Morphological Features of Diamond Crystals Dissolved in Fe<Subscript>0.7</Subscript>S<Subscript>0.3</Subscript> Melt at 4 GPa and 1400°C

An experimental study of the dissolution of natural and synthetic diamonds in a sulfur-bearing iron melt (Fe_0.7S_0.3) with high P–T parameters (4 GPa, 1400°С) was performed. The results demonstrated that under these conditions, octahedral crystals with flat faces and rounded tetrahexahedral diamond crystals are transformed into rounded octahedroids, which have morphological characteristics similar to those of natural diamonds from kimberlite. It was suggested that, taking into account the complex history of individual natural diamond crystals, including the dissolution stages, sulfur-bearing metal melts up to sulfide melts were not only diamond-forming media during the early evolution of the Earth, but also natural solvents of diamond in the mantle environment before the formation of kimberlitic melts.     

Diamond formation in sulfide pyrrhotite-carbon melts: Experiments at 6.0–7.1 GPa and application to natural conditions

Experiments at 6.0–7.1 GPa and 1500–1700°C were carried out to explore the boundary conditions of diamond nucleation and growth in pyrrhotite-carbon melt-solutions. Pyrrhotite is one of the main sulfide minerals of the pyrrhotite-pentlandite-chalcopyrite assemblage of mantle rocks and primary inclusions in diamond. Solutions of carbon in sulfide melts oversaturated with respect to diamond at the expense of the dissolution of starting graphite (thermodynamically unstable phase) are formed owing to the difference between the solubilities of graphite and diamond, which increases under the influence of temperature gradients in experimental samples. We determined the fields of carbon solutions in pyrrhotite melt showing labile and metastable oversaturation with respect to diamond, which correspond to the spontaneous nucleation of the diamond phase and diamond growth on seeds, respectively. The linear growth rate of diamond in sulfide-carbon melts is rather high (on average, 10 μ/min during the first 1–2 min from the onset of spontaneous crystallization). The nucleation density is estimated as 180 grains per cubic centimeter. Diamonds crystallized from sulfide melts show octahedral and spinel twin shapes. Diamond polycrystals were synthesized for the first time from a sulfide medium as intergrowths of skeletal (edge) or “cryptocrystalline” microdiamonds, from 1 to 100 μm in size, their spinel twins and, occasionally, polysynthetic (star-shaped) twins. During diamond growth from sulfidecarbon melts on smooth faces of cuboctahedral diamond seeds synthesized in metal systems, smooth-faced layer-by-layer step-like growth was observed on their octahedral (111) faces, whereas growth on the (100) cubic faces produced rough-surfaced layers of intergrown micropyramids, whose axes were oriented normal to the (100) face. The obtained experimental results were applied to the problem of diamond genesis under the conditions of the Earth’s mantle in the framework of the model of carbonate-silicate parental melts with blebs of immiscible sulfide melts.     

Formation of epigenetic graphite inclusions in diamond crystals: experimental data

To elucidate the conditions of formation of epigenetic graphite inclusions in natural diamond, we carried out experiments on high-temperature treatment of natural and synthetic diamond crystals containing microinclusions. The crystal annealing was performed in the CO–CO₂ atmosphere at 700–1100 °C and ambient pressure for 15 min to 4 h. The starting and annealed diamond crystals were examined by optical microscopy and Raman spectroscopy. It has been established that the microinclusions begin to change at 900 °C. A temperature increase to 1000 °C induces microcracks around the microinclusions and strong stress in the diamond matrix. The microinclusions turn black and opaque as a result of the formation of amorphous carbon at the diamond–inclusion interface. At 1100 °C, ordered graphite in the form of hexagonal and rounded plates is produced in the microcracks. A hypothesis is put forward that graphitization in natural diamond proceeds by the catalytic mechanism, whereas in synthetic diamond it is the result of pyrolysis of microinclusion hydrocarbons. The obtained data on the genesis of graphite microinclusions in diamond are used to evaluate the temperature of kimberlitic melt at the final stage of formation of diamond deposits.     

Distribution of structural impurities and fluid microinclusions in cubic and coated diamond crystals from the Udachnaya pipe,Yakutia, Russia

FTIR microspectroscopic data were used to construct two-dimension maps showing the distribution of structural impurities and mineral microinclusions in cubic and coated octahedral diamond crystals from the Udachnaya kimberlite pipe in Yakutia. Elevated concentrations of hydrogen and total nitrogen are detected in parts corresponding to the early growth of single-episode growth regions of diamond crystals. These concentrations decrease toward the peripheral portions of these regions. The microinclusions contain water and polyphase mineral associations that preserve a high residual pressure. Microinclusions in the coats of octahedral diamond crystals are dominated by silicates, in which the intensity of IR spectral bands increases toward the peripheries, whereas the cubes posses irregularly distributed domains rich in these phases. The carbonate phases of the microinclusions are distributed according to growth zones of the crystals, and their distribution is often not correlated with the concentrations of structural impurities. The facts that microinclusions in the diamond cuboids are dominated by carbonates and that the rims of the octahedra are dominated by silicates suggest that the diamonds crystallized from dominantly carbonate and silicate fluids/ melts, respectively. The chemical composition of the microinclusions point to an eclogitic paragenesis of the crystals. Facts are obtained that provide support for the earlier hypothesis that cubic diamond crystals and coated octahedral crystals grow at metasomatic interaction between deep fluids and eclogitic rocks in the lithospheric mantle.     

Fe–S melt as a likely solvent of diamond under mantle conditions

The first results of experimental study of diamond dissolution in a S-bearing Fe melt at high P–T parameters are reported and the morphology of partially dissolved crystals is compared with that of natural diamonds. Our results show that under the experimental conditions (4 GPa, 1400°C), flat-faced octahedral diamond crystals are transformed into curve-faced octahedroids with morphological features similar to those of natural diamonds.     

浅海微孔泥晶碳酸盐岩储层研究进展

微孔泥晶灰岩储层是一种重要的油气储层类型,尤其是对于中东地区白垩系碳酸盐岩储层,这明显不同于典型碳酸盐岩储层—古风化壳型岩溶储层、礁滩相储层及层状白云岩储层等三大类型。泥晶基质具有致密镶嵌结构和白垩质结构两种结构类型。泥晶晶体具有微菱形、圆形和它形三种晶体形态,以圆形泥晶储层物性最好,微菱形泥晶储层其次,它形泥晶为致密储层。微孔（直径小于10 μm）是泥晶基质中孔隙的主体,除此还发育铸模孔、海绵状基质溶孔和溶蚀沟道等。微孔的形成主要受原始矿物组分和成岩条件的共同制约:低镁方解石灰泥是微孔泥晶灰岩形成的先决原始矿物条件,相对矿物学稳定性可促进原始组构包括原生晶间微孔的保存。特殊的成岩环境是微孔泥晶灰岩发育的必要改造条件,泥晶灰岩中孔隙的形成是早期浅层埋藏下大气淡水淋滤及埋藏成岩期间有机酸溶蚀作用等两期溶蚀叠加的结果。在浅埋藏淡水透镜体内,方解石次生加大（奥斯特瓦尔德成熟过程）形成了沉积物早期胶结,防止压实的同时部分保留了原始结构和晶间微孔网络,还通过消除小晶体提高了渗透性;淡水选择性淋滤也形成了较广泛发育的铸模孔。在埋藏成岩过程中,有机酸性流体溶蚀作用形成海绵状基质溶孔及溶蚀沟道,也导致了圆形泥晶晶体的形成。     

Some specific features of genesis of microdiamonds of octahedral and cubic habit from kimberlites of the Udachnaya pipe (Yakutia) inferred from carbon isotopes and main impurity defects

Microdiamonds (crystals smaller than 1 mm) of octahedral and cubic habit from Udachnaya kimberlite pipe (Yakutia) have been compared in order to distinguish genetic features inferred from carbon isotopic composition and impurity defects. Microdiamonds of cubic habit from the Udachnaya kimberlite pipe have a fibrous internal structure and a high content of nitrogen impurity (400–3000 ppm). Octahedral microdiamonds from the same deposit are distinguished by a low nitrogen content of 0 to 500 ppm and zoning structure. The isotopic composition of carbon (δ¹³C is –4.7‰ for octahedra and –4.5‰ for cuboids) suggests a common source of carbon for these morphologic groups. The studied characteristics can be due to crystallization of octahedra from carbon dissolved in the melt, and cuboids, under the conditions of the hampered diffusion of carbon.     

Morphological Features of Diamond Crystals Resulting from Dissolution in a Fe–Ni–S Melt under High Pressure

Doklady Earth Sciences - The primary results are presented on the dissolution of plane-faced diamond crystals of octahedral habit in a Fe–Ni–S melt under 3.5 GPa and 1400°C. It was...     

Growth of diamond in hydrous silicate melts

This study demonstrates that a hydrous, halide bearing silicate melt is a viable medium for diamond growth. Experiments were conducted in the MgO–SiO₂–H₂O–C ± KCl ± NaCl system, which was used as a model for harzburgitic mantle. In no case did we observe crystals that could be interpreted as spontaneously nucleated, but growth of diamond on seed crystals at 1,400–1,600°C and 7 GPa in experiments of 4 h duration was observed. The addition of KCl to the system produced crystallization of diamond at temperatures as low as 1,400°C. At higher temperatures, larger growth features were produced than those that seen in the KCl-free system at the same conditions. The NaCl-bearing system is different; in these experiments, the diamond seed crystals show evidence of possible dissolution and layer growth, albeit more subdued growth than in the KCl system. Therefore, NaCl may be an inhibitor of diamond growth in a hydrous silicate melt. Based on these results, hydrous silicate melts could play a role in formation of diamond in either deep subduction zones, or above slabs imbricated against a lithospheric ‘root’ in the sub-continental lithospheric mantle. The water and halide necessary for their formation could be transported into the mantle in hydrous phases such as serpentine in subducting lithospheric slabs. Dehydration of serpentine at >200 km depth would release hydrous, halide-bearing fluids into the overlying mantle wedge or lithospheric root, triggering melting at conditions similar to those of the formation of natural diamond.     

Effect of oxygen fugacity on the etching rate of diamond crystals in silicate melt

Experimental data on the etching of diamond crystals in basaltic melt at 1130°C with variable oxygen fugacity in the environment are considered. The oxygen fugacity was set with the HM and NNO buffers. The study was carried out on a 0.6–0.8 mm fraction (powder) of natural diamond crystals. It has been established that, at the same temperature, the rate of diamond etching (oxidation) in silicate melt depends on the oxygen fugacity in the environment. The etching rate decreases with decline in the oxygen fugacity from the case where the melt comes into contact with atmospheric air to the conditions controlled by the HM and NNO buffers. Under the conditions of the HM and NNO buffers, oxidation was accompanied by surface graphitization of diamond crystals.     

Spectroscopic constraints on growth of Siberian mixed-habit diamonds

Notable within-crystal variability of mineralogical and geochemical properties of single natural diamonds are commonly attributed to changing chemistry of parental fluids, sources of carbon and redox conditions of diamond precipitation. A distinct type of compositional heterogeneity (mixed-habit structure) is well-known to occur in diamonds as well as in many other minerals due to purely “structural” reasons that are unequal crystal chemistry of crystallographically different faces and selective absorption and fractionation of impurities between adjacent growth pyramids. Based on the combined cathodoluminescence, Fourier-transformed infrared spectroscopy and photoluminescence spectroscopy, study of nine diamond crystals with different growth histories and external morphology, but all showing mixed-habit patterns at different growth stages, we show that mixed-diamonds may grow in closed system conditions or with a slowly decreasing growth rate from a media with a much lower impurity content than previously thought. Intracrystal nitrogen distribution seems to be a function of growth rate even in the cases of unusual impurity partitioning between growth sectors. Generally poor with IR-active hydrogen at moderate nitrogen aggregation parameters, studied diamonds likely resemble the low hydrogen content from the growth medium that, for cubic diamonds, was typically suggested hydrogen-rich and a crucial factor for growth of cubic and mixed-habit diamonds. We also show that mixed-habit diamond growth may occur not only in peridotitic suite but also in an extended field of geochemical affinities from high-Ni to low-Ni or maybe even Ni-free environments, such as pyroxenitic or eclogitic.     

On the kinematics of zircon growth and its petrogenetic significance: a cathodoluminescence study

The relative growth rates of zircon crystal faces are recorded by growth zoning, which is recognizable in cathodoluminescence photographs of oriented crystal sections. The kinematics of zircon growth is graphically presented by the slopes of Grabahnen between crystal sectors. The relative velocities of advance of crystal faces correlate to the pattern of growth zoning. Widely spaced zoning, interrupted by surfaces of dissolution, on the one hand, and narrowly spaced uninterrupted oscillatory zoning, on the other hand, are interpreted as markers of low and high zircon-supersaturation of the melt, respectively. The following model is deduced from this correlation. The prism {110} is that crystal form, the growth rate of which reacts most sensitively to the zircon-supersaturation of the melt. The growthrate of the steep pyramid {211} is not primarily controlled by zircon-supersaturation. Instead, the growth of {211} is delayed by the adsorption of foreign atoms on its faces. The kinematics of zircon growth in anatectic, plutonic and meta-rhyolitic host rocks in the pre-Mesozoic basement of the Tauern Window (Eastern Alps) is explained by this model. Results show that the kinematics of zircon growth carry a much greater petrogenetic significance than has been previously suggested for the final shape of crystals alone.     

山东郯城砂矿金刚石特征及包裹体研究

通过对115粒山东郯城砂矿金刚石样品进行矿物学和光谱学特征研究,结果显示郯城金刚石的粒径集中在1. 0~4. 0mm之间,晶体形态以菱形十二面体为主,其次八面体与菱形十二面体聚形,八面体较少;晶面形貌除倒三角凹坑、塑性变形滑移线、熔蚀沟、生长丘、生长阶梯、叠瓦状蚀象、滴状丘、晕线等原生形貌发育外,小部分发育有次生形貌 绿色色斑,且大多数金刚石的边棱清晰,磨圆程度不高。研究首次测得了郯城金刚石的拉曼特征峰的半高宽数据和金刚石包裹体拉曼谱图,显示郯城砂矿金刚石结晶程度差异较大,暗示其形成的金刚石地质生长条件和环境的复杂性;金刚石包裹体有橄榄石、黄铜矿、针铁矿、石墨矿物,其中橄榄石包裹体占比较高,表明郯城金刚石包裹体类型以橄榄岩型为主,测试结果与华北东部古老克拉通之下的岩石圈地幔大部分由橄榄岩组成的结论一致。对比郯城金刚石与蒙阴金刚石特征的异同,初步探讨了金刚石砂矿的物质来源,为揭示郯城砂矿金刚石的形成及演化提供了金刚石及其包裹体的新的证据。     

The relationship between the distribution of nitrogen impurity centres in diamond crystals and their internal structure and mechanism of growth

Fifty diamond crystals of different morphological types (octahedra, dodecahedroids, cubes and single tetrahexahedroid) with differing internal structures were examined using methods of cathodoluminescence (CL), anomalous birefringence and local infrared (IR) analysis. The main objective of the study was to examine the regularities of nitrogen impurity distribution in diamond with differing internal structures. Almost all the analyzed octahedra, as well as dodecahedroids with zonal structures and the blocky dodecahedroids, are characterized either by nearly isothermic growth conditions or by a decrease in formation temperature during the crystallization process. In contrast to zoned octahedra and dodecahedroids, dodecahedroids with zonal–sectorial and sectorial internal structures show a notably different distribution of nitrogen defects, with N_tot generally decreasing from crystal cores to marginal areas, and degree of nitrogen aggregation increasing in the same direction. From this, it would follow that in these crystals, the temperature of diamond formation of the outer crystal zones is approximately 40–50 °C higher than that of the inner zones. The same result (15 to 80 °C) was obtained for diamond crystals with cubic habit, which generally show a fibrous internal structure, reflecting normal mechanisms of growth. The anomalous distribution of nitrogen centres in diamond crystals that grew through the normal mechanism, with a high rate of growth and in an oversaturated medium, might point to non-equilibrium relationships between the concentrations of different nitrogen centres. It is likely that in crystals of this type, the rate of growth is higher than the rate of structural nitrogen aggregation. Thus, it appears that in these peculiar crystals of diamond we deal with non-equilibrium concentrations of nitrogen B centres and, consequently, with anomalous, non-actual diamond formation temperatures.     

Cambrian skeletal halite crystals and experimental analogues

A suite of unusual highly skeletal halite pseudomorphs is described from the lower Middle Cambrian rocks that crop out at Ardmore in the Georgina Basin, northern Australia. The pseudomorphs are preserved as both moulds and casts within a dark and light colour-banded chert. They are orientated parallel to bedding and represent halite crystal growth within a brine pool. Laterally equivalent rocks are characterized by sedimentary textures and structures indicative of periodic emergence and desiccation. Laboratory grown halite crystals are also described and compared with the pseudomorphs from Ardmore. Experimentally produced halite crystals formed from either: (1) brine solutions obtained as residues after organic matter extraction from phosphate rocks, or (2) solutions of NaCI and distilled water. Both pyramidal rafted hoppers and floor nucleated cuboids formed in solutions containing only NaCI and distilled water; whereas in the residue solutions, that contained humic acids, rafted pyramidal forms were absent and crystal nucleation was restricted to the floor of the evaporative dish. As brine depth decreased the halite precipitation rate increased and resulted in a suite of excrescent, highly skeletal crystals that formed as a result of brine evaporation to dryness. The variable crystal morphology depended upon both the brine depth and slope of the evaporative dish floor. Horizontally orientated chevron halite crystals formed where the evaporative dish was inclined and precipitation preferentially occurred on cube faces. Pagoda, reticulate ridge and dendritic forms represent an increasingly skeletal crystal suite characterized by the preferential precipitation of NaCI on cube edges and corners rather than faces. Using the experimentally grown crystals as analogues the pseudomorphs at Ardmore are interpreted as forming in very shallow brine pools that evaporated to dryness.     

Chromite growth,dissolution and deformation from a morphological view point: SEM investigations

The SEM investigations on some chromite crystals show significant morphological differences between cumulate chromites (from stratiform complexes and from ophiolites) and chromites from podiform deposits (ophiolites). The later exhibit a rounded habit and abundant pits which are both the result of dissolution processes. Accordingly, the interpretation of the silicate inclusions within these chromites and the interpretration of the trace-elements distribution need great care. The nodules, a typical structure of the podiform chromites, are explained by a magmatic growth followed by a progressive recrystallization. The accessory chromites which define the mineral lineation in the tectonite peridotites from ophiolites have been deformed by processes of intracrystalline gliding, stretching and dissolution. In the harzburgites, the orthopyroxene has progressively exsolved at sub-solidus (symplektites) vermicular to massive chromite displaying euhedral forms. In the dunitic lenses, the idiomorphic chromites which are aligned parallel to the lineation may have also recrystallized in a solid state.     

The first finding of graphite inclusion in diamond from mantle rocks: The result of the study of eclogite xenolith from Udachnaya pipe (Siberian craton)

A xenolith of eclogite from the kimberlite pipe Udachnaya–East, Yakutia Grt+Cpx+Ky + S + Coe/Qtz + Dia + Gr has been studied. Graphite inclusions in diamond have been studied in detail by Confocal Raman (CR) mapping. The graphite inclusion in diamond has a highly ordered structure and is characterized by a substantial shift in the band (about 1580 cm^–1) by 7 cm^–1, indicating a significant residual strain in the inclusion. According to the results of FTIR spectroscopic studies of diamond crystals, a high degree of nitrogen aggregation has been detected: it is present mainly in form A, which means an “ancient” age of the diamonds. In the xenolith studied, the diamond formation occurred about 1 Byr, long before their transport by the kimberlite melt, and the conditions of the final equilibrium were temperatures of 1020 ± 40°C at 4.7 GPa. Thus, these graphite inclusions found in a diamond are the first evidence of crystallization of metastable graphite in a diamond stability field. They were formed in rocks of the upper mantle significantly below (≥20 km) the graphite-diamond equilibrium line.     

Formation of metastable graphite inclusions during diamond crystallization in model systems

Metastable graphite inclusions have been studied in diamond, forsterite, and orthopyroxene synthesized in silicate-carbonate-fluid and aqueous chloride systems at 6.3–7.5 GPa and 1400–1600°C. The graphite inclusions were studied using optic microscopy and Raman spectroscopy. It has been established that graphite in diamond and liquidus silicate minerals is represented by a highly ordered variety. Depending on parameters of runs, the graphite inclusions are hexagonal, irregular polygonal, or rounded in shape. The morphology of graphite inclusions involving metastable graphite in run products is compared with previously established crystallization sequence of carbon phases. It has been revealed that the protogenetic graphite inclusions in diamond are rounded, and this shape was caused by dissolution of the newly formed graphite. Polygonal graphite inclusions are syngenetic and represented by metastable graphite that crystallized contemporaneously with diamond.     

Linear growth rate and sectorial growth dynamics of diamond crystals grown by the temperature-gradient techniques (Fe–Ni–C system)

The paper reports data on the linear growth rates of synthetic diamond single crystals grown at high P–T parameters by the temperature-gradient technique in the Fe–Ni–C system. Techniques of stepwise temperature changes and generation of growth microzoning were applied to evaluate the growth rates of various octahedral and cubic growth sectors and variations in these rates with growth time. The maximum linear growth rates of the order of 100–300 µm/h were detected at the initial activation of crystal growth, after which the growth rates nonlinearly decreased throughout the whole growth time to 5–20 µm/h. The fact that the linear growth rates can broadly vary indicates that the inner structure and growth dynamics of single diamond crystals grown by the temperature-gradient technique should be taken into account when applied in mineral–geochemical studies (capture of inclusions, accommodation of admixture components, changes of the defective structure, etc.).     

Chromatographic study of formation conditions of rhombododecahedral diamond crystals

The results of chromatographic study of the formation of rhombododecahedral diamonds synthesized in the Fe-Ni-(Ti)-C system at 5.5–6.0 GPa and 1350–1450°C are presented, including crystals with rounded surfaces of the rhombododecahedron with parallel striation, which are morphological analogues of natural diamonds abundant at various kimberlite, lamproite, and placer deposits. Chromatography was performed at 150°C with mechanical breakup of diamonds. The stable release of methane when diamonds of habit {110} are crushed is established. It is concluded that the appearance of the habit rhombododecahedron may be related not only to the effect of temperature and pressure on crystal growth but also to reductive conditions of crystallization. At the same time, the appearance of significant amounts of hydrocarbons in the system probably results in stopping of the growth of faces {110} and {100} and, instead, formation of specific surfaces that are composed of microscopic accessories faced by planes {111}.