different mechanisms; octahedral and cuboid growth. This type of crystal growth is known to preferentially partition nitrogen into the octahedral growth sectors during growth. To identify if this fractionation occurs for any other trace element, this study applies in situ laser ablation mass spectrometry to a well-studied collection of mixed-habit diamonds. The results show high concentrations of nickel (3.6–36.4ppm) and cobalt (0.108–1.25ppm) in the cuboid sectors, whereas the octahedral sectors contain only <0.05–0.35ppm nickel and <0.014–0.03ppm cobalt. The relative enrichment of nickel in the cuboid sectors varies between 10 and 460 times. For cobalt, where the concentrations in the octahedral sector are commonly below the limit of quantification, the enrichment is estimated to be in the range of 10–100 times. There appears to be a rough linear relationship between the concentrations of the two elements in the cuboid sectors, with nickel concentrations approximately 27 times that of cobalt. This finding of nickel fractionation into the cuboid sectors is the opposite of what is seen in synthetic diamonds grown from FeNi solvents, where Ni is only found in the octahedral growth sectors. We propose that this is because the inherent lattice strain in the rough cuboid diamond growth of natural mixed-habit diamonds is greater than that in the smooth octahedral growth, making it easier to fit the large Ni and Co ions into a divacancy. In diamonds grown from metal solvents (i.e. industrial synthetic diamonds), the cube face is a smooth one, therefore the partitioning of trace elements is simply a result of the octahedral surface offering more bonding opportunities than the cube surface.Due to the extremely low concentrations of trace elements in these gem quality diamonds, many of the elements are commonly below the limit of quantification (LOQ). While we are confident that Pr, Sr, Pb, Zr, V, Cr and Zn are not fractionated by the crystal growth mechanisms of mixed-habit diamonds that affect Ni and Co, this remains to be proven for the remaining trace elements analysed. Data on the fractionation of copper are inconclusive, with 75% of data from the cuboid sectors above the LOQ and all data from the octahedral sectors below the LOQ. However, the values of the LOQ for the octahedral data are often higher than those of the cuboid data, so further investigation of Cu is necessary to confirm its behaviour.

A spectroscopic and carbon-isotope study of mixed-habit diamonds: Impurity characteristics and growth environment

Mixed-habit diamonds have experienced periods of growth where they were bounded by two surface forms at the same time. Such diamonds are relatively rare and therefore under-investigated. Under certain physical and chemical conditions, smooth octahedral faces grow concurrently with rough, hummocky cuboid faces. However, the specific conditions that cause this type of growth are unknown. Here we present a large array of spectroscopic data in an attempt to investigate the impurity and carbon-isotope characteristics, as well as growth conditions, of 13 large (>6 mm diameter) plates cut from mixed-habit diamonds. The diamonds all generally have high nitrogen concentrations (>1400 ppm), with the octahedral sectors enriched by 127-143% compared to their contemporary cuboid sectors. Levels of nitrogen aggregation are generally low (2-23% IaB) with no significant difference between sectors. IR-active hydrogen features are predominantly found in the cuboid sectors with only very small bands in the octahedral sectors. Platelet characteristics are variable; only one sample shows a large B′ band intensity in the octahedral sector, with no platelets occurring in the cuboid sector. Other samples either show a small B′ band in both sectors, or just in the cuboid sector, or none at all. These data support a model that shows the concentration-adjusted aggregation rate of nitrogen to be the same in both sectors, whereas the subsequent platelet development is reduced in the cuboid sectors. This is because the interstitial carbon atoms have interacted with disk-crack-like defects only found in cuboid sectors, which in turn reduces their chances of aggregating to form platelets. These disk-crack-like defects are also thought to be the most likely site for the IR-active hydrogen features and they maybe intrinsic to cuboid growth in mixed-habit diamonds. When they are graphitized, as they are in all of the diamonds in this study, this may reflect a heating event prior to volcanic exhumation. Spectroscopic analysis of the green cathodoluminescence exhibited by all of the diamonds shows nickel centers to be present in only the cuboid sectors. Carbon isotope data, obtained by secondary ion mass spectrometry, show very little variation in seven of the diamonds. The total range of 217 analyses is -7.94 to -9.61 (±0.15)‰, and the largest variation in a single stone is 0.98‰. No fractionation in carbon isotopes is seen between octahedral and cuboid sectors at the same growth horizon. These data suggest that the source fluid chemistry, as well as pressure, temperature, and oxygen fugacity were very stable over time, allowing such large volumes of mixed-habit growth to occur. The high concentration of impurities, namely nitrogen and hydrogen, is probably the critical factor required to cause mixed-habit growth. The impurity and isotopic data fall in line with previous modeling based on diamond growth from reduced carbonates with the loss of a 13C-enriched CO2 component.

Platelet development in cuboid diamonds: insights from micro-FTIR mapping

Fourier transform infrared mapping of diamonds can reveal detailed information on impurities, with a spatial context. We apply this technique, combined with in situ isotopic analysis of carbon, to the study of cuboid diamond growth in a sample that exhibits some mixed-habit growth. While there has been some uncertainty in the literature regarding sectoral differences in nitrogen aggregation and subsequent platelet development, the data from this study appear far more conclusive. We show that despite nitrogen being concentrated in octahedral sectors, there is no detectable difference in the concentration-adjusted rate of nitrogen aggregation within octahedral and cuboid sectors. However, the resultant platelet development is significantly reduced in cuboid sectors compared to contemporaneously formed octahedral sectors. This finding has significant implications for the classification of diamonds using the relationship between their platelet intensity and the absorption caused by B centres. It means that cuboid diamonds naturally fall below the linear relationship that has been termed regular, which would lead to them being incorrectly interpreted as having experienced heating or deformation. The data also support earlier suggestions that large hydrogen concentrations in the diamond-forming fluid may be required for cuboid growth. We further suggest that high nitrogen and hydrogen concentrations are required for mixed-habit diamond growth, which might be the product of specific fluid chemistries that occur in reducing mantle environments.

Monitoring diamond crystal growth, a combined experimental and SIMS study

Detailed ion microprobe measurements have been made on two synthetic diamond crystals in order to investigate how variations in chemical and isotopic compositions are related to growth sectors and overall growth history. The crystals were grown by the metal catalyst technique under identical T - P conditions of 1450 °C and 5.5 GPa, but with different source nitrogen abundances. Measurements for carbon and nitrogen isotope compositions and nitrogen abundance were made in traverses across the crystal sectors, which included cubic sectors and octahedral sectors of both relatively rapid and relatively slow growth. In both crystals an early phase of growth dominated by falling δ 13 C and rising of N ppm is followed by an extensive phase of growth with moderately constant δ 13 C and gradually descending N ppm . The change from falling to stable δ 13 C ratios has been numerically modelled on the basis of the carbon isotope fractionation between the carbon solution in metal melt and the growing diamond in closed system; the stabilization of the diamond carbon isotope composition is achieved once a steady state is attained and diamond grows with the same carbon isotope composition as the graphite source. The decreasing N ppm values appear to be a product of Rayleigh fractionation. Carbon isotope compositions show little difference between different growth sectors, and δ 15 N values in any given sector show no change with time of growth. However, the nitrogen isotope compositions show major differences of ca. 30 ‰ between octahedral and cubic sectors. These differences are not related to growth rate or time of growth and appear to represent a consistent difference in N isotope adhesion between the two faces.

On the interaction of molecular hydrogen with diamonds: An experimental study using nuclear probes and thermal desorption

Hydrogen behavior in monocrystalline diamonds with different concentrations and types of nitrogen defects was studied using Nuclear Reaction and micro-Elastic Recoil Detection Analyses and Thermal Desorption Spectroscopy. Diamonds were studied in as-received state, after HPHT treatment and after hydrogenation in molecular hydrogen. A considerable amount of hydrogen was found to be bonded to diamond surfaces. Kinetics of surface hydrogen desorption is similar to what was reported for plasma-hydrogenated diamonds. The solubility of hydrogen in type IIa diamonds is very low. The efficiency of hydrogen traps in diamond bulk varies with the dominant type of nitrogen-related defects. The cross-section of traps decreases in row Ib > IaA > IaB diamonds, though binding energy in type IaB crystals may be higher, than in type IaA. Dislocations may promote hydrogen diffusion. A marked dependency of the hydrogen content and diffusivity between diamond growth sectors were observed for some samples. The total hydrogen content is higher in octahedral sectors.

Three historical ‘asteriated’ hydrogen-rich diamonds: growth history and sector-dependent impurity incorporation

Three historical ‘asteriated’ diamond slices present mixed-habit growth with contemporaneous lobe-shaped cuboid sectors and octahedral sectors. The lobe shape is explained by continuous variation of the relative growth rates. Nitrogen concentration in octahedral sectors is higher than in cuboid sectors, whereas hydrogen is incorporated mostly in cuboid sectors. The H-related IR peak at 3236 cm −1 peak, and the two peaks at 3050 and 3154 cm −1 (which may be completely absent from the spectra) are independent from the well-known 1405–3107 cm −1 system. Nitrogen aggregation during thermal history seems partially inhibited in cuboid sectors by the large amount of hydrogen, as platelets and N3 centres are present in octahedral sectors but not in cuboid sectors. Cuboid, H-rich growth sectors are thought to appear when the amount of hydrogen in the medium is too high to be incorporated in the octahedral sectors only. The definition of ‘H-rich diamonds’ is revisited. The first-order Raman peak at 1332 cm −1 is slightly growth-sector dependent; cuboid sectors show a sharper and more Lorentzian peak than octahedral sectors.

Microscale variations of δ13C and N content within a natural diamond with mixed-habit growth

The variations of carbon isotope composition and nitrogen content in a natural diamond with mixed octahedral and cuboid habit have been studied with the ion microprobe technique (SIMS). The systematic variations of δ 13C and Nppm in continuous growth zones belonging to both octahedral and cubic sectors have been measured. In general the octahedral growth sector in comparison with the cuboid one is richer in nitrogen by 90–300 ppm and has a slightly heavier δ 13C by 0.3–1.5‰. These data show that simultaneous mixed growth of cuboid and octahedral sectors in natural diamond entails little fractionation of carbon isotopes. The observed variations are most likely due to the different nature and kinetics of growth of natural cuboid and octahedral diamonds.

Kinetics of Ib to IaA nitrogen aggregation in diamond

Diamonds containing nitrogen impurities in the form of singly substituted C centres (type Ib diamond) are rare in nature because lb nitrogen atoms aggregate to form A centres (type IaA diamond) during the long-term (∼0.5 to 3 Gyr) upper mantle residence period experienced by most diamonds. Preservation of N-rich diamond with a type Ib component requires a young age of crystallization, or unusually cool conditions of mantle storage, or both. Quantitative constraints on the origin of Ib diamonds are possible if the kinetics of nitrogen aggregation are accurately known. However, previous experimental studies have produced significantly different values for the activation energy of Ib → IaA conversion (Ea) which is at least partly due to sector dependence of the aggregation rate in the starting materials. We report a series of high-pressure (P), temperature (T) experiments on synthetic and natural type Ib diamond in which we have used infrared (IR) microspectrometry techniques to unravel the sector dependency. The results show that cube sectors have an Ea of 6.0 ± 0.2 eV, which is significantly greater than the Ea of 4.4 ± 0.3 eV determined for the octahedral sectors. Both values are in excellent agreement with a recent theoretical study in which the sector dependency is ascribed to different mechanisms of nitrogen migration. The higher Ea is most relevant to natural Ib diamond which are invariably of cube or cuboid habit. Application to the Kokchetav microdiamonds of metamorphic origin indicate that their aggregation state is consistent with peak temperatures of 950°C and a burial to exhumation history of ∼17 Myr. Application to yellow cubes and “coat” from Yakutia having ∼20% Ib component, indicate this diamond generation grew_ˇMyr before kimberlite eruption for mantle T_˘950°C but did not grow directly from the transporting kimberlite magma.

Disequilibrium carbon and oxygen isotope variations in natural calcite

OXYGEN isotope fractionation in the calcite–water system is widely used to investigate the temperature, fluid composition and reaction rate of calcite precipitation, under the assumption that equilibrium is maintained during precipitation. Here I present analyses of coarsely crystalline, inorganic calcite displaying combined growth and sector zoning, which show this assumption to be untrue. Variations in the isotope composition of successive growth zones can be interpreted as due to changing conditions of precipitation, with equilibrium being maintained throughout. But variations across different sectors of the same synchronous growth surface (∼2‰ for δ13C and ∼0.9‰ for δ18O) record disequilibrium in the same crystal. This type of disequilibrium may be common, as calcite precipitates in a great variety of crystal form combinations; moreover, many noncarbonate minerals also grow as crystals with two or more crystallographic forms. Indeed, Boyd et al.1 reported fractionation of nitrogen isotopes between cubic and octahedral sectors of a synthetic diamond, but I believe that crystallographi-cally controlled isotope fractionation of stoichiometric ions has not hitherto been recognized.

The segregation of nickel-related optical centres in the octahedral growth sectors of synthetic diamond

Abstract The vibronic absorption bands with zero-phonon lines at 1·40, 1·883, and 2·51 eV, the broad absorption band with a maximum near 1·4eV and the series of absorption lines near 3·1 eV, the vibronic luminescence bands with zero-phonon lines at 1·40eV and 2·56 eV, all associated with nickel-related optical centres in synthetic diamond, are shown in this investigation to be restricted exclusively to the octahedral growth sectors. In addition, absorption in a sharp line at the Raman frequency (1332cm−1), at the high-frequency limit of the defect-induced one-phonon region, is present only in the octahedral sectors. The relative intensities of the different absorption bands depend on the nitrogen concentration in the diamond. The 1·883 and 2·51 eV systems are strongest in samples containing a high nitrogen concentration, whereas the 1·4 eV broad band and 1·40 eV vibronic system are strongest in diamonds with a low nitrogen concentration.

A study of diamonds of cube and cube-related shape from the Jwaneng mine

The Jwaneng mine in southwest Botswana is unusual inasmuch as about 8% of the diamonds produced have shapes which may be loosely described as “cubes”. Such shapes are normally very rare. The shapes vary from stones with heavily re-entrant cube faces and trigonally-symmetric Y-shaped octahedral faces, through more or less flat-faced cubes, to rounded cubes with growth protuberances at the cube corners. X-ray and cathodoluminescence topographic studies show that re-entrant cubes experienced mixed-habit growth with normal, faceted octahedral growth being confined to rather narrow sectors between which are regions of hummocky, non-crystallographic growth of mean orientation {100}, known as cuboid growth. Cuboid growth sectors contain regions with cloud-like light-scattering defects. Microscopic infrared absorption measurements and microscopic photoluminescence measurements, respectively, have shown that the occurence of the cloud-like defects correlates with the strength of the 3107 cm -1 absorption line and with a yellowish green luminescence band with a peak at about 525nm. There seems to be no correlation with the total nitrogen content as measured by the absorption coefficient at 1282 cm -1. The photoluminescence spectra measured at 77 K reveal sharp lines due to the S1 centre, which has zero-phonon lines at 503.4 nm (2.643 eV) and 510.7 nm (2.429 eV), and the S3 centre, which has a zero-phonon line at 496.7 nm (2.495 eV). Topographic evidence shows that rounded cubes are composed of normal, single-habit, octahedral growth surrounded by an overgrowth of cuboid form with narrow octahedral sectors producing the protuberances at the cube corners. Microscopic infrared measurements in the one-phonon region show that in the cuboid overgrowth the nitrogen is practically exclusively in the A form whereas in the octahedral core there is a significant amount of nitrogen in the B form, together with a platelet peak. These results suggest that rounded cubes underwent two distinct phases of growth separated, presumably, by a large time interval.

Stacking fault formation due to the presence of liquid inclusions in barium nitrate crystals

Liquid inclusions on the cubic and octahedral faces of barium nitrate crystals show different formation mechanisms. Using X-ray diffraction topographs, we have established that the defects which appear on the inclusions in octahedral sectors are of two kinds: dislocations having large size Burgers vectors formed by the association of several coplanar 〈110〉 vectors and stacking faults created by the glide of a {111} plane, when the macrosteps which border the inclusion are parallel to the three other {111} planes. The nature of the glide vectors have been analysed and interpreted in terms of the atomic structure. The images on the section and traverse topographs have been compared.

Quantitative lattice parameter mapping in Sr(NO 3) 2 and Ba(NO 3) 2 crystals

A plane wave imaging technique has been applied to the characterization of growth defects in Ba(NO 3) 2 and Sr(NO 3) 2 crystals. Precise values of lattice parameter differences between cubic and octahedral sectors have been obtained in the case of Sr(NO 3) 2. For Ba(NO 3) 2, plane wave observation confirms the crystalline perfection revealed by Lang topography.