Chemistry of micas from kimberlites and xenoliths—I. Micaceous kimberlites

Micaceous kimberlites from South Africa and Canada contain two types of groundmass mica less than 1 mm across. Very rare Type I micas are relatively iron-rich with $\text{mg [ = Mg/(Mg + Fe)] 0.45–0.65}$, TiO₂ 3–6 wt%, Al₂O₃ 14–16wt%, no Fe³⁺ required in tetrahedral sites, low NiO (~0.02 wt%), and relatively high na [Na₂O/(Na₂O + K₂O)] 0.02–0.03. The much more abundant Type II micas are variable in composition, but relative to Type I micas are more magnesium (mg 0.80-0.93), lower in TiO₂ (0.7–4.0 wt%) and Al₂O₃ (6.8–14.2 wt%), have substantial Fe³⁺ in tetrahedral sites, and have relatively low na. Both types may have rims with compositions indicative of mica-‘serpentine’ mixtures resulting from reaction with a highly aqueous fluid. The petrographically-determined ‘serpentine’ is chemically of two types: Fe-rich serpentine and Fe-rich talc. Associated phases in the ground-mass vary from one kimberlite to another: calcite, dolomite, diopside, chromite, Mg-ilmenite, perovskite, barite, pyrite, pentlandite, millerite?, heazlewoodite?, quartz.Inter-grain variations in composition of Type II micas may result from establishment of local reservoirs on a mm scale, consequent upon mechanical mixing and competition of other phases for minor elements (e.g. chromite for Cr, serpentine for Ni).Type I micas may result from an intrusive precursor (carbonatitic?) to kimberlite, perhaps genetically related, which was incorporated into a later pulse of kimberlite from which the Type II micas crystallized.     

Eclogite xenoliths from Wajrakarur kimberlites, southern India

Summary Mineralogical characteristics of eclogite xenoliths from three kimberlite pipes (KL2, P2 and P10) of the Proterozoic Wajrakarur kimberlite field of southern India have been studied. In a rare sample of enstatite eclogite from the KL2 pipe garnet contains microscopic triangular arrays of needles or blebs of omphacite, enstatite and rutile consistent with an origin by exsolution parallel to the isometric form {111}. Discrete omphacite grains in the sample contain exsolved needles or blebs of enstatite and garnet. Kyanite eclogites are abundant in the KL2 pipe which occasionally show a secondary ring of pure celsian around kyanite grains. Omphacite Na₂O contents in the eclogites of the KL2 and P2 pipes are typically between 3 and 6 wt%, and garnet has widely variable composition with end member ranges of Prp22-81Grs0-47Alm10-30Sps0-1Adr0-5Uv0-3. Eclogites of the P10 pipe comprise chromian omphacite and garnet. Phase relations in the ACF projection exhibit systematic increase of the Ca-Tschermak’s component in omphacite from enstatite eclogite through biminerallic eclogite to kyanite eclogite. Garnet-clinopyroxene Fe–Mg geothermometry yields temperatures mostly in the range of 900–1100 °C. A formerly supersilicic nature of garnet in enstatite eclogite as inferred from exsolution mineralogy indicates minimum peak pressure of 5 GPa.     

Nature and origin of eclogite xenoliths from kimberlites

Eclogites from the Earth's mantle found in kimberlites provide important information on craton formation and ancient geodynamic processes because such eclogites are mostly Archean in age. They have equilibrated over a range of temperatures and pressures throughout the subcratonic mantle and some are diamond-bearing. Most mantle eclogites are bimineralic (omphacite and garnet) rarely with accessory rutiles. Contrary to their overall mineralogical simplicity, their broadly basaltic-picritic bulk compositions cover a large range and overlap with (but are not identical to) much younger lower grade eclogites from orogenic massifs. The majority of mantle eclogites have trace element geochemical features that require an origin from plagioclase-bearing protoliths and oxygen isotopic characteristics consistent with seawater alteration of oceanic crust. Therefore, most suites of eclogite xenoliths from kimberlites can be satisfactorily explained as samples of subducted oceanic crust. In contrast, eclogite xenoliths from Kuruman, South Africa and Koidu, Sierra Leone stem from protoliths that were picritic cumulates from intermediate pressures (1–2 Ga) and were subsequently transposed to higher pressures within the subcratonic mantle, consistent with craton growth via island arc collisions. None of the eclogite suites can be satisfactorily explained by an origin as high pressure cumulates from primary melts from garnet peridotite.     

Rare-earth abundances in kimberlites from Greenland and Zambia

Ten rare-earth elements (La, Ce, Nd, Sm, Eu, Gd, Tb, Tm, Yb and Lu) and Ta, Th and Hf contents in eight kimberlites and inclusions from Greenland and Zambia have been determined by instrumental neutron activation. All the samples have highly fractionated rare-earth (REE) distribution patterns. La/Yb ratios in the Greenland kimberlites (hypabyssal facies) vary from 111.8 to 188.4, and the total rare-earth contents range from 204.8 to 380.3 ppm. No europium anomaly is present. The Zambian kimberlites (diatreme facies) are altered and carbonated. Rare-earth patterns in these are also light REE-enriched. A significant difference is shown to exist between the diatreme and hypabyssal facies of kimberlites.     

Mineralogical studies of pyropes in kimberlites from China

Based on the measurements of refractive index,specific gravity,unit cell parameter,and mineral chemistry and infrared absorption spectrum analyses of pyropes in kimberlites from China,systematic studies of the Physical properties and compositional variations of pyropes of different colors and diverse paragenetic types,within and between kimberlite provinces have been undertaken,The origin of pyropes in the Kimberlites and the depth of their formation have been discussed.Pyropes of the purple series are different from those of the orange series in physical and chemical properties,for exaple,pyropes of the puple series are higher in α0,RI,SG,Cr2O3,MgO,Cr/(Cr Al),Mg/(Mg Fe),and Mg/(Mg Ca),and lower in Al2O3,Fe2O3 FeO than those of the orange series.The classification of garnets in kimberlites from china by the Dawson and Stephens‘ method(1975) has been undertaken and clearly demonstrates that pyropes of diamond-rich kimberlites contain much more groups than those of diamond-poor,especially diamond-free kimberlites.The higher in α0,RI,SG,Cr2O(3.Cr/(Cr Al),knorringite and Cr-component the pyropes are ,the richer in diamond the kimberlites will be.The infrared absorption spectrum patterns of pyropes change with their chemical composition regularly,as reflected in the shape and position of infrared absorption peaks.Two absortpion bands at 862-901 cm^-1 will grade into degeneration from splitting and the absorption band positions of pyropes shift toward lower frequency with increasing Cr2O3 content and Cr/(Cr Al) ratio of pyropes,LREE contents of orange pyrope megacrysts are similar to those of porple pyrope macrocrysts,but the former is higher in HREE than the latter,showing their different chondrite-normalized patterns.The formation pressures of pyropes calculated by Cr-component,Ca-component,knorringite molecules of pyropes show that some pyropes of the purple series in diamondiferous kimberlites fall into the diamond stability field.but all pyropes of diamond-free kimberlites lie outside the diamond stability field.The megacrysts were formed through early crystallization of kimberlites magma at high pressure condition,the majority of the purple pyrope macrocrysts have been derived from disaggregated xenoliths but the minoirty of them appear to be fragments of the discrete megacryst pyropes,or phenocrysts.     

The age of unusual xenogenic zircons from Yakutian kimberlites

Several spindle-shaped grains of zircon, which have a small size (<0.25 mm) and a distinct purplish pink coloration were found in the crushed samples of kimberlites from the Aykhal, Komsomolskaya-Magnitnaya, Botuobinskaya (Siberian platform), and Nyurbinskaya (Yakutia) pipes and olivine lamproites of the Khani massif (West Aldan). U-Pb SHRIMP II zircon dating performed at the VSEGEI Center for Isotopic Research yielded the ages of 1870–1890 Ma for the pipes of the Western province (Aykhal and Komsomolskaya) and 2200–2750 Ma for the pipes of the eastern province (Nyurbinskaya and Botuobinskaya), which allowed us to consider these zircons to be xenogenic to kimberlites. Although these zircons resemble in their age and color those from the granulite xenoliths in the Udachnaya pipe [2], no other granulite minerals are found there. Thus, major geological events in the mantle and lower crust, which led to the formation of zircon-bearing rocks, happened at 1800–1900 Ma in the northern part of the kimberlite province, whereas in the Eastern part of the province (Nakyn field) these events were much older (2220–2700 Ma). It is known that the period of 1800–1900 Ma in the Earth’s history was accompanied by intense tectonic movements and widespread alkaline-carbonatite magmatism. This magmatism was related to plume activity responsible for overheating the large portions of the mantle to the temperatures at which some diamonds in mantle rocks would burn (northern part of the kimberlite province). In the Nakyn area, the mantle underwent few or no geological processes at that time, and perhaps for this reason this area hosts more diamondiferous kimberlites. The age of olivine lamproites from the Khani massif is 2672–2732 Ma. Thus, these are some of the world’s oldest known K-alkaline rocks.     

Exsolved garnet-bearing pyroxene megacrysts from some South African kimberlites

Clinopyroxene and orthopyroxene megacrysts containing garnet lamellae up to 1.2 mm thick as an exsolved phase are found rarely in kimberlites from Frank Smith and Bellsbank. Chemically the clinopyroxenes are characteristically subcalcic, being within the range of 100 Ca/Ca + Mg + Fe = 27 to 36, and the orthopyroxenes are characterized by high Al₂O₃ and Cr₂O₃. Immediately after crystallization during very slow cooling, clinopyroxene and orthopyroxene exsolve wide-spaced orthopyroxene and clinopyroxene phases parallel to (100) of the host phases, respectively, then both host and exsolved phases exsolve garnet lamellae. Topotactic relations between pyroxenes and garnet are determined by X-ray for the first time. Partitioning of major and minor elements among the coexisting clinopyroxene, orthopyroxene and garnet in pyroxene megacrysts is the same as that of the granular-type garnet peridotite xenoliths in Lesotho and South African kimberlies. Mineralogy and chemistry indicate that subcalcic clinopyroxene and orthopyroxene megacrysts contain respectively about 10 and 3 mole % of the garnet molecule in solid solution.     

Petrology of highly aluminous xenoliths from kimberlites of Yakutia

Highly aluminous xenoliths include kyanite-, corundum- and coesite-bearing eclogites, grospydites and alkremites. These xenoliths are present in different kimberlites of Yakutia but have most often been found in Udachnaya and other pipes of the central Daldyn–Alakitsky region. Kimberlites of this field also contain eclogite-like xenoliths with kyanite and corundum that originate in the lower crust or the lower crust–upper mantle transition zone. Petrographic study shows that two rock groups of different structure and chemistry can be distinguished among kyanite eclogites: fine- to medium-grained with mosaic structure and coarse-grained with cataclastic structure. Eclogites with mosaic structure are characterized by the occurrences of symplectite intergrowths of garnet with kyanite, clinopyroxene and coesite; only in this group do grospydites occur. In cataclastic eclogites, coarse-grained coesite occurs, corresponding in size to other rock-forming minerals. Highly aluminous xenoliths differ from bimineralic eclogites in their high content of Al₂O₃ and total alkali content. Coesite-bearing varieties are characterized by low MgO content and higher Na/K and Fe²⁺/Fe³⁺ ratios, as well as high contents of Na₂O. Geochemical peculiarities of kyanite eclogites and other rocks are exhibited by a sloping chondrite-normalized distribution of rare earth elements (REE) in garnets and low Y/Zr ratio, in contrast to bimineralic rocks. Coesite is found in more than 20 kyanite eclogites and grospydites from Udachnaya. Grospydites with coesite from Zagadochnaya pipe are described. Three varieties of coesite in these rocks are distinguished: (a) subhedral grains with size of 1.0–3.0 mm; (b) inclusions in the rock-forming minerals; (c) sub-graphic intergrowths with garnet. The presence and preservation of coesite in eclogites indicate both high pressure of formation (more than 30 kbar) and set a number of constraints on the timing of xenolith cooling during entrainment and transport to the surface. Different ways of formation of the highly aluminous eclogites are discussed. Petrographic observations and geochemistry suggest that some highly aluminous rocks have formed as a result of crystallization of anorthosite rocks in abyssal conditions. δ¹⁸O-estimations and other petrologic evidence point out the possible origin of some of these xenoliths as the result of subduction of oceanic crust. Diamondiferous samples have been found in all varieties except alkremites. Usually these eclogites contain cubic or coated diamonds. However, two sample corundum-bearing eclogites with diamonds from the Udachnaya pipe contain octahedra that show evidence of resorption.     

Garnet lherzolites from the Hanaus-I and Louwrensia kimberlites of Namibia

The Gibeon cluster of Namibian kimberlites is emplaced into the Orange River Belt which has accreted to the Kaapvaal Craton. These offcraton kimberlites lack diamonds and are younger than the diamondiferous on-craton kimberlites. The Hanaus-I and Louwrensia kimberlites each contain a bimodal suite of upper-mantle-derived garnet lherzolite xenoliths characterized by a coarse granular or mosaic porphyroclastic texture. The Louwrensia pipe in addition contains garnet harzburgites. Deformed lherzolites are not iron-enriched relative to the coarse types. Conditions of equilibration calculated by the Wells-Wood method are 841–1,013° C at 25.6–36.3 kbars, and 869–1,195° C at 23.9–39.4 kbars, for coarse lherzolites from Louwrensia and Hanaus respectively, and from 1,080–1,112° C at 31.6–34.5 kbars, and 983–1,228° C at 24.7–35.2 kbars, for mosaic porphyroclastic types from Louwrensia and Hanaus respectively. The coarse varieties from both localities have similar equilibration conditions to coarse lherzolites from on-craton kimberlites and define the lower limb of a perturbed geotherm. The upper high temperature limb of the Namibian geotherm is considered to be an apparent geotherm generated by the deformation and metasomatism of the upper mantle by a rising diapir. Such geotherms, being the result of kimberlite-xenolith interactions, provide no stratigraphic or thermal information concerning the site of kimberlite or diamond formation.     

Chrome-rich garnets from the kimberlites of yakutia and their parageneses

The chrome-rich magnesian garnets (6.6–18.9% Cr₂O₃) of kimberlitic concentrates and some peridotite xenoliths contain variable admixtures of CaO: from 0.69 to 26.0% (1.7–72% Ca-component). All the garnets both in respect of Ca and Cr-contents make up a continuous series.The variability in the Ca-content is caused by differences in paragenesis. Most of the Ca-poor pyropes are related to a paragenesis without clinopyroxene (mostly dunitic). Garnets rich in calcium are related to a paragenesis without entstatite. All the parageneses listed are of an ultramafic type, i.e. contain magnesian olivine. The solubility of knorringite—Mg₃Cr₂(Si₃O₁₂)—in kimberlitic garnets is possibly limited by pressure and does not exceed 50–60% mol.     

Petrogeochemical characteristics of the kimberlites from the Middle Markha region with application to the problem of the geochemical heterogeneity of kimberlites

A comparative geochemical investigation of kimberlites from the Middle Markha region and traditional diamond-bearing areas of Yakutia supported the division of kimberlites into two geochemical types. One of them includes kimberlites from the traditional diamond-bearing areas of Yakutia, and the other is represented by the kimberlites of the Middle Markha region and the rocks of the Zolotitsa field and V. Grib pipe of the Arkhangelsk province. The obtained representative parameters of the two kimberlite types indicate a sharp geochemical contrast between them, and the individual features of correlation relationships between the elements in the rocks of the two types suggest that these differences are related to fundamental genetic factors, which concern primarily the group of highly charged incompatible trace and radioactive elements. The presence of geochemically contrasting rocks within a rather uniform petrochemical association and the geochemically specialized occurrences of kimberlites and related rocks are consequences of repeated metasomatic transformations of mantle rocks under the influence of deep-derived fluids or volatiles released during the recycling of subneous source and subsequent derivation of geochemically specialized types of deep magmas showing signatures of individual mantle sources.     

寻找含金刚石金伯利岩的地质经济学

地质经济学关注地质科学研究和矿产勘查经费使用的有效性,是使科研的成果更有意义、更具创新性、找矿更具有针对性。含金刚石的金伯利岩体是世界上分布较少和较难寻找的岩体,但地质学家却做了大量的和深入的研究。从消化全球科学研究成果、建立金刚石矿勘查靶区、利用指示矿物追踪、消除干扰体,进而用多种手段确定靶位,建立了一套科研成果用于找矿的行之有效的步骤,进行每一个步骤时都要考虑到资金使用的有效性。本文讨论地质经济学在寻找含金刚石的金伯利岩全过程中的应用,抛砖引玉,认为地质经济学应当贯穿在科研和找矿的各个方面。本文还对中国寻找金刚石矿的远景区进行了分析。     

Isotope-geochemical systematics of kimberlites and related rocks from the Siberian Platform

Using the ICP-MS method we have studied the isotope systematics of Sr and Nd as well as trace element composition of a representative collection of kimberlites and related rocks from the Siberian Platform. The summarized literature and our own data suggest that the kimberlites developed within the platform can be divided into several petrochemical and geochemical types, whose origin is related to different mantle sources. The petrochemical classification of kimberlites is based on persistent differences of their composition in mg# and in contents of indicator oxides such as FeO_tot, TiO₂, and K₂O. The recognized geochemical types of kimberlites differ from one another in the level of concentration of incompatible elements as well as in their ratios.Most of isotope characteristics of kimberlites and related rocks of the Siberian Platform correspond to the earlier studied Type 1 basaltoid kimberlites from different provinces of the world: Points of isotopic compositions are in the field of primitive and weakly depleted mantle. An exception is one sample of the rocks from veins of the Ingashi field (Sayan area), which is characterized by the Sr and Nd isotopic composition corresponding to Type 2 micaceous kimberlites (orangeites).The most important feature of distribution of isotopic and trace-element compositions (incompatible elements) is their independence of the chemical rock composition. It is shown that the kimberlite formation is connected with, at least, two independent sources, fluid and melt, responsible for the trace-element and chemical compositions of the rock. It is supposed that, when rising through the heterogeneous lithosphere of the mantle, a powerful flow of an asthenosphere-derived fluid provoked the formation of local kimberlite chambers there. Thus, the partial melting of the lithosphere mantle led to the formation of contrasting petrochemical types of kimberlites, while the geochemical specialization of kimberlites is due to the mantle fluid of asthenosphere origin, which drastically dominated in the rare-metal balance of a hybrid magma of the chamber.     

Geochemistry of magnesian ilmenites from kimberlites in South Africa and Lesotho

Magnesium ilmenite from discrete nodules and lamellar intergrowths with pyroxene from the Kao, Sekameng, Frank Smith, and Monastery kimberlites has been analysed for Ti, Fe, Mg, Nb, Zr, Cr, Cu, Zn, Mn, Co, and Ni. Each kimberlite contains discrete ilmenites which exhibit a wide compositional range within the ilmenite-geikeilite series. Lamellar ilmenites from Frank Smith and Monastery differ in composition but both show a limited range in composition which lies within the compositional range shown by discrete ilmenites from these pipes. The ilmenites are enriched in Nb, Zr, Cr, Co, and Ni and depleted in Mn, Cu, and Zn relative to Mg-poor ilmenites from basic intrusions. Nb, Zr, and Ni correlate with Fe, Ti and Mg variations but the abundances of the other trace elements are independent of major element variation. R-mode factor analysis is interpreted to imply that the geochemistry cannot be interpreted in terms of a differentiation hypothesis in which trace elements are removed from or concentrated in residua. Factor scores and major element abundances indicate that each pipe is characterized by a particular suite of discrete ilmenite nodules, which are considered to be phenocrysts in a proto-kimberlite magma. Lamellar ilmenite-pyroxene intergrowths are unlikely to have had a eutectic origin, and show no simple relationship to the discrete ilmenites.     

Hydroxyl in olivines from mantle xenoliths in kimberlites of the Siberian platform

From a total of 335 olivine crystal grains, crystallographically orientated platelets and, where possible, parallelepipeds were prepared, chemically analysed by electron microprobe, examined under the polarisation microscope, and studied by polarised FTIR microscope-absorption-spectrometry in the _OH vibrational range, 3,000–3,800 cm^–1. The 335 crystal grains were extracted from 174 different specimens of Yakutian upper mantle material, including 97 xenoliths that represent all the rock types occurring in all the kimberlites of the Siberian shield. The other specimens were mega- and macrocrysts or inclusions in diamonds and garnets. Analysis of the polarised _OH-spectra allowed distinction between hydroxyl in non-intrinsic separate inclusions, NSI, and in isolated local or condensed extended defects, intrinsic to the olivines, ILD or CED, respectively. As the two latter types cannot be distinguished by vibrational spectroscopy, and as they are presumably interconnected by intracrystalline condensation reactions of the type n [ILD][CED]_n, we propose to symbolise them as [ILD/CED]. Of the total of 70 polarised _OH-bands that were found in the whole set, 17 are caused by NSI, 53 by [ILD/CED]. Total mean integrated _OH-band intensities, (̄_int)_total with ̄_int=(_||a+_||b+_||c)_int/3, were determined from the spectra. They yielded the contents of structurally unallocated water, using the recent calibration of the IR-method (Bell et al. 2003). The range is 0<wt. ppm (H₂O)_total<419. Olivines included in diamonds were found to be free of hydroxyl (detection limit of the single crystal IR-spectrometry, ca. 1 wt. ppm water). The total water contents of the different types of olivines increase in the sequence groundmass crystals < megacrysts < macrocrysts. NSI are: (1) Serpentine plus talc with _OH in the range 3,704–3,657 cm^–1, either polarised along a of the olivine matrix (Pbnm setting) or unpolarised. Approximately 232 olivines out of the 335 contain such NSI. Serpentine and talc occur mostly together, in rare cases one of them alone and if so, mostly talc. (2) Mg-edenite or Mg-pargasite occur rarely and with _OH at 3,709–3,711 cm^–1. NIS types (1) and (2) are presumably formed by metasomatic alterations of the host olivines, the orientated ones probably in the mantle, the unorientated ones during later stages. (3) The spectra of 23 olivine crystals, displayed specifically a _OH-band, polarised c>a>b, at 3,327–3,328 cm^–1, an energy typical of _OH in hydrous wadsleyite. We assume this phase to be present as NIS in the respective olivines, possibly as relic phase. (4) Weak bands between 3,175 and 3,260 cm^–1 polarised along c, are tentatively assigned to molecular water NSI with relatively strong hydrogen bonds to the matrix. We did not find larger clusters of molecular water, i.e. liquid-like water with its characteristic broad band centred at ca. 3,400 cm^–1. We did also not find any humite minerals as an NSI. Of the 53 _OH-bands intrinsic to olivine, the 29 most abundant and strong ones were subject to further analysis in terms of OH^–-bearing structural defects [ILD/CED]. Nearly all these bands are strongly polarised along a. Two bands at 3,672 and 3,535 cm^–1 are assigned to boron-related defects, [ILD/CED]_B. Five bands at 3,573, 3,563, 3,541, 3,524 and 3,512 cm^–1 are intensity-correlated and are assigned to Si-depleted titan-clinohumite-like defects, [ILD/CED]_(thl). The other, so far unidentified _OH of [ILD/CED] are suggested to originate from OH^– in different types of (Mg, Fe)-depleted defects recently predicted and discovered by TEM. These are called [ILD/CED]_(KWK). Eight mostly strong bands of them occur at energies higher than 3,573 cm^–1, [ILD/CED]_(KWK)-H, 13 strong to medium strong bands occur below 3,500 cm^–1, [ILD/CED]_(KWK)-L. Such intrinsic defects may occur alone, [ILD/CED]_(thl) and [ILD/CED]_(KWK)-H, or in different combinations with each other, [ILD/CED]_(KWK)-H+[ILD/CED]_(thl), [ILD/CED]_(KWK)-H+[ILD/CED]_(KWK)-L and [ILD/CED]_(KWK)-H+[ILD/CED]_(thl)+[ILD/CED]_(KWK)-L. Though there are indications that the occurrences of such types and combinations of the intrinsic OH^–-bearing defects in the olivines are related to the types and genetic peculiarities of their host rocks, straightforward and simple correlations do not exist. The reasons for this and also for the great number of varieties of intrinsic [ILD/CED] are discussed.Editorial responsibility: J. Hoefs

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Composition of chromites from kimberlites of the Botuobinskaya pipe in Yakutia

A representative collection (138 analyses) of chromites from kimberlites of the Botuobinskaya pipe in Yakutia was studied. With allowance for the Cr, Ti, and Al contents, the chromites are subdivided into the low-Cr aluminous group A and the high-Cr and high-Ti group B. The chromites of group A with their compositional variations controlled by the Al³⁺-Cr³⁺ isomorphism are not related to kimberlite in composition and reveal attributes of restites. The chromites of group B with heterovalent Ti⁴⁺ + Fe²⁺ + Fe³⁺-3Cr³⁺ isomorphism vary in their composition in line with the compositional variations of kimberlites, thus demonstrating their primary magmatic origin. The chromites of the second group crystallized simultaneously with olivines from kimberlites, and both minerals could have formed nodules of spinel dunite.