Bradleyite, a sodium phosphate-magnesium carbonate, Na3Mg(PO4)(CO3), occurs in sedimentary salt rocks and in igneous, carbonatitic, and kimberlitic rocks. In this paper, we present the characteristics of a bradleyite sample found in a new geological environment as an inclusion in a diamond from the C & oacute;rigo Sorriso placer deposit in Mato Grosso State, Brazil, where other unusual mineral inclusions in diamond were earlier identified. Bradleyite is part of a polymineral inclusion, comprising a porous aggregate of grains <150 nm in size, hosted within a dolomite crystal. The studied bradleyite is characterized by the highest MgO+FeO concentrations and the lowest Na content among bradleyites from other localities. It demonstrates significant variability in composition, particularly Na (28.75-37.84 mass % Na2O). Nitrogen was also detected by EDS analysis. We report for the first time the ab initio crystal structure of natural bradleyite. It has monoclinic symmetry, with cell parameters a = 8.684 & Aring;, b = 6.804 & Aring;, c = 5.074 & Aring;, and beta = 90.34 degrees. The structure was solved ab initio and refined using dynamical scattering theory in space group P2(1)/m, confirming the model obtained from powder XRD analysis of synthetic analogs. The final structure model converged to a formula Na-3(Mg0.86Fe0.14)(PO4)(CO3), Z = 2. Bradleyite is a polygenetic mineral. In continental salt deposits, it forms under atmospheric pressure during sedimentation. In deep-formed igneous rocks, such as kimberlites and carbonatites, bradleyite occurs as a product of late-stage crystallization of carbonatitic melt and as a primary-crystallized phase in deep-seated minerals, such as olivine, ilmenite, chrome spinel, and magnetite. Our findings demonstrate its stability in diamond and diamond-forming environments and that it may be considered a product of crystallization from a primary melt inclusion.

The redox state of the Earth’s interior (i.e., the oxygen fugacity, fo2) is related to the Fe speciation (Fe2+, Fe3+) in mantle rock-forming minerals and controls the speciation of volatiles like carbon at depth. To date, the fo2 of the lower mantle has been mostly constrained by HP-T experiments, due to the extreme rarity of natural samples represented by mineral inclusions in sub-lithospheric diamonds. Experimental evidence suggests that the lower mantle is reduced and saturated in Fe(-Ni) metal (about 1 wt%). However, coexisting minerals like ferropericlase and bridgmanite are predicted to contain 0.02 and 0.6 of Fe3+ /∑Fe, respectively. A slight increase of Fe3+ /∑Fe (less than 1 wt%) is expected in the case of fo2 > iron-wüstite buffer. This would imply the complete oxidation of Fe(Ni) alloys promoted by reduction of carbonates (either fluids or melts). The finding of carbonates trapped in sub-lithospheric diamonds is natural evidence of the (local) oxidative redox state of the deep and inaccessible lower mantle and this is enhanced by the lack of metallic inclusions coexisting with Fe3+-poor ferropericlase in sublithospheric diamonds. Moreover, the variation of Fe3+ in bridgmanite appears, at least currently, to be better explained by its crystal chemistry while the effect of pressure and fo2 remains unclear, mainly due to the lack of oxybarometers applicable to lower mantle assemblages.In this study, we combined an experimental investigation of the Fe3+/∑Fe in ferropericlase and bridgmanite equilibrated at known high pressure, temperature and oxygen fugacity conditions with Fe3+ /∑Fe measurements conducted on bridgmanite(-like) and ferropericlase inclusions in sublithospheric diamonds from Rio Sorriso and São Luís (Brazil) and Kankan (Guinea). Some inclusions are composite for which the Fe3+/∑Fe was determined by in situ synchrotron Mössbauer source spectroscopy and the bulk Fe3+ /∑Fe determined.Our preliminary results show a discrepancy between natural inclusions and experimental products in terms of i) modal abundance of ferropericlase and bridgmanite, likely related to their diverse role in diamond formation (redox) processes; ii) chemical compositions expected for both peridotitic and metabasaltic parageneses; and iii) Fe3+ /∑Fe content.

A set of ten diamonds from different sources representing the main types of physical classification (IaA, IaAB, IaB and IIa) has been first explored by FTIR spectroscopy and SIMS calibrated by hydrogen ion implantation. The concentrations of hydrogen in the studied diamonds measured by SIMS range between 9.98 and 47.6 at.ppm or from 1.76 × 1018 to 8.40 × 1018 at./cm3. The 3107 cm−1 absorption line has been detected by FTIR spectroscopy (resolution near 1 cm−1) in five studied diamonds (IaB and IaAB). The 3107 cm−1 IR absorption lines due to H defects in these diamonds have been successfully described by the Lorentzian shape and their parameters have been evaluated. Based on the linear regression equation relating intensities of the 3107 cm−1 IR absorption lines in the studied diamonds with their H defect concentrations, the H defect IR absorption at 3107 cm−1 in diamond has been quantified. The absorption calibration constant for diamond's 3107 cm−1 absorption line is 386 ± 64 ppb cm2. The corresponding cross-section per one H defect σ3107 (cm2) = (9.34 ± 1.25) × 10−18 /Γ (cm−1), Γ is the full width at half maximum (FWHM) of the 3107 cm−1 absorption line. Based on a widely accepted interpretation that the H defect resulting in the 3107 cm−1 IR absorption peak in diamonds is the N3VH defect, the values of the obtained calibration constant and cross-section have been assigned to the N3VH defect in diamond. The equations relating concentration of the N3VH defect ([N3VH]) with the 3107 cm−1 IR absorption line integrated intensity (I3107) and intensity (α3107) are as follows: [N3VH] (ppb) = (386 ± 64) I3107; [N3VH] (ppm) = (0.607 ± 0.080) α 3107 (cm−1)Γ (cm−1). Hydrogen impurities that do not contribute to the IR absorption at the 3107 cm−1 peak were detected in all studied diamonds.

Diamonds provide unique information on the deep Earth’s mantle through the investigation of entrapped minerals and fluids from which pressure-temperature-oxygen fugacity are determined. In this study, we investigated a diamond from the Rio Sorriso area, Juina (Brazil), a site known for the high abundance of discovered sublithospheric diamonds. The studied diamond contains both colorless and greenish optically visible inclusions of Cr-diopside, high-Mg olivine, and enstatite. Thermobarometric estimates of the polished and entrapped inclusions suggest that the diamond likely formed between 4 and 5 GPa, and in the T range 1050–1150°C; in contrast, major and trace elements data from one polished clinopyroxene provide evidence of interaction between the local peridotite and a Na-rich carbonated melt, the growth medium from which the diamond crystallized. Our study, thus, demonstrates that diamonds from underneath the Amazonian craton did not originate solely at lower mantle depths but also within a metasomatized lithospheric mantle.

Ballas is a rare polycrystalline diamond variety characterized by a radially oriented internal structure and spheroidal outer shape. The origin of natural ballases remains poorly constrained. We present the results of a comprehensive investigation of two classic ballas diamonds from Brazil. External morphology was studied using SEM, high-resolution 3D optical microscopy, and X-ray tomography. Point and extended defects were examined on polished central plates using infra-red, photo- and cathodoluminescence spectroscopies, and electron back-scattering diffraction; information about nanosized inclusions was inferred from Transmission Electron Microscopy. The results suggest that fibrous diamond crystallites comprising ballas are split with pronounced rotation, causing concentric zoning of the samples. Pervasive feather-like luminescing structural features envelop single crystalline domains and most likely represent fibers with non-crystallographic branching. These features are enriched in N3 point defects. Twinning is not common. The nitrogen content of the studied samples reaches 700 at.ppm; its concentration gradually increases from the center to the rim. Annealing of the ballases took place at relatively high temperatures of 1125–1250 °C; the annealing continued even when the samples were fully grown, as suggested by the presence of the H4 nitrogen-related defects in the outer rim. Presumably, the ballas diamond variety was formed at high supersaturation but in conditions favoring a small growth kinetic coefficient. The carbon isotopic composition of the studied ballases (δ13C = −5.42, −7.11‰) belongs to the main mode of mantle-derived diamonds.

Subduction related to the ancient supercontinent cycle is poorly constrained by mantle samples. Sublithospheric diamond crystallization records the release of melts from subducting oceanic lithosphere at 300–700 km depths 1 , 2 and is especially suited to tracking the timing and effects of deep mantle processes on supercontinents. Here we show that four isotope systems (Rb–Sr, Sm–Nd, U–Pb and Re–Os) applied to Fe-sulfide and CaSiO 3 inclusions within 13 sublithospheric diamonds from Juína (Brazil) and Kankan (Guinea) give broadly overlapping crystallization ages from around 450 to 650 million years ago. The intracratonic location of the diamond deposits on Gondwana and the ages, initial isotopic ratios, and trace element content of the inclusions indicate formation from a peri-Gondwanan subduction system. Preservation of these Neoproterozoic–Palaeozoic sublithospheric diamonds beneath Gondwana until its Cretaceous breakup, coupled with majorite geobarometry 3 , 4 , suggests that they accreted to and were retained in the lithospheric keel for more than 300 Myr during supercontinent migration. We propose that this process of lithosphere growth—with diamonds attached to the supercontinent keel by the diapiric uprise of depleted buoyant material and pieces of slab crust—could have enhanced supercontinent stability.

Diamonds from the Rio Sorriso placer in the Juina area, Mato Grosso State, Brazil, contain mineral inclusions of ferropericlase associated with MgSiO3, CaSiO3, magnesite, merrillite, and other minerals. The ferropericlase inclusions in Rio Sorriso diamonds are resolved into two distinct genetic and compositional groups: (1) protogenetic, high-Ni and low-Fe (Ni = 8270–10,660 ppm; mg# = 0.756–0.842) ferropericlases, and (2) syngenetic, low-Ni and high-Fe (Ni = 600–3050 ppm; mg# = 0.477–0.718) ferropericlases. Based on the crystallographic orientation relationships between natural ferropericlase inclusions and host diamonds, high-Ni and low-Fe ferropericlases originate in the upper part of the lower mantle, while low-Ni and high-Fe ferropericlases, most likely, originate in the lithosphere. Mineral inclusions form the ultramafic lower-mantle (MgSiO3, which we suggest as bridgmanite, CaSiO3, which we suggest as CaSi-perovskite, and high-Ni and low-Fe ferropericlase) and lithospheric (CaSiO3, which we suggest as breyite, Ca(Si,Ti)O3, and low-Ni and high-Fe ferropericlase) associations. The presence of magnesite and merrillite inclusions in association with ferropericlase confirmed the existence of a deep-seated carbonatitic association. Diamonds hosting high-Ni and low-Ni ferropericlase have different carbon-isotopic compositions (δ13C = −5.52 ± 0.75‰ versus −7.07 ± 1.23‰ VPDB, respectively). It implies the carbon-isotopic stratification of the mantle: in the lower mantle, the carbon-isotopic composition tends to become isotopically heavier (less depleted in 13C) than in lithospheric diamonds. These regularities may characterize deep-seated diamonds and ferropericlases not only in the Juina area of Brazil but also in other parts of the world.

Abstract Merrillite, ideally Ca18Na2Mg2(PO4)14 (Dana No: 38.03.04.04; Strunz No: 08.AC.45), an analog to synthetic tricalcium phosphate β-Ca3(PO4)2, was identified as an inclusion in lower-mantle diamonds from the Rio Soriso area, Brazil. It was associated with former bridgmanite, CaSi- and CaTi-perovskites, and ferropericlase. This is the first report of merrillite in a terrestrial environment; previously, it was known only in meteorites and lunar rocks. The compositions of merrillite vary in different localities; the Rio Soriso sample was enriched in SO3 (2.03 wt%). Merrillite from lower-mantle diamonds may be a retrograde phase of the tuite [γ-Ca3(PO4)2]. Owing to their crystal structures, both merrillite and tuite may be important potential hosts for rare earth elements (REE) and large ion lithophile elements (LILE), including Sr and Ba, in the deep Earth. The find of merrillite suggests a larger variety of mineral species in the lower mantle than previously assumed.

Abstract A set of natural diamonds, representing the main types of physical classification (IaA, IaAB, IaB and IIa), was studied with the utilisation of FTIR and SIMS with the implantation of hydrogen into a standard sample. The volumetric concentrations of hydrogen in the studied diamonds are between 5-24 at.ppm. According to their FTIR spectra, the diamonds form two groups: low-hydrogen (0.07‒1.33 cm-1 arbitrary units) and hydrogen-rich (8.4‒15.2 cm-1 arbitrary units) diamonds. The volumetric concentrations of hydrogen, established with the use of SIMS correspond to the FTIR data: low-hydrogen diamonds have 5‒11 at.ppm of hydrogen, and hydrogen-rich diamonds have 13-24 at.ppm of hydrogen. There is a correlation between SIMS and FTIR determinations of hydrogen concentrations in diamonds. Each group is characterised by its trend. The correlation between volumetric hydrogen concentration and FTIR characteristics for low-hydrogen diamonds is expressed by the equations: [H]at.ppm = 10.71*[H]IR + 3.79 and for high-hydrogen diamonds and [H]at.ppm = 1.57*[H]IR-0.3. Our observations confirm bonding hydrogen with nitrogen in H-rich diamonds. Another structure may be suggested for the hydrogen impurity in low-H diamonds. It is also possible that the instrumentally determined hydrogen concentrations in diamond reflect IR-active and other hydrogen centres.

The mineralogical and geochemical features of titanite and associated minerals in a rare sample of kyanite-coesite-rutile-bearing eclogite from the Udachnaya-East (Vostochnaya) kimberlite pipe have been studied in detail. Subidiomorphic titanite grains (100–300 μm) were identified in the intergranular space. The composition of individual grains of titanite is characterized by a constant presence of Al2O3, F, P2O5, Zr, and Sr impurities but varies within the xenolith. Based on the absence of titanite inclusions in the rock-forming minerals and their presence in the intergranular space, titanite was formed in the studied sample at a late stage of its formation, most likely in the process of metasomatic action of the fluid/melt. Crystallization of rock-forming minerals (garnet + omphacite + kyanite) and accessory rutile occurred jointly at 3.5 ± 0.32 GPa and 920 ± 65°С. The value of Eu/Eu* = 1.06 in the reconstructed bulk composition of the rock, the high modal content of kyanite (~17 vol %), and the value of Ca# = Ca/(Ca + Mg + Fe + Mn) > 0.5 in garnet indicate a subduction nature of the studied eclogite. Most likely, the formation of titanite in the studied sample occurred as a result of the metasomatic action of a fluid/melt enriched in calcium, strontium, large lithophilic elements, and lead, by a mechanism similar to the formation of eclogites in the units of the Western Tien Shan.

Among placer diamond occurrences in Ukraine, a group of microdiamonds have been distinguished that have specific morphological, color and spectral characteristics, not observed in other natural diamonds. These diamonds, termed “Dniester–type diamonds”, have tetrahexahedroidal and rhombododecahedroidal morphologies, green coloration, and high concentrations of single–atom, unaggregated nitrogen in the form of C–centers (66–74% of all N atoms), along with low ratios of nitrogen aggregation (0–13% agrregation ratio) and high total nitrogen content (892–1493 atomic ppm). With these characteristics, Dniester–type diamonds are approximate the Type Ib-Iab classification. The predominance of single–atom, unaggregated nitrogen indicates a short residence time under high–temperature conditions. These Dniester–type diamonds have a narrow range of carbon isotopic compositions, from δ13С = -10.52‰ VPDB tо -12.82‰ VPDB (average δ13С = -11.85‰ VPDB). They are distributed in Quaternary and Neogene sediments of the southwestern part of the Ukrainian Shield. This distribution forms a local halo within the Dniester and Southern Bug rivers interfluve and Black Sea beach sediments, approximately 650 km in length. This implies their endemic character and the likely nearby presence of primary source(s) of unknown, possibly non–kimberlitic type.

Ellinaite, a natural analog of the post-spinel phase β-CaCr2O4, was discovered at the Hatrurim Basin, Hatrurim pyrometamorphic formation (the Mottled Zone), Israel, and in an inclusion within the super-deep diamond collected at the placer of the Sorriso River, Juína kimberlite field, Brazil. Ellinaite at the Hatrurim Basin is confined to a reduced rankinite–gehlenite paralava, where it occurs as subhedral grains up to 30 µm in association with gehlenite, rankinite and pyrrhotite or forms the rims overgrowing zoned chromite–magnesiochromite. The empirical formula of the Hatrurim sample is (Ca0.960Fe0.0162+Na0.012Mg0.003)0.992(Cr1.731V0.1833+Ti0.0683+Al0.023Ti0.0034+)2.008O4. The mineral crystallizes in the orthorhombic system, space group Pnma, unit-cell parameters refined from X-ray single-crystal data: a 8.868(9), b 2.885(3), c 10.355(11) Å, V 264.9(5) Å3 and Z=4. The crystal structure of ellinaite from the Hatrurim Basin has been solved and refined to R1=0.0588 based on 388 independent observed reflections. Ellinaite in the Juína diamond occurs within the micron-sized polyphase inclusion in association with ferropericlase, magnesioferrite, orthorhombic MgCr2O4, unidentified iron carbide and graphite. Its empirical formula is Ca1.07(Cr1.71Fe0.063+V0.06Ti0.03Al0.03Mg0.02Mn0.02)Σ1.93O4. The unit-cell parameters obtained from HRTEM data are as follows: space group Pnma, a 9.017, b 2.874 Å, c 10.170 Å, V 263.55 Å3, Z=4. Ellinaite belongs to a group of natural tunnel-structured oxides of the general formula AB2O4, the so-called post-spinel minerals: marokite CaMn2O4, xieite FeCr2O4, harmunite CaFe2O4, wernerkrauseite CaFe23+Mn4+O6, chenmingite FeCr2O4, maohokite MgFe2O4 and tschaunerite Fe(FeTi)O4. The mineral from both occurrences seems to be crystallized under highly reduced conditions at high temperatures (>1000 ∘C), but under different pressure: near-surface (Hatrurim Basin) and lower mantle (Juína diamond).

Diamonds were lately identified in chromitites from ophiolites and in volcanic rocks. Although the tectonic settings of diamonds found in these rocks are different, the diamonds are identical in small size, cuboctahedral habit, sets of minor admixture elements, and isotopic characteristics. A model is suggested for their formation during various stages of a single evolutionary cycle of the oceanic lithosphere, in relation to the geochemical and dynamic features of an ascending flow of mantle material, which produces the oceanic lithosphere at mid-oceanic ridges. In contrast to the continental lithosphere, in which mantle diamonds are usually related to kimberlite and lamproite magmatism in the presence of abundant CO2-rich fluid, diamonds in the oceanic lithosphere crystallize in environments poor in fluid and containing carbon mostly in its reduced forms. In the asthenospheric part of the ascending flow, carbon may occur in the form of nanometer-sized diamonds. In the upper parts of the oceanic lithosphere, the diamonds are overgrown and become microdiamonds (0.2–0.7 mm) within chromitites. After basaltic magma is derived from pyrolite, the residual harzburgites with lenses of diamondiferous chromitites are brought (at spreading) to the convergent boundaries of oceanic lithospheric plates, where the following two processes can proceed. If the oceanic lithosphere collides with a continental plate, the obducted material of the oceanic lithosphere is transferred to the surface of the continental margin and forms ophiolite massifs hosting diamond-bearing chromitites. If the oceanic lithosphere is subducted, the residual peridotite already enriched in volatiles is remelted. The arc magmas thus derived host diamond microcrystals, which have been formed in the chromitites and are sometimes found in volcanic lavas and ashes.

The best-known, most well-studied diamondiferous rocks are kimberlites and lamproites. Diamonds are also found in impactites, metamorphic rocks, ophiolites, and modern volcanic rocks. Diamonds from these rocks differ from kimberlitic diamonds in size, morphology, trace-element and isotope composition, and physical properties. Differences in these characteristics are related to their different mechanisms of origin. In some cases, diamonds can be formed in “metastable” conditions under disequilibrium thermodynamic parameters, supporting the conclusion that diamond is a polygenetic mineral, formed in nature under different physicochemical and geodynamic conditions. According to thermodynamic considerations and calculations, “metastable” crystallization of diamond is mainly controlled by the size of the forming crystallites. The main effectors in decreasing the energetic barrier for nanosized diamonds are surface tension and related surface energy.

The volumetric concentration of hydrogen in two Brazilian diamonds is determined using secondary ion mass spectrometry and implantation of hydrogen into an external standard sample (with a dose of 1 × 16 at/cm 2 and energy of 120 KeV). The diamonds studied differ noticeably in their intensities of IR-active hydrogen from 0 to 1.5 cm –1 according to the analyses of their IR spectra. The results demonstrate that for both samples studied, the volumetric concentration of hydrogen does not exceed the reached detectable level of (1–2) × 10 18 at/cm 3 or 1.7–3.3 at. ppm; i.e., it is lower by an order of magnitude than in the early chemical analysis and by 2–3 orders of magnitude lower than the results of the ion-beam spectrochemical, nuclear-physical, and ERDA analyses. Only a part of the hydrogen forms optically active impurities in diamond crystals and can be determined by spectral methods.

Article On “Kamchatite” diamond aggregate from northern Kamchatka, Russia: New find of CVD-formed diamond in nature—Reply to K.D. Litasov, T.B. Bekker, and H. Kagi was published on January 1, 2020 in the journal American Mineralogist (volume 105, issue 1).

—We have studied volcanogenic diamonds in the context of a discussion of their genesis, including some assumption on their artificial origin. The carbon isotope composition of diamonds collected from the eruption products of Tolbachik volcano (δ13CVPDB from –22 to –29‰) is within the range of the δ13CVPDB values of natural diamonds, including those from kimberlites. The δ15NAir values of the Tolbachik diamonds, measured for the first time (–2.58 and –2.32‰), correspond to δ15NAir of volcanic gases and differ from that of atmospheric nitrogen (δ15NAir = 0‰), which may be expected in synthetic diamonds. In the studied volcanogenic diamonds, as in synthetic ones, the nitrogen impurity is unaggregated. However, such an unaggregated form of nitrogen is specific to many natural diamonds (e.g., variety II diamonds, according to Orlov’s classification). Impurity elements (Cl, F, O, S, Si, Al, Ca, and Na) are locally concentrated in volcanogenic diamonds; they are a constituent of micro- and nanoinclusions in them. The high contents of F and Cl in the studied diamonds are correlated with the composition of volcanic gases; there is no reason to expect a similar correlation in synthetic diamonds. Moreover, the studied cube-octahedral Tolbachik diamonds have a number of accessory forms, some of which are not observed in synthetic diamonds. Their surfaces are frequently covered with films composed of Mg–Fe and Ca–Mg silicates, aluminosilicates, sulfates, metal alloys, and native Al. Mineral inclusions in the studied diamonds are Mn–Ni–Si alloys and silicides varying in composition from (Mn,Ni)4Si to (Mn,Ni)5Si2, Mn5Si2, and pure Mn silicide MnSi. Summing up the obtained data, we conclude that volcanogenic diamonds form in a strongly reducing environment, in which silicides and native metals and their alloys are stable. The predominant cube-octahedral morphology of these diamonds and the unaggregated nitrogen impurity point to their short-term residence under high-temperature conditions. This makes them similar, to some extent, to synthetic diamonds. There are, however, clear differences as well. Volcanogenic diamonds are similar in compositional peculiarities, including isotope compositions, to natural diamonds that form under most unfavorable conditions, such as cuboids, balases, carbonado, and some diamonds of the eclogite paragenesis. They also resemble diamonds found in situ in harzburgite and chromitite of ophiolites. This suggests a specific mechanism of formation of both volcanogenic and ophiolitic diamonds in the oceanic lithosphere.