Origin of zoned spinel by coupled dissolution–precipitation and inter-crystalline diffusion: evidence from serpentinized wehrlite, Bangriposi, Eastern India

Textural and mineral–chemical characteristics in the Bangriposi wehrlites (Eastern India) provide insight into metamorphic processes that morphologically and chemically modified magmatic spinel during serpentinization of wehrlite. Aluminous chromite included in unaltered magmatic olivine is chemically homogenous. In sub-cm to 10s-of-micron-wide veins, magnetite associated with antigorite and clinochlore comprising the serpentine matrix is near-stoichiometric. But Al–Cr–Fe³⁺ spinels in the chlorite–magnetite veins are invariably zoned, e.g., chemically homogenous Al-rich chromite interior successively mantled by ferritchromite/Cr-rich magnetite zone and magnetite continuous with vein magnetite in the serpentine matrix. In aluminous chromite, ferritchromite/Cr-rich magnetite zones are symmetrically disposed adjacent to fracture-controlled magnetite veins that are physically continuous with magnetite rim. The morphology of ferritchromite–Cr-rich magnetite mimics the morphology of aluminous chromite interior but is incongruous with the exterior margin of magnetite mantle. Micropores are abundant in magnetite veins, but are fewer in and do not appear to be integral to the adjacent ferritchromite–Cr-rich magnetite zones. Sandwiched between chemically homogenous aluminous chromite interior and magnetite mantle, ferritchromite–Cr-rich magnetite zones show rim-ward decrease in Cr₂O₃, Al₂O₃ and MgO and complementary increase in Fe₂O₃ at constant FeO. In diffusion profiles, Fe₂O₃–Cr₂O₃ crossover coincides with Al₂O₃ decrease to values <0.5 wt% in ferritchromite zone, with Cr₂O₃ continuing to decrease within magnetite mantle. Following fluid-mediated (hydrous) dissolution of magmatic olivine and olivine + Al–chromite aggregates, antigorite + magnetite and chlorite + magnetite were transported in 10s-of-microns to sub-cm-wide veins and precipitated along porosity networks during serpentinization (T: 550–600 °C, f(O₂): ?19 to ?22 log units). These veins acted as conduits for precipitation of magnetite as mantles and veins apophytic in chemically/morphologically modified magmatic Al-rich chromite. Inter-crystalline diffusion induced by chemical gradient at interfaces separating aluminous chromite interiors and magnetite mantles/veins led to the growth of ferritchromite/Cr-rich magnetite zones, mimicking the morphology of chemically modified Al–Cr–Fe–Mg spinel interiors. Inter-crystalline diffusion outlasted fluid-mediated aluminous chromite dissolution, mass transfer and magnetite precipitation.     

Solid inclusions in chrome-spinels and platinum group element concentrations from the hochgrössen and kraubath ultramafic massifs (Austria)

A great variety of platinum group mineral, sulfide and silicate inclusions in chrome spinel from Hochgrössen and Kraubath ultramafic massifs, and platinum group element contents of three different rock types have been investigated. Both ultramafic massifs are tectonically isolated bodies, variably serpentinized and metamorphosed (greenschist to lower amphibolite facies), and show ophiolitic geochemical affinities. The chromite from massive chromitites and disseminated in serpentinized dunites and serpentinites, exhibits compositional zonation as the result of alteration during serpentinization and metamorphism. Three distinctive alteration stages are indicated in the chrome-spinels from the Hochgrössen, whereas alteration is less significant in chromites from Kraubath: The core of chrome spinel represents the least altered part, surrounded by an inner rim characterized by slight compositional differences in Cr, Mn, Fe²⁺ and Al with respect to the core. The outer rim is formed by ferritchromite with a sharp boundary to the inner rim and shows a significant decrease of Al, Mg, Cr and increase of Fe²⁺, Fe³⁺ and Ni compared to the core. Two different groups of inclusions in chrome-spinel are present: the first group occurs within the chromite core, and comprises olivine, orthopyroxene, amphibole, sulfides and platinum-group minerals, i.e. dominated by Ru-Os-Ir-sulfides. The second group is formed by chlorite, serpentine, galena, pyrite, arsenopyrite, Pt-Pd-Rh-dominated sulfarsenides and sperrylite. In particular the abundance of Pt-Pd-Rh-sulfarsenides and arsenides is typical of both ultramafic massifs and is very unusual for chromitites from ophiolites. Morphology, paragenesis and chemical composition indicate a different origin for these two groups of inclusions. The first group is intimately related to the crystallisation of the chromite host. The second group of inclusions clearly displays a secondary formation during serpentinization and metamorphism, closely related to the alteration of chrome-spinel and the development of ferritchromite. The distribution patterns of the platinum group elements from massive chromitites, disseminated chrome-spinel bearing serpentinites and serpentinites exhibit variable enrichment of Rh, Pt and Pd, Rh, Pt for the Hochgrössen and Kraubath massifs, respectively. These results are in accordance with the occurrence and distribution of platinum-group mineral phases. A remobilisation of Pt, Pd, and Rh, together with Ni, Cu and possibly Fe as bisulfide and/or hydroxide complexes and deposition of metals by the reaction of the metal bearing hydrothermal fluid with chromite is proposed.     

Chromite-bearing spessartites from Kasuga-mura,Japan, and their bearing on possible mantle origin andesite

An unusual association of chromite and hornblende was found in the spessartites of andesite composition, occurring as a dike swarm associated with a Cretaceous granite batholith. The spessartites are largely porphyritic with phenocrysts of either hornblende or augite. One dike, comprising a finegrained spessartite, exhibits distinct chilled selvages of aphanitic facies. The chromites in the fine-grained and augite-spessartites are significantly higher in Cr/ (Cr+Al) than those occurring rarely as inclusions in the phenocrystic hornblendes in the hornblende spessartite, although both are similar in Mg/ (Mg+Fe), Fe₂O₃, and TiO₂. The phenocrystic hornblendes are titaniferous pargasite with high Mg/ (Mg+Fe), and differ in their higher octahedral Al from the groundmass hornblendes including those in the fine-grained spessartite. The crystallization sequence in the phenocrystic hornblende-bearing spessartites is Al-rich chromite, phenocrystic hornblende, and plagioclase without pyroxene, suggesting a high water content in the magma and the start of the crystallization at relatively high pressures. The finegrained spessartite from which the porphyritic spessartites have been derived by fractionation of dominant mafic minerals, has the high Mg-value and Cr content equivalent to those in primitive, undifferentiated basalts, although still andesitic in SiO₂ content. Chemically similar magnesian andesites, although uncommon, found in some orogenic calc-alkalic suites may represent a magma composition in equilibrium with mantle peridotite under the condition of high water pressures.     

变质橄榄岩中尖晶石的分形结构因子类型及其成因意义

利用沈步明（１９９３）提出的分形结构因子的新方法，计算了橄榄岩包体、方辉橄榄岩、纯橄榄岩和铬铁矿等六种尖晶石的分形结构因子。划分了四种尖晶石分形结构因子类型：Ａ１和Ｃｒ强变异型、Ａ１和Ｃｒ变异型、Ｍｇ和Ｆｅ２－变异型、Ｍｇ和Ｆｅ２－强变异型，结合尖晶石的地质产状讨论了这四种类型的成因意义。通过大量分形结构子的计算和图表．有力说明分形结构因子对于表征物质在空间分布的结构性和数据之间的相关性，是一种有效的定量化手段。在推测尖晶石和主岩的成因方面，尖晶石的Ｃｒ（Ａ１）和Ｍｇ（Ｆｅ２＋）的ＦＳＦ值图解更优于尖晶石的Ｃｒ／（Ｃｒ＋Ａ１）－Ｍｇ／（Ｍ＋Ｆｅ２－）图解。     

Melting and Melt Segregation in the Mantle Wedge above a Subduction Zone: Evidence from the Chromite-bearing Peridotites of the Miyamori Ophiolite Complex, Northeastern Japan

Chromite-bearing peridotites of the Ordovician Miyamori ophiolitecomplex exhibit spatial mineralogical variations on scales rangingfrom several centimeters to a few kilometers. The largest variationscorrespond to the entire structure of the complex, which featuresa layered zone of interstratified harzburgite, wehrlite, andvarious pyroxenites sandwiched between zones of unlayered harzburgiteand dunite containing only minor pyroxenite bands. All zonesexhibit the same deformation microstructures, tabular equigranularto porphyroclastic textures, and strong mineral aggregate lineation.Harzburgite from the unlayered zones is characterized by olivinevalues of 100Mg/(Mg+Fe)=91–93.5 and chromite values of100Cr/(Cr+Al+Fe³⁺)=40–75. These variables exhibit a positivecorrelation, which is typical of harzburgites and lherzolitesfrom the basal units of ophiolites and from xenoliths in alkalibasalts and kimberlites. The harzburgite is therefore interpretedas a residue from partial melting in the mantle. By contrast,harzburgite in the interlayered zone features olivine valuesof 100Mg/(Mg+Fe)=88–92 and chromite values of 100Cr/(Cr+Al+Fe3+)=40–60,and in this case the variables tend to show a negative correlationin any given locality and they partly overlap data from theintercalated wehrlite and dunite. The harzburgite of the layeredzone is interpreted as residual mantle that reacted with evolvedmelts that then crystallized as wehrlite and dunite. The harzburgitein the unlayered zones is more refractory than that in the layeredzone, even after removing effects of reaction. This differencecan be explained either by enhanced partial melting and meltextraction in the unlayered zones, possibly owing to the preferentialintroduction of a waterrich fluid, or by melt segregation fromthe unlayered zones and transfer to the layered zone in responseto a piezometric pressure gradient and compaction of a solidresidual matrix. Mineralogical evidence suggests that evolvedmelts migrated through conduits formed in the layered zone byfracturing or diapirism.     

铬尖晶石的晶胞参数与成因特征

铬尖晶石成分与晶胞、比重的关系1955年米赫耶夫提出尖晶石类总的晶胞公式为: (单位kx) 两价平均离子半径 三价平均离子半径。     

The origin of chromitites and related PGE mineralization in the Bushveld Complex: new mineralogical and petrological constraints

This article reports a study of chromitites from the LG-1 to the UG-2/3 from the Bushveld Complex. Chromite from massive chromitite follows two compositional trends on the basis of cation ratios: trend A—decreasing Mg/(Mg + Fe) with increasing Cr/(Cr + Al); trend B—decreasing Mg/(Mg + Fe) with decreasing Cr/(Cr + Al). The chromitites are divided into five stages on the basis of which trend they follow and the data of Eales et al. (Chemical Geology 88:261–278, 1990) on the behaviour of the Mg/Fe ratio of the pyroxene and whole rock Sr isotope composition of the environment in which they occur. Following Eales et al. (Chemical Geology 88:261–278, 1990), the different characteristics of the stages are attributed to the rate at which new magma entered the chamber and the effect of this on aAl₂O₃ and, in the case of stage 5, the appearance of cumulus plagioclase buffering the aAl₂O₃. The similarity of PGE profiles across the MG-3 and MG-4 chromitites that are separated laterally by up to 300 km and the variation in V in the UG-2 argue that the chromitites have largely developed in situ. Modelling using the programme MELTS shows that increase in pressure, mixing of primitive and fractionated magma, felsic contamination of primitive magma or addition of H₂O do not promote crystallization of spinel before orthopyroxene (in general they hinder it) and that the Cr₂O₃ content of the magma was of the order of 0.25 wt.%. Less than 20% of the chromite in the magma is removed before orthopyroxene joins chromite, which implies a >13-km thickness of magma for the Critical Zone. It is suggested that the large excess of magma has escaped up marginal structures such as the Platreef. The PGE profile of chromitites depends on whether sulphide accumulated or not along with chromite. Modelling shows that contamination of Critical Zone magma with a felsic melt will induce sulphide immiscibility, although not chromite precipitation. The LG-1 to LG-4 chromitites developed without sulphide, whilst those from the LG-5 upwards had associated liquid sulphide. Much of the sulphide originally in the LG-5 and above has been destroyed as a result of reaction with the chromite.     

Relict chromian spinels in Tulu Dimtu serpentinites and listvenite,Western Ethiopia: implications for the timing of listvenite formation

Serpentinites (massive and schistose) and listvenite occur as tectonic sheets and lenses within a calcareous metasedimentary mélange of the Tulu Dimtu, western Ethiopia. The massive serpentinite contains high-magnesian metamorphic olivine (forsterite [fo] ~96 mol%) and rare relict primary mantle olivine (Fo_90–93). Both massive and schistose serpentinites contain zoned chromian spinel; the cores with the ferritchromite rims preserve a pristine Cr/(Cr+Al) atomic ratio (Cr# = 0.79–0.87), suggesting a highly depleted residual mantle peridotite, likely formed in a suprasubduction zone setting. Listvenite associated with serpentinites of smaller ultramafic lenses also contain relict chromian spinel having identical Cr# to those observed in serpentinites. However, the relict chromian spinel in listvenite has significantly higher Mg/(Mg+Fe²⁺) atomic ratios. This suggests that a nearly complete metasomatic replacement of ultramafic rocks by magnesite, talc, and quartz to prevent Mg–Fe²⁺ redistribution between relict chromian spinel and the host, that is, listvenite formation, took place prior to re-equilibration between chromian spinel and the surrounding mafic minerals in serpentinites. Considering together with the regional geological context, low-temperature CO₂-rich hydrothermal fluids would have infiltrated into ultramafic rocks from host calcareous sedimentary rocks at a shallow level of accretionary prism before a continental collision to form the East African Orogen (EAO).     

Reequilibration of chromite within Kilauea Iki lava lake,Hawaii

Chromite mainly occurs as tiny inclusions within or at the edges of olivine phenocrysts in the 1959 Kilauea Iki lava lake. Liquilus chromite compositions are only preserved in scoria that was rapidly quenched from eruption temperatures. Analyses of drill core taken from the lava lake in 1960, 1961, 1975, 1979, and 1981 show that chromite becomes richer in Fe⁺², Fe⁺³, Ti and poorer in Mg, Al, Cr than the liquidus chromite. The amount of compositional change depends on the time elapsed since eruption, the cooling history of the sample, the extent of differentiation of the interstitial melt, and the position of the chromite inclusion within the olivine phenocryst. Compositional changes of the chromite inclusions are thought to be a result of reequilibration with the residual melt by cationic diffusion (Mg, Al, Cr outwards and Fe⁺², Fe⁺³, Ti inwards) through olivine. The changing chemical potential gradients produced as the residual melt cools, crystallizes and differentiates drives the reequilibration process. Major and minor element zoning profiles in olivine phenocrysts suggest that volume diffusion through olivine may have been the major mechanism of cationic transport through olivine. The dramatic compositional changes observed in chromite over the 22 years between eruption and 1981 has major implications for othe molten bodies.     

Base metal mineral segregation and Fe-Mg exchange inducing extreme compositions of olivine and chromite from the Xiadong Alaskan-type complex in the southern part of the Central Asian Orogenic Belt

The Xiadong Alaskan-type complex shares much in common with typical Alaskan-type complexes worldwide, while showing some unique features in terms of mineral compositions. Olivine from the Xiadong dunites is characterized by extremely high Fo component of 91.7–96.7 and anomalously negative correlation of Fo with NiO, while chromite is featured by high 100 × Fe³⁺/(Fe³⁺ + Cr + Al) (>70), high 100 × Fe²⁺/(Fe²⁺ + Mg) (>70), high 100 × Cr/(Cr + Al) (>90), low MnO (<0.6 wt%) and TiO₂ contents (<0.5 wt%). To investigate these particular features, we conducted petrographic observation and mineral composition analyses for the Xiadong dunite. A number of Fe and/or Ni sulfides and alloys occurring as inclusions in olivine and chromite indicate that base metal mineral segregation took place prior to crystallization of olivine and chromite and probably induced Fe and Ni depletions in olivine. The FeO and MgO variations in profile analyses from chromite to adjacent olivine are compatible with Fe-Mg exchange. The diffusion mechanism of Fe from olivine to chromite and Mg from chromite to olivine may have elevated both Fo of olivine and 100 × Fe²⁺/(Mg + Fe²⁺) ratio of chromite and further enhanced the decoupling of Fo and NiO in olivine. We thus suggest that base metal mineral segregation and Fe-Mg exchange play important roles in the extreme compositions of the Xiadong dunite. The Ni depletion of olivine and degree of Fe-Mg exchange between olivine and chromite may be used as indicators of mineralization in mafic-ultramafic intrusions.     

Alteration Mechanisms of Chromian-Spinel during Serpentinization at Wadi Sifein Area, Eastern Desert, Egypt

Wadi Sifein podiform chromite deposits, Central Eastern Desert of Egypt, are hosted by fully serpentinized peridotite that is a part of the dismembered Pan‐African ophiolite complexes. Relics of primary minerals and the chemical characters indicate that the ophiolitic rocks were derived from depleted mantle peridotite of harzburgite and subordinate dunite compositions. The mantle rocks were initially formed at a mid‐oceanic ridge and subsequently thrust at a supra‐subduction zone. The chromite mineralization at Wadi Sifein area displays either pod‐shaped bodies with massive and lumpy chromitite appearance or dissemination of chromian‐spinel in serpentinite matrix. The podiform chromitite exhibits a very limited compositional range in terms of Cr# [Cr/(Cr + Al) atomic ratio] and Mg# [Mg/(Mg + Fe) atomic ratio]. The chromian‐spinel, however, frequently displays optical and geochemical zoning. Four zones can be identified from core to edge: inner core representing the original composition of the chromian‐spinel; narrow Cr‐rich ferritchromit zone; wide ferritchromit zone; and outer Cr‐magnetite/magnetite zone. The zonation of chromian‐spinel is interpreted to be a result of serpentinization rather than magmatic or metamorphic processes. The geochemical data obtained from the chromitite and chromian‐spinel was statistically processed using discriminant and R‐mode factor analyses. Two trends, minor and major, were achieved considering the formation of ferritchromit. The minor trend is controlled by the redistribution of trivalent cations, where Cr₂O₃ increased on the expense mainly of Al₂O₃ and to less extent Fe₂O₃ to form zone II during the peak of serpentinization. The major trend of alteration, however, is explained by the exchange between Mg‐Fe²⁺ rather than Cr, Al, and Fe³⁺ to form zone III. Kammererite formation was accompanied the formation of zones III and IV at a 314°C temperature of formation.     

Podiform Chromitites in the Luobusa Ophiolite (Southern Tibet): Implications for Melt-Rock Interaction and Chromite Segregation in the Upper Mantle

The Luobusa ophiolite in the Indus—Yarlung Zangbo sutureof southern Tibet hosts the largest known chromite deposit inChina. The podiform chromitites occur in a well-preserved mantlesequence consisting of harzburgite with abundant lenses of dunite.The harzburgites have relatively uniform bulk-rock compositionswith mg-numbers [100 Mg/(Mg + Fe)] ranging from 89 to 91 andshow flat, unfractionated, chondrite-normalized platinum groupelement (PGE) patterns. Their accessory chromite varies widelyin cr-number [100Cr/(Cr + Al)] (18–66). These rocks areessentially residua left after extraction of mid-ocean ridgebasalt (MORB)-type magmas. The podiform chromitites displaynodular, massive, disseminated and banded textures and typicallyhave dunite envelopes that grade into the surrounding harzburgiteand diopsidic harzburgite with increasing pyroxene contents.They consist of relatively uniform chromite with high cr-numbers(74–82), have strongly fractionated, chondrite-normalizedPGE patterns with enrichment in Os, Ir and Ru relative to Rh,Pt and Pt, and are believed to have formed from a boniniticmagma produced by a second stage of melting. Dunites containaccessory chromite intermediate in composition between thoseof harzburgite and chromitite and are believed to be the productsof reaction between new boninitic magmas and old MORB-type peridotites.The melt-rock reaction removed pyroxene from the peridotitesand precipitated oli-vine, forming dunite envelopes around thechromitite pods. The melts thus became more boninitic in compositionand chromite saturated, leading to precipitation of chromitealone. The interplay of melt-rock interaction, chromite fractionationand magma mixing should lead to many fluctuations in melt composition,producing both massive and disseminated chromitites and phaselayering within individual podiform bodies observed in the Luobusaophiolite. KEY WORDS: boninitic magmas; dunite envelope; melt—rock interaction; MORB peridotities; podiform chromitites ^*Corresponding author. Present address: Department of Geology, Laurentian University, Sudbury, Ont, Canada P3E 2C6.     

Analysis of cation distribution in the octahedral sheet of dioctahedral 2:1 phyllosilicates by using inverse Monte Carlo methods

An inverse Monte Carlo (MC) method was developed to determine the distribution of octahedral cations (Al³⁺, Fe³⁺, and Mg²⁺) in bentonite illite–smectite (I–S) samples (dioctahedral 2:1 phyllosilicates) using FT–IR and ²⁷Al MAS NMR spectroscopies. FT–IR allows determination of the nature and proportion of different cation pairs bound to OH groups measuring the intensities of OH-bending bands. ²⁷Al MAS NMR data provide information about cation configuration because ²⁷Al MAS NMR intensity depends on Fe distribution. MC calculations based on FT–IR data alone show Fe segregation by short-range ordering (Fe clusters within 9 to 15?Å from a given Fe atom). Fe segregation increases with illite proportion. MC calculations based on IR and ²⁷Al NMR simultaneously yield similar configurations in which Fe clusters are smaller. The latter calculations fail to build appropriate cation distributions for those samples with higher number of illite layers and significant Fe content, which is indicative of long-range Fe ordering that cannot be detected by FT–IR and ²⁷Al MAS NMR. The proportion of Mg–Mg pairs is negligible in all samples, and calculations, in which the number of Mg atoms, as second neighbours, is minimised, create appropriate configurations.     

Formation of ferrian chromite in podiform chromitites from the Golyamo Kamenyane serpentinite,Eastern Rhodopes,SE Bulgaria: a two-stage process

The Golyamo Kamenyane serpentinite is a portion of a metaophiolite, located in the Upper High-Grade Unit of the metamorphic basement of the Eastern Rhodope Metamorphic Complex, SE Bulgaria. It consists of metaharzburgite and metadunite hosting layers of metagabbro and some chromitite bodies. All these lithologies were affected by ultrahigh-pressure (UHP) metamorphism and subsequent retrograde evolution during exhumation. Chromite from chromitites can be classified into four textural groups: (1) partly altered chromite, (2) porous chromite, (3) homogeneous chromite and (4) zoned chromite. Partly altered chromite shows unaltered, Al-rich cores with unit cell size of 8.255 Å and Cr# [Cr/(Cr + Al) atomic ratio] = 0.52–0.60, Mg# [Mg/(Mg + Fe²⁺) atomic ratio] = 0.65–0.70 and Fe³⁺/(Fe³⁺ + Fe²⁺) = 0.20–0.30, surrounded by porous chromite, with a cell size of 8.325 Å, Fe³⁺/(Fe³⁺ + Fe²⁺) < 0.20 and values of Cr# and Mg# evolving from 0.60 to 0.91 and 0.65–0.44, respectively, from core to rim. The chemical composition of porous chromite varies within the following ranges: Cr# = 0.93–0.96, Mg# = 0.48–0.35 and Fe³⁺/(Fe³⁺ + Fe²⁺) = 0.22–0.53. Its unit cell size is very constant (8.350 Å). Most pores in porous and partly altered chromite are filled with chlorite, which also occurs between chromite grains. Homogeneous chromite has Fe³⁺/(Fe³⁺ + Fe²⁺) = 0.55–0.66, Cr# = 0.96–0.99, Mg# = 0.32–0.19 and a cell size of 8.385 Å. The cores of zoned chromite are similar to those of partially altered chromite, but the rims are identical to homogeneous chromite. Although chlorite predominates in the silicate matrix of homogeneous and zoned chromite, it coexists with some antigorite, talc and magnesiohornblende. Mineral data and thermodynamic modeling allow interpretation of the alteration patterns of chromite as the consequence of a two-stage process developed during retrograde metamorphic evolution coeval with fluid infiltration. During the first stage, chromite reacts in the presence of fluid with olivine to produce chlorite and Cr- and Fe²⁺-rich residual chromite (ferrous chromite) at ~700 to ~450 °C. This dissolution–precipitation reaction involves continuous chromite mass loss resulting in the development of a porous texture. This stage takes place progressively on cooling under water-saturated and reducing conditions. The second stage mainly consists of the formation of homogeneous chromite with ferrian chromite composition by the addition of magnetite to the porous ferrous chromite during a late oxidizing hydrothermal event.     

The cumulus and post-cumulus evolution of chrome-spinels in ultrabasic layered intrusions: evidence from the Cuillin Igneous Complex,Isle of Skye,Scotland

Chrome-spinels from the layered Peridotilte Series of the unmetamorphosed, anorogenic 60 Ma Cuillin Igneous Complex, Isle of Skye, display a wide variety of compositions. Cumulus (within seams) chrome-spinels from the lowest exposed portion of the Peridotite Series exhibit features indicative of textural equilibrium, are rich in Al and Mg, and have low values of the ratio Cr/(Cr+Al). Cumulus chrome-spinels from higher up in the series are different from these: particularly, textural disequilibrium is evident, intercumulus plagioclase and olivine are present, and the chrome-spinels are rich in Cr, Fe and Ti, with high values of the ratio Cr/(Cr+Al). Intercumulus (dispersed) chrome-spinels tend towards anhedral forms and define enrichment trends towards Fe (both Fe²⁺ and Fe³⁺) with decreasing Mg, Cr and Al, and towards Al, with decreasing Fe²⁺ and Cr (and increasing Mg). Individual crystals are completely homogeneous and are devoid of reaction rims. The observed textural characteristics and compositional data of the chrome-spinels documented here suggest that the semi-quantitative peritectic reaction: aluminous chrome-spinel + meltplagioclase + olivine + chromian chrome-spinel, is responsible for the observed parageneses, and that both the environment of crystallization (eumulus or intercumulus) and the role of plagioclase ±olivine crystallization are critical parameters for this geochemical trend in spinels within upper crustal magmatic systems. The effects of pyroxene crystallization on the development of this geochemical trend are also considered. This investigation highlights the need to consider the role of post-cumulus mineral-melt reactions and their influences upon the final compositions of major oxide and silicate phases within layered intrusions.     

Contribution to the crystal chemistry of beryl

Chemical analyses and crystallographic and some optical data have been obtained for 28 samples of beryl from Bahia State, Brazil. The larger range of variability in the chemical composition is shown by Mg, Fe and Li. Sodium is the more diffuse alkali element. Potassium is always very limited. Calcium appears in noticeable amounts only in three samples. The Ti, Cr, Rb, and Cs elements were also tested. The samples studied here can be defined as sodium-potassium beryls with low alkali content. Unit cell parameters show the following ranges: a=9.210–9.245 and c=9.190–9.220 Å. From a statistical analysis of these data it may be seen that: an increase of Fe and (Fe+Mn+Mg) percentage has a positive correlation with a, but no influence on c, which in turn has a close positive correlation with Li and is negatively correlated with Be. Less negative correlations also exist between the pairs {Be, Li}, {a,Al}, {Al, Fe} and {Al, (Fe+Mn+Mg)}. A positive correlation also exists between sodium and the parameter a.     

阿尔巴尼亚布尔其泽纯橄岩壳中橄榄石的包裹体研究

阿尔巴尼亚布尔其泽纯橄岩壳非常新鲜,主要由橄榄石、尖晶石和单斜辉石等矿物组成.其中橄榄石存在单斜辉石和铬尖晶石（磁铁矿）共生包裹体现象,包裹体矿物粒度在1~10 μm,有些甚至为纳米级200~500 nm.纯橄岩橄榄石的Fo值为94.7~96.0,铬尖晶石的Cr#为76.5~82.4,远高于蛇绿岩地幔橄榄岩中常见纯橄岩的铬值（Cr#>60）.基于前人研究结果,提出这种现象是由于亏损方辉橄榄岩与含钛、铬、铁熔体发生交代作用,从而形成橄榄石的固溶体并存在Ti4+、Al3+、Ca2+、Fe3+,而部分Cr3+进入铬尖晶石结晶.后期由于岩体在抬升过程中降温,橄榄石中混溶的组分析出包裹体形成磁铁矿和铬尖晶石.并且依据铬尖晶石-橄榄石的矿物化学成分,识别出岩体内方辉橄榄岩相对较低的部分熔融程度约为30%~40%,纯橄岩部分熔融程度约为40%,表明不同岩相间其形成背景存在明显差异.因此,认为布尔奇泽蛇绿岩具有多阶段的过程,首先是在洋中脊环境下经历部分熔融作用形成了方辉橄榄岩,后受到俯冲环境（SSZ）的岩石-熔体反应生成更富Mg、Si和Cr等的熔体,致使地幔橄榄岩高度部分熔融,形成此类纯橄岩.      

Distribution of major and minor constituents between mica and host ultrabasic rocks,and between zoned mica and zoned spinel

Ultrabasic rocks with Niggli values for si between 26 and 100, and k between 0.6 and 1 have been chosen for the study of the distribution of 23 major and minor constituents between phlogopite and the host rock. The rocks include kimberlites, carbonatites and lamprophyres which contain abundant zoned micas and a few zoned spinels. Chemical variations in zoned mica and spinel were determined by electron probe microanalysis, and distribution coefficients for Al, Ti, Fe, Cr, Mg, Mn, and Ni were calculated between mica and spinel for the average composition of the mica and spinel, and for chemically-related and adjacent zones. The spinel changes from chromite in the centre to chromian ulvöspinel in the rim, and the mica from chromian phlogopite, through titaniferous biotite to chromium-poor phlogopite. Distribution coefficients for Cr and Fe between adjacent zones in spinel (0.8 and 0.02 for Cr; 1.1 and 3.1 for Fe) and in mica (0.4 and 0.1 for Cr; 2 and 0.3 for Fe) show more variation than the distribution coefficients between the cogenetic phlogopite and chromite (0.04 for Cr, 0.2 for Fe), and biotite and ulvöspinel (0.08 for Cr; 0.2 for Fe). It is concluded that distribution coefficients for major and minor constituents between related zones are more meaningful than those calculated from the average chemical composition of zoned minerals.The results indicate that phlogopite is the principal carrier of potassium, rubidium, aluminium, fluorine and primary water in kimberlite and carbonatite. Minor elements, such as chlorine, lithium, barium, nickel, chromium, titanium and zinc are present mainly in the mica, whereas sodium, strontium, calcium, carbon and manganese are more abundant in other minerals.     

Surface chemical mechanisms of inadvertent recovery of chromite in UG2 ore flotation: Residual layer identification using statistical ToF-SIMS analysis

In the UG2 ore (Bushveld Complex, South Africa) flotation, normally more than 3% of the gangue minerals, principally chromite with talc and pyroxene, report to the concentrate diluting the PGM recovery and contributing to subsequent processing costs. Previous studies have identified residual talc-like layers on orthopyroxene surfaces in Merensky ore flotation contributing to inadvertent flotation of relatively large particles (20–150 µm) of this mineral. Chromite (75–150 µm) from flotation of UG2 ore has been similarly examined. Statistical comparison of ToF-SIMS analysis of particles from concentrate and tails reveals no significant difference in Cu, Pb, Ni and collector (IBX and DTP) signals between these streams but surface exposure of Mg and Si is favoured in the concentrate. The flotation rate of coarse chromite correlates with the exposures of magnesium and silicon in patches on the chromite surface; higher exposures give earlier flotation. Conversely, there is a negative correlation with signals corresponding to the chromite surface, i.e. Cr, Fe, Al. Flotation of chromite without collector has confirmed this statistical discrimination. Hydrophobic talc-like residual layers, similar to those found on orthopyroxene surfaces, probably from partial alteration, explain this flotation mechanism.     

Chromite deposits in the ophiolite complex of Northern Oman

In the ophiolite series of N. Oman numerous small and large elongated podiform chromite deposits occur within the peridotite complex at a certain level of 100–200 meters below the layered zone, which grades into the overlying gabbro complex. The chromite grains were corroded after their deposition; their composition shows a large variation in Cr/Al ratio and a small variation in Cr/Fe ratio. The chromium is thought to have been liberated from clino-pyroxenes during partial melting of lherzolitic upper mantle material. Precipitation of large quantities of chromite at the specific level below the gabbro complex is related to the plagioclase- /spinel-lherzolite transition, but the influence of a change in oxygen fugacity is also considered.