Fossil coal samples from various deposits were studied using the Raman scattering. An unusual Raman spectrum was discovered for coal graphite from the Seregen deposit (Taimyr). The spectrum consists of intense narrow bands, usually characteristic for single crystals, unlike the standard spectra for fossil coals—broad D and G bands and a weak second-order Raman spectrum. Research has shown that there is no direct analogy between the spectra of the Taimyr sample of coal graphite and the Raman spectra of other allotropic forms of carbon. Based on the experiments we posit that there is a possible new allotropic form of carbon.

As shown in previous chapters, nanodiamonds can form in the stability field of graphite in the presence of organic matter and water. Such conditions characterize different shallow-seated geological contexts, including oceanic serpentinite systems. In fact, nanodiamond clusters were found in some serpentinite xenoliths from Sicily. The discovery was made by HRTEM electron microscopy observations coupled with Raman spectroscopy analyzes. NDs formed in the temperature interval of 150–350 °C and at pressures lower than 2 kbar from an organic-water system. As indicated by an increasing set of literature reports, microdiamonds occur in various ophiolite complexes around the world. Although the origin in deep oceanic mantle is the most popular hypothesis for ophiolitic microdiamonds, it is also possible that they were formed in the stability field of graphite on the seeds of nanodiamonds carried out in supercritical hydrothermal fluids circulating in oceanic serpentinite systems, particularly during hydrodynamic cavitation episodes. It is also highlighted that nanodiamonds in oceanic serpentinite systems possibly stimulated the assemblage of complex bioorganic molecules relevant to the emergence of life on early Earth. Nano- and micron-sized diamonds were also found in garnet peridotite xenoliths from the Salt Lake Crater (Hawaii). Although a deep mantle origin for the Hawaiian diamonds is generally indicated, here, it is suggested that they precipitated metastably under low-pressure conditions from reduced C-O-H fluids during the interaction between the upwelling basaltic magma and a serpentinized section of the oceanic crust. Accordingly, the enigmatic microdiamond aggregates worldwide known as “carbonados” possibly formed during the final stage of serpentinization processes in the primary oceans of Earth from an organic-water system. Although the origin of nonkimberlitic continental microdiamonds may be related to buried serpentinite bodies cross-cut by upwelling magma, this hypothesis cannot yet be proven due to the lack of relevant data.

The model of nanosized diamond particle formation at metastable P‒T parameters from a C–H–O fluid system is presented. This explains the hydrothermal formation and growth of diamond and the specifics of CVD diamond synthesis gas mixtures at low P‒T parameters. The present model makes the simplest possible assumptions about the key processes and is then able to account for various tendencies seen in experimental and natural data. The role of carbon isotopes and nitrogen in diamond nucleus formation is also discussed. The determined relations may help to develop new models needed for diamond formation and deposition.

The results of experiments at low P‒T parameters on nanosized diamond, diamond-like phases, and fullerene-like structure formation from organics are presented. The products of experiments at 500 °C and 1 kbar include diamond, carbine, cubic carbon, and lonsdaleite particles 70–80 nm in size, as indicated by Raman observations and TEM analyzes. Fullerene and fullerene-like phases were synthesized at 700–750 °C and 5 kbar, resembling onion-like structures in wood charcoals carbonatized at 700 °C. The experimental data prove that nano- and microdiamonds can originate from fluids without crystal seeds at P‒T parameters corresponding to graphite stability, according to the theoretical assumption provided in Chap. 2 . Experiments also show that fullerenes, carbon nanotubes, and onion-like carbon structures can be formed from reduced hydrocarbon fluids at temperatures lower than 1000 °C.

Samples of anthraxolites, graphite coals from Taimyr, shungites from Karelia, and anthracites from Donbass and Kuzbass were studied using Raman spectroscopy. In summary, peaks at 1311–1326 cm–1 corresponding to nanodiamonds were recorded in all studied samples. This shows that nanocrystalline diamonds can form both in the late stages of lithogenesis (catagenesis and metagenesis) and during contact metamorphism. The experimental results reported in Chap. 3 show that rare onion-like carbon structures from Karelian shungites could be formed from hydrocarbon gases under pyrometamorphic conditions. Mineralogical, geological, and geochemical data on the Kokchetav massive allow us to conclude that Kokchetav microdiamonds were formed during crustal metamorphic processes from hydrothermal fluids bearing hydrocarbon and nitrogen at P‒T parameters corresponding to graphite stability and that their UHP origin is questionable.

This book explains the properties and formation mechanisms of various types of diamonds, especially nanodiamond

The large body of published mineralogical and experimental data and P‒T- $$f_{{O_{2} }}$$ estimates were used to establish the processes of mantle and kimberlitic diamond formation. As a result, three main stages of diamond formation should be highlighted: 1. Asthenospheric melt-driven; 2. Lithospheric fluid-driven; 3. Shallow mantle or crustal hydrothermal-driven. Diamond formation in the upper mantle is generally related to aqueous fluids at pressures and temperatures corresponding to “cold” geotherms. Macrodiamonds can have multiple origins. Extralarge Type IIa diamonds can originate in a P‒T range from lower mantle to crustal depths, and their main build-up takes place from fluids during the solidification of kimberlite magma at crustal depths.

On nowadays multiphase and the facies heterogeneity of the formations are distinguished at the study of kimberlite pipes. Most researchers associate the formation of diamonds only with the mantle source. To date, satellite minerals with specific compositions associated with kimberlite diamonds have been identified as deep mantle diamond association. They are extracted from the concentrate of the kimberlites heavy fraction and may reflect the diamond grade of the pipe. For some minerals in the diamond association, however, they can not be reliable. Some researchers also revealed shallow diamond associations, related to the formation of serpentine, calcite, apatite, and phlogopite. There is recent data on the formation of diamonds in rocks of the oceanic crust. In the last years microdiamonds were identified in chromites of the oceanic crust in association with antigorite formed at 350-650 °C and 0.1-1.6 GPa. As a result, the authors established a postmagmatic kimberlitic stage of diamond formation associated with secondary mineral associations based on the experimental and mineralogical data for the conditions of the shallow upper mantle and crust. Mineralogical and petrographic studies of Angolan kimberlite pipe show that antigorite is the indicator mineral of this stage.

При изучении кимберлитовых тел выделяется многофазность кимберлитового вулканизма и фациальная неоднородность образований, слагаемых кимберлитовые трубки. Большинство исследователей связывают образование алмазов только с мантией. На сегодняшний день выделены минералы-спутники, ассоциирующие с кимберлитовыми алмазами, обладающие специфическими составами, объединенные в глубинную мантийную «алмазную ассоциацию». Они извлекаются из концентрата тяжелой фракции кимберлитов и могут отражать степень их потенциальной алмазоносности. При этом для некоторых минералов алмазной ассоциации они не всегда достоверны. Рядом исследователей кроме глубинных алмазных ассоциаций выделялась и малоглубинная, связанная с образованием серпентина, кальцита, апатита и флогопита из собственно кимберлита. Недавно появились данные об образовании алмазов в породах океанической коры. Установлены находки микроалмазов в хромитах в равновесии с антигоритом, образовавшемся при температурах 350-650 °C и давлениях 0,1-1,6 ГПа. Авторами на основе имеющихся экспериментальных и минералогических данных в кимберлитах выделена постмагматическая стадия формирования алмазов для условий малоглубинной верхней мантии и земной коры, связанная со вторичным минеральным образованием. Проведенные минералого-петрографические исследования кимберлитовой трубки в Анголе свидетельствуют, что индикационным минералом такой стадии в кимберлитовых породах является антигорит.

Rock fragments from the deepest parts of a buried hydrothermal system belonging to the Mesozoic Tethys Ocean entered as xenoliths in a Miocenic diatreme, hence brought to the surface, in the Hyblean Plateau (Sicily). Some xenoliths consist of strongly serpentinized ultramafic rocks bearing blebs of abiotic organic matter, where clusters of amorphous carbon nanoparticles, including nanodiamonds, are immersed. Such an occurrence conjures up established hypotheses that diamond surfaces are suitable catalytic platforms stimulating the assemblage of complex bio-organic molecules relevant to the emergence of life on Earth. The appearance of bio-organic molecules under primitive Earth conditions is one of the major unsolved questions on the origin of life. Here we report new micro-Raman spectra on blebs of abiotic organic matter from a selected xenolith. Diamond bands were related to hydrogenated nanocrystalline diamonds, with size of nearly 1–1.6 ​nm, formed from organics at low pressures and temperatures. In particular, diamond surfaces can give rise to crystalline interfacial water layers that may have played a fundamental role in the early biosphere evolution as a good medium for rapidly transporting positive charges in the form of hydrated protons. Nowadays, proton gradients in alkaline hydrothermal vents along oceanic ridges are generally viewed as key pre-biotic factors. In general, serpentinites span the entire geological record, including prebiotic times. These hydrous ultramafic rocks often display evidence of abiotic carbon species, both organic and inorganic, including nanodiamonds, being also capable to give rise to chemiosmotic processes and proton gradients necessary to the organisms, such as the “Last Universal Common Ancestor” (LUCA), in the prebiotic Earth.

Pyrite and organic matter closely coexist in some hydrothermally-altered gabbroic xenoliths from the Hyblean Plateau, Sicily. The representative sample consists of plagioclase, Fe-oxides, clinopyroxene, pyrite and minor amounts of many other minerals. Plagioclase displays incipient albitization, clinopyroxene is deeply corroded. Pyrite grains are widely replaced by spongy-textured magnetite, which locally hosts Ca-(and Fe-)sulfate micrograins and blebs of condensed organic matter. Whole-rock trace element distribution evidences that incompatible elements, particularly the fluid-mobile Ba, U and Pb, are significantly enriched with respect to N-MORB values. The mineralogical and geochemical characteristics of the sample, and its U-Pb zircon age of 216.9 ± 6.7 MA, conform to the xenolith-based viewpoint that the unexposed Hyblean basement is a relict of the Ionian Tethys lithospheric domain, mostly consisting of abyssal-type serpentinized peridotites with small gabbroic intrusions. Circulating hydrothermal fluids there favored the formation of hydrocarbons trough Fischer-Tropsch-type organic synthesis, giving also rise to sulfidization episodes. Subsequent variations in temperature and redox conditions of the system induced partial de-sulfidization, Fe-oxides precipitation and sulfate-forming reactions, also promoting poly-condensation and aromatization of the already-formed hydrocarbons. Here we show organic matter adhering to a crystal face of a microscopic pyrite grain. Pyrite surfaces, as abiotic analogues of enzymes, can adsorb and concentrate organic molecules, also acting as catalysts for a broad range of proto-biochemical reactions. The present data therefore may support established abiogenesis models suggesting that pyrite surfaces carried out primitive metabolic cycles in suitable environments of the early Earth, such as endolithic recesses in mafic rocks permeated by hydrothermal fluids.

Samples of gilsonite from Adzharia, anthraxolite and graphite of coal from Taimyr, shungite from Karelia, and anthracite from Donbass are studied using Raman spectroscopy. Peaks at 1600 cm−1, indicating the presence of nanographite, are recorded in all samples. The anthracite sample from Donbass, 1330 cm−1, corresponds to the sp3-line of carbon hybridization conforming to a nanodiamond. It is concluded that in nature diamonds can be formed at late stages of lithogenesis (catagensis, metagenesis), and for coals, it can occur at the zeolite stage of regional metamorphism of rocks, before the green schist stage.

It is known that the Р – Т parameters of diamond-bearing kimberlite xenoliths correspond to subductive paleogeotherms lying between the 36 and 41 mW/m 2 conductive models. There are some studies showing the correlation of diamond ability with oxygen fugacity and the fluid composition of mantle xenoliths. The most diamondiferous samples correspond to the water compositions of the calculated O–H–C fluid with a minimum atomic carbon content in it. From the calculations it follows that the fluid carbon atomic content increases with a temperature increase and with the pressure decreasing. The most minor C contents have the 35 mW/m 2 conductive model in comparison with the 40 and 45 mW/m 2 models. As a result, it is possible to conclude that the low temperature fields (less than 1100°C) of the “cold” geotherms have the highest diamondiferous ability.

The petrological model of type IIa diamond formation is proposed. By the model these diamonds were formed from fluids on the last stages of kimberlite magma formation at 600–700°C and in the range of 20–30 kbar. The model explains the main specific of the type IIa diamonds. They are the enrichment of light carbon, low nitrogen content, the absence of silicate inclusions, the large size and cleanliness, a high degree of desorption.

The processes of formation of some diamond types still raise contentious issues, mainly on the origin of the largest diamond crystals recovered from kimberlites. These diamonds constitute less than 2% of worldwide resources and correspond to rare type IIa. They possess some peculiar features: (i) silicate and oxide inclusions are extremely rare, (ii) their delta C-13 ranges from -17 to -21 parts per thousand. The detailed estimation of the Premier pressure temperature-oxygen fugacity parameters and the physic-chemical modeling of diamond growth-dissolution processes suggest that extra-large diamonds have multiple origins. Their formation may occur from lower mantle to crustal depths. Their main building-up takes place from fluids in the pegmatitic veins solidified along the contacts of kimberlite magma at a crustal depth. The model explains the main features of the largest kimberlitic diamonds, i.e. their great sizes, light delta C-13 signatures, low nitrogen contents, high degree of resorption, absence of mantle-derived mineral inclusions and their occurrence in the form of rare isolated crystals in the host kimberlite.

There are four main types of natural diamonds and related formation processes. The first type comprises the interstellar nanodiamond particles. The second group includes crustal nano- and micron-scale diamonds associated with coals, sediments and metamorphic rocks. The third one includes nanodiamonds and microndiamonds associated with secondary alteration and replacing of mafic and ultramafic rocks. The fourth one includes macro-, micron-and nano-sized mantle diamonds which are associated with kimberlites, mantle peridotites and eclogites. Each diamond type has its specific characteristics. Nano-sized diamond particles of lowest nanometers in size crystallize from abiotic organic matter at lower pressures and temperatures in space during the stages of protoplanetary disk formation. Nano-sized diamonds are formed from organic matter at P-T exceeding conditions of catagenesis stage of lithogenesis. Micron-sized diamonds are formed from fluids at P-T exceeding supercritical water stability. Macrosized diamonds are formed from metal-carbon and silicate-carbonate melts and fluids at P-T exceeding 1150 degrees C and 4.5 GPa. Nitrogen and hydrocarbons play an important role in diamond formation. Their role in the formation processes increases from macro-sized to nano-sized diamond particles. Introduction of nitrogen atoms into the diamond structure leads to the stabilization of micron-and nano-sized diamonds in the field of graphite stability. (C) 2017, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license.