Crystal morphology and surface structures of orthorhombic MgSiO3 in the presence of divalent impurity ions

Many rheological and transport properties of rocks are determined by the grain boundary structures of their constituent minerals. These grain boundaries often also hold a high concentration of dopant ions. Here, as a first step towards modelling the transport and rheological behaviour of the lower mantle, we report the results of lattice static simulations on the surface structures of Fe²⁺ and Ca²⁺-doped orthorhombic MgSiO₃-perovskite. For all the surfaces we studied, the energies of the doped structures are lowered, sometimes by more than 1 J/m², with respect to the pure surfaces. From our calculated crystal morphologies, we predict that the grains become more tabular as the concentration of Fe²⁺ ions increases, while under equilibrium conditions the grains are cubic. By calculating the replacement energies of Mg²⁺ by Fe²⁺ and Ca²⁺ ions in the six outermost surface layers, we conclude that these divalent ions would tend to segregate onto the crystal surfaces. We suggest, therefore, that the grain boundary structure and rheology of MgSiO₃-perovskite dominated rocks will be strongly affected by the presence of minor elements in the lower mantle.     

EPR of V4+ and Fe3+ in titanites

EPR investigations of yellow-green titanites from four localities showed presence of V⁴⁺ and Fe³⁺. They were investigated at X-band and room temperature. For V⁴⁺ the principal values of the g matrix and ⁵¹V hyperfine splitting and the directions of their principal axes indicate presence of a VO²⁺ ion substituting for Ti. Due to a high zero-field splitting only approximate values of the fine structure parameters of Fe³⁺ could be determined. With directions of their principal axes very similar to those of V⁴⁺ this ion must also substitute for Ti. Unusually large linewidths for both ions with little variation for samples from different localities are ascribed to the reported domain structure of titanites and accumulation of impurities in the domain boundaries. While for Fe³⁺ a small variation of the fine structure parameters explains this broadening, for V⁴⁺ a distribution of g-factors equal to its total anisotropy must be responsible whereas the V=0 bond length is remarkably constant. Due to preferential incorporation of impurities in the grain boundaries a contribution of dipolar broadening cannot be excluded.     

Iron-Titanium Oxide Minerals in the Bjerkrem-Sogndal Massif, South-western Norway

Ilmenite and magnetite are investigated from the point of viewof their distribution, microtexture, and chemical composition(major and minor elements) in the Bjerkrem-Sogndal massif (Egersundarea, South-Rogaland, SW. Norway). This massif is an igneouslayered synkinematic lopolith made up of cumulates of the anorthosite-mangeritesuite. The lower part of the massif presents a rhythmic structure. The microtextures of ilmenite result from simple exsolutionof ilmenite-hematite solid solutions. Magnetite contains intergrowthsof ilmenite formed by oxidation-exsolution of ulv?spinel-magnetitesolid-solutions. In the stratigraphic sequence, on a large scale, ilmenite appearsfirst alone, and is then accompanied by magnetite; its hematitecontent decreases towards the top of the massif, while the titaniumcontent of the magnetite increases. On the scale of the rhythms,similar trends but of lesser amplitude are also observed. Evidence of deuteric readjustment of the orthomagmatic compositionof the two oxides is provided (1) by the observation of microtexturesat the contact between grains (zoning of primary ilmenite andrim of secondary ilmenite) (2) by the existence of differencesin chemical composition between isolated grains and grains incontact, and (3) by the determination of the equilibrium temperatureby means of the Buddington and Lindsley geothermometer. Reconstitution of the T-fo₂ orthomagmatic conditions in twoparticular levels of the massif shows that the reducing characterof the magma increases during differentiation. The sudden changesin the oxide assemblage at the base of the rhythms reflect asudden increase in the fo₂ of the magma. These increases, asshown by variation in Cr, Ni, and Co, are due to recurrencesof the basic character of the magma. The variations of the minor elements Mn, V, Ga, and Zn are interpretedin terms of the influence of the deuteric readjustment. It followsthat the ratios Mn/Fe²⁺, Ga/Fe³⁺, and Zn/Fe²⁺ increase and thatthe ratio V/Fe³⁺ decreases in the magma in the course of differentiation.The distribution of Mn between ilmenite and magnetite is discussed. Intermittent supplies of undifferentiated magma are proposedas the geological mechanism controlling the chemical recurrencesassociated with the rhythmic structure.     

Weathering of ilmenite from Chavara deposit and its comparison with Manavalakurichi placer ilmenite,southwestern India

The magnetic fractions of ilmenite from the beach placer deposit of Chavara, southwest India have been studied for mineralogical and chemical composition to assess the range of their physical and chemical variations with weathering. Chavara deposit represents a highly weathered and relatively homogenous concentration. Significant variation in composition has been documented with alteration. The most magnetic of the fractions of ilmenite, separated at 0.15 Å, and with a susceptibility of 3.2 × 10^?6 m³ kg^?1, indicates the presence of haematite–ilmenite intergrowth. An iron-poor, titanium-rich component of the ilmenite ore has been identified from among the magnetic fractions of the Chavara ilmenite albeit with an undesirably high Nb₂O₅ (0.28%), Cr₂O₃ (0.23%) and Th (149 ppm) contents. The ilmenite from Chavara is compared with that from the nearby Manavalakurichi deposit of similar geological setting and provenance. The lower ferrous iron oxide (2.32–14.22%) and higher TiO₂ (56.31–66.45%) contents highlight the advanced state of alteration of Chavara. This is also evidenced by the relatively higher Fe³⁺/Fe²⁺ ratio compared to Manavalakurichi ilmenite. In fact, the ilmenite has significantly been converted to pseudorutile/leucoxene.     

Location, valence states, and oxidation mechanisms of iron in eudialyte-group minerals from Mössbauer spectroscopy

A representative collection of structurally characterized eudialyte-group minerals (EGM) with varying relative concentrations of Fe²⁺ and Fe³⁺ ions from several localities was investigated at room temperature by ⁵⁷Fe Mössbauer spectroscopy coupled with magnetometric, optical, and X-ray powder diffraction methods. To refine the Mössbauer parameters of isomer shift and quadrupole splitting for Fe²⁺ and Fe³⁺ in different types of coordination polyhedra (planar quadrangle, square pyramid, and distorted octahedron) in EGM structures, we also collected Mössbauer parameters for gillespite and labuntsovite. The main purpose of this work is to determine the location of Fe³⁺ in different sites in typical eudialyte, rastsvetaevite, georgbarsanovite, and some of their naturally hydrated and heat-treated analogs, and investigate the kinetics and oxidation mechanisms of iron ions in their structures. Our study has confirmed the presence of Fe²⁺ ions in the planar quadrangle and square pyramid in primary eudialytes, as well as the presence of Fe³⁺ ions in the square pyramid and distorted octahedron in primary, naturally hydrated, and heat-treated eudialytes. According to this study, hydrated eudialytes are characterized by the location of Fe³⁺ ions mainly in octahedra with OH groups and/or water molecules at trans vertices, while heat-treated eudialytes are characterized by their location in square pyramids with an O^2? anion at the apical vertex.     

Coexisting hornblendes and biotites from Precambrian gneisses of the south coast of Western Australia

The compositions of coexisting hornblendes and biotites from amphibolite and granulite facies gneisses from the south coast of Western Australia were controlled by host rock composition, paragenesis, metamophic grade, pressure, and oxygen fugacity. The Mg/(Mg + Fe²⁺) and Mn/Fe²⁺ ratios in both minerals and possibly the Al^vi contents of the hornblendes are related to host rock compositions. Metamorphic grade appears to influence, perhaps only indirectly, the Ti, Mn, and Fe³⁺ contents of both minerals and possibly the hornblende Ca content. The higher Ti and lower Mn contents of the granulite facies hornblendes and biotites are attributed to their coexistence with pyroxenes, whereas their lower Fe³⁺/(Fe²⁺ + Fe³⁺) ratios are probably due to lower oxygen fugacity in the granulite facies environment. Grade-related colour variations in both minerals were controlled by their Ti/Fe²⁺ and Fe³⁺/(Fe²⁺ + Fe³⁺ ratios. The relatively low Al^vi contents of the hornblendes suggest low- to moderate-pressure metamorphism.Variations in element distribution coefficients are related to variations in mineral compositions rather than metamorphic grade. Thus K_{D(Al^iv ?Si)} is related to the Al^iv andedenite alkali contents of the hornblendes, K_{D(Fe²⁺ ?Mg)} to the distributions of Al^iv ?Si and Al^vi + Ti + Fe³⁺, K_D(Mn) to the Mn contents of both minerals, and K_D(Al^vi) to the Al^vi contents of the biotites.     

Fe sorption onto nontronite (NAu-2)

The sorption of ferrous iron to a clay mineral, nontronite (NAu-2, a ferruginous smectite), was investigated under strictly anoxic conditions as a function of pH (3-10), Fe²⁺ concentration (0.01-50 mM), equilibration time (1-35 days), and ionic strength (0.01-0.5 M NaClO₄). The surface properties of NAu-2 were independently characterized to determine its fixed charge and amphoteric site density in order to interpret the Fe²⁺ sorption data. Fe²⁺ sorption to NAu-2 was strongly dependent on pH and ionic strength, reflecting the coupled effects of Fe²⁺ sorption through ion exchange and surface complexation reactions. Fe²⁺ sorption to NAu-2 increased with increasing pH from pH 2.5 to 4.5, remained constant from pH 4.5 to 7.0, increased again with further increase of pH from pH 7.0 to 8.5, and reached a maximum above pH 8.5. The Fe²⁺ sorption below pH 7.0 increased with decreasing ionic strength. The differences of Fe²⁺ sorption at different ionic strengths, however, diminished with increasing equilibration time. The Fe²⁺ sorption from pH 4.5 to 7.0 increased with increasing equilibration time up to 35 days and showed stronger kinetic behavior in higher ionic strength solutions. The kinetic uptake of Fe²⁺ onto NAu-2 is consistent with a surface precipitation mechanism although our measurements were not able to identify secondary precipitates. An equilibrium model that integrates ion exchange, surface complexation and aqueous speciation reactions reasonably well describes the Fe²⁺ sorption data as a function of pH, ionic strength, and Fe²⁺ concentration measured at 24 h of equilibration. Model calculations show that the species Fe(OH)⁺ was required to describe Fe²⁺ sorption above pH 8.0 satisfactorily. Overall, this study demonstrated that Fe²⁺ sorption to NAu-2 is affected by complex equilibrium and kinetic processes, likely caused by surface precipitation reactions.     

The crystal chemistry and mössbauer study of schorlomite

Six schorlomite samples with TiO₂ contents varying between 9.70 and 15.34 weight percent were studied by means of Mössbauer spectroscopy and chemical analysis. The measured Mössbauer spectra have complex shapes. The spectra of these samples were fitted with six doublets, which can be assigned to ^VIIIFe²⁺, ^VIFe²⁺, ^VIFe³⁺, ^IVFe³⁺ and two electron delocalizations, ^IVFe³⁺ ? ^VIIIFe²⁺ and ^IVFe³⁺ ? ^VIFe²⁺, respectively. The assignment of iron absorption doublets and their Mössbauer parameters are discussed in terms of the single crystal structure data of one of the samples studied in this work. Cation distributions are also given. The occupancies of cations at the tetrahedral (Z) site are Fe³⁺>Al³⁺, Ti⁴⁺, and the relative enrichments at Z site are always Fe³⁺>Ti⁴⁺. Most of the six samples contain Ti³⁺. Ti³⁺/ΣTi ratios range from 1.43 percent to 6.40 percent. Fe²⁺/ΣFe ratios vary from 8.84 percent to 11.31 percent. Four types of substitution must be considered for Ti entering the garnet structure.     

Optical absorption spectra and Fe distribution in the structures of Li-Fe micas

The paper proposes pioneering data on the polarized optical absorption spectra of Li-Fe micas: intermediate members of the siderophyllite-zinnwaldite-polylithionite and annite-protolithionite-zinnwaldite-trilithionite series with variable Fe and Li proportions and Li- and Fe-bearing muscovite (phengite). Based on the analysis of structural data, the complicated structure of the Fe²⁺ → Fe³⁺ charge transfer band in the mica structures is explained, and arguments are presented to justify the ascribing of its shortwave component (CTB-1, ν = 17200-14900 cm^?1) to charge transfer in the pair Fe²⁺(M2) → Fe³⁺(M2) and the longwave component (CTB-2, ν = 14200-13600 cm^?1) to charge transfer in the pair Fe²⁺(M1) Fe³⁺(M2). It is demonstrated that the anomalous shift of the superposition of two-component CTB toward the shortwave region, to 17000 cm^?1, results from a decrease in the length of oxygen edges between adjacent M2M2 and M1M2 tetrahedrons when Li is accommodated in the mica structure. The first data are presented on the spectrum of Fe²⁺ ions in large distorted M3(M1) tetrahedrons (OAC Fe²⁺ II) in hetero-octahedral Li micas (zinnwaldite), and the behavior of the corresponding absorption bands at 11400 and 8000 cm^?1 is determined. It is proved that characteristics of the optical spectra of Fe²⁺ ions can be used as an indicator of the structure of the octahedral layer in the mica structures. Results of the comparative analyses of spectral parameters of the Fe²⁺ → Fe³⁺ charge transfer, crystal field spectra of Fe²⁺ ions, and the crystal-chemical characteristics controlling them in micas of the polylithionite-siderophyllite series are completely consistent with the character of cation ordering in the crystal structures of these micas determined by X-ray diffraction analysis.     

Macrocrystic corundum and Fe–Ti oxide minerals entrained in alkali basalts from the Eger (Ohře) Rift: Mg−Fe3+-rich ilmenite as tracer of an oxidized upper mantle

Macrocrysts of corundum, ilmenite, and spinel-group minerals from alluvial deposits of the Eger Rift were studied for composition, texture, and mineral inclusions. All macrocrysts show usually magmatic corrosion textures indicating disequilibrium with the transporting alkali-basalt magma. Corundum grains, exclusively sapphires, were classified by trace-element signatures as magmatic and metamorphic types. Some sapphire grains show erratic compositions that may have resulted from a metasomatic overprint. The inclusion inventory of magmatic corundum suggests crystallization from a differentiated alkaline silicate melt. Corundum itself was never observed as an inclusion mineral. Magnesium- and Fe³⁺-rich ilmenite, described as typical mantle-derived species, is the dominant heavy mineral in almost all alluvial deposits of the Eger Rift. Most discrete macrocrysts are similar in appearance and composition to kimberlite- and basanite-related ilmenite. Ilmenite included in alluvial corundum and zircon grains differ from the bulk of discrete ilmenite grains by larger concentrations of Nb and Mn. The mantle origin of the Mg–Fe³⁺-rich ilmenite is confirmed by compositional and thermo-barometric comparison with ilmenite from clinoproxenitic and hornblenditic xenoliths, which probably originated in the Moho region. The Fe–Ti two-oxide geothermometry and oxygen-barometry of coexisting ilmenite–magnetite pairs yield equilibrium temperatures between 900 and 1,080 °C and oxygen fugacities log₁₀fO₂ between ?0.1 and 1.1 (relative to the NNO buffer), which indicate that the upper mantle as well as the mantle/crust transition zone below the rift is at least partially oxidized. The ilmenite macrocrysts were transported from the source region to the surface by explosive alkali-basalt magmas, as implied by the presence of basaltic-pipe breccias in close vicinity to some placer deposits.     

Partition of major and minor elements and equilibration in coexisting pyroxenes

The partition of Si, Al, Ti, Fe³⁺, Mg, Fe²⁺, Mn, Ca and Na between coexisting Ca-rich and Ca-poor pyroxenes from a wide variety of igneous and metamorphic rocks have been investigated systematically. Many of the distributions, and especially those for the partition of Ti, Mg, Fe²⁺, Mn and Na, indicate characteristic trends for pyroxenes from the various petrologic groups identified. The partition of Mg, Fe²⁺ and Mn correlate with inferred cooling rates, the partition co-efficients of pyroxenes from extruded and other quickly cooled rocks most nearly approaching unity. In contrast, the partition of Si and Ti and the absolute amounts of Al may be related to the physicochemical conditions prevailing during original crystallisation; Ti being particularly relatively enriched in Ca-rich pyroxenes of ultramafic associations. The trends of the compositions of the Ca-rich pyroxenes plotted in the pyroxene quadrilateral also correlate with cooling rates and comparison with the limited data available on the phase relations of coexisting pyroxenes suggests that sub-solidus chemical readjustments have occurred in both phases.     

Optical absorption of electronic Fe–Ti charge-transfer transition in natural andalusite: the thermal stability of the charge-transfer band

Differently colored natural Brazilian andalusite crystals heat-treated under reducing and oxidizing conditions were analyzed by optical spectroscopy. The intensity of a broad intense band at around 20,500 cm⁻¹ in the optical absorption spectra of all color zones of the sample is proportional to the product of Ti- and Fe-concentrations and herewith proves its attribution to electronic Fe²⁺/Ti⁴⁺ IVCT transition. The band is strictly E||c-polarized, causing an intense red coloration of the samples in this polarization. The polarization of the Fe²⁺/Ti⁴⁺ IVCT band in andalusite, E||c, shows that the electronic charge-transfer process takes place in Al–O octahedral groups that share edges with neighbors on either side, forming chains parallel to the c-axis of the andalusite structure. Under thermal treatments in air, the first noticeable change is some intensification of the band at 800°C. However, at higher temperatures its intensity decreases until it vanishes at 1,000°C in lightly colored zones and 1,100°C in darkly colored ones. Under annealing in reducing conditions at 700 and 800°C, the band also slightly increases and maintains its intensity at treatments at higher temperatures up to 1,000°C. These results demonstrate that weakening and disappearance of the Fe²⁺/Ti⁴⁺ IVCT band in spectra of andalusite under annealing in air is caused by oxidization of Fe²⁺ to Fe³⁺ in IVCT Fe²⁺/Ti⁴⁺-pairs. Some intensification of the band at 800°C is, most probably, due to thermally induced diffusion of Fe²⁺ and Ti⁴⁺ in the structure that leads to aggregation of “isolated” Ti⁴⁺ and Fe²⁺ ions into Fe²⁺–Ti⁴⁺-pairs. At higher temperatures, the competing process of Fe²⁺ → Fe³⁺ oxidation overcomes such “coupling” and the band continues to decrease. The different thermal stability of the band in lightly and darkly colored zones of the samples evidence some self-stabilization over an interaction of Fe²⁺/Ti⁴⁺-pairs involved in IVCT process.     

Magnetite, ilmenite and ulvite in rocks and ore deposits: petrography, microprobe analyses and genetic implications

Summary ?Detailed petrographic studies and microchemical analyses of titanomagnetite from igneous and metamorphic rocks and ore deposits form the basis of this investigation. Its aim is to compare the data obtained and their interpretations with the experimentally deduced subsolidus oxidation-exsolution model of Buddington and Lindsley (1964). The results are also considered relevant for the interpretation of compositional variations in black sands which are recovered for titanium production. The arrangement of the samples investigated is in accordance with textural stages C1 to C5 caused by subsolidus exsolution with increasing degrees of oxidation (Haggerty, 1991). Stage 1 is represented by two types of optically homogeneous TiO₂-rich magnetite: a. An isotropic type considered to represent solid solutions of magnetite and ulvite containing between 5.2 to 27.5 wt% TiO₂ corresponding to about 14.7 to 77.7 mol% Fe₂TiO₄ in solid solution with magnetite. The general formula of this type is Fe²⁺ _1+x Fe³⁺ _2−2x Ti _x O₄ (x = 0.0–1.0). b. The second type which has not been reported so far is anisotropic and shows complex internal twinning resembling inversion textures. It is thus attributed to inversion of a high-temperature ilmenite modification (with statistical distribution of the cations) which forms solid solutions with magnetite. TiO₂ varies between 9.3 and 24.5 wt% corresponding to about 17.2 to 43.6 mol% ilmenite in solid solution with magnetite. This type is interpreted as a cation-deficient spinel with the general formula Fe²⁺ _{12/12 + 1/4x}Fe³⁺ _{24/12 − 3/2x} □ _{0 + 1/4x} Ti _x O₄ (x = 0.0–16/12). Isotropic and anisotropic homogeneous magnetites occur in volcanic rocks only; the homogeneity of the solid solutions was explained by fast cooling which prevented the development of exsolution textures. Stages 2 and 3 are represented by magnetite with or without ulvite. The magnetite host contains ilmenite lamellae forming trellis and sandwich textures. In contrast to the requirement of the oxidation-exsolution model, the ilmenite lamellae are concentrated exclusively in the cores of the host crystals. The reverse host-guest relationship may also occur. Stages 4 and 5 are identical with thermally generated martite (= martite due to heating). The textures are characterized by very broad lamellae of ferrian ilmenite or titanohematite dominantly concentrated along the margins of the host crystals. Thermally generated martite is restricted to subsolidus-oxidation reactions. The ilmenite lamellae of trellis and sandwich textures contain low Fe₂O₃-concentrations (average 4.8 mol%; to a maximum of 8.3), whereas the Fe₂O₃-content of thermally generated martite is between 32 to 71 mol%. With respect to the Fe₂O₃-concentrations in the ilmenite lamellae, no transition between the two types was observed. The results of this paper show that the widely accepted oxy-exsolution model of Buddington and Lindsley (1964) which is based on experimental results can – with the exception of thermally generated martite – not explain the tremendous variety of magnetite–ilmenite–ulvite relationships in natural rocks and ore deposits. Received October 16, 2001; accepted May 2, 2002     

Thermodynamic and magnetic properties of surface Fe<Superscript>3+</Superscript> species on quartz: effects of gamma-ray irradiation and implications for aerosol–radiation interactions

Samples of a natural amethyst, pulverized in air, and irradiated for gamma-ray doses from 0.14 to 70 kGy, have been investigated by powder electron paramagnetic resonance (EPR) spectroscopy from 90 to 294 K. The powder EPR spectra show that the surface Fe³⁺ species on the gamma-ray-irradiated quartz differ from its counterpart without irradiation in both the effective g value and the observed line shape, suggesting marked radiation effects. This suggestion is supported by quantitatively determined thermodynamic properties, magnetic susceptibility, relaxation times, and geometrical radius. In particular, the surface Fe³⁺ species on gamma-ray-irradiated quartz has larger Gibbs and activation energies than its non-irradiated counterpart, suggesting radiation-induced chemical reactions. The shorter phase-memory time (T _m) but longer spin–lattice relaxation time (T ₁) of the surface Fe³⁺ species on the gamma-ray-irradiated quartz than that without irradiation indicate stronger dipolar interactions in the former. Moreover, the calculated geometrical radius of the surface Fe³⁺ species on the gamma-ray-irradiated quartz is three orders of magnitude larger than that of its counterpart on the as-is sample. These results provide new insights into radiation-induced aerosol nucleation, with relevance to atmospheric cloud formation and global climate changes.     

加热白云母的穆斯堡尔效应及光学吸收光谱的研究

近年来,层状硅酸盐加热后氧化态的变化以及阳离子在各晶位的迁移引起了人们的兴趣,由于穆斯堡尔效应和光学吸收光谱是测定铁以及某些过渡族离子的氧化状态变化及位置分布的最有效的方法,因此,一些研究主要集中在黑云母等层状硅酸盐的穆斯堡尔效应和光学吸收光谱的研究上(Tripathi et al.,1978;Hogg et al.,1975)。     

Is iron redox cycling in a high altitude watershed photochemically or thermally driven?

Many studies have shown that the concentration of aqueous Fe²⁺ increases in surface waters during exposure to sunlight and attribute this phenomenon either to photoreductive dissolution of ferric minerals/colloids or to ligand-to-metal charge transfer within organic complexes of Fe³⁺. In a multi-summer study of iron redox cycling in a relatively high pH stream (Middle Crow Creek, MCC) that drains a mostly-granitic watershed at an altitude of 2400 m, aqueous Fe³⁺ (not Fe²⁺) concentrations were correlated with both sunlight and temperature. A steady state model fails to explain the [Fe²⁺] and [Fe³⁺] data from this stream. However, Fe²⁺ concentrations can be explained using a simple kinetic model in which rate constants for oxidation and reduction were obtained by fitting data from in situ oxidation experiments, including first-order thermal (nonphotochemical) reduction of Fe³⁺. Rate constants obtained from experiments in the dark result in too much Fe²⁺ to match the data from illuminated experiments, requiring a net photooxidation process to explain [Fe³⁺] measured in MCC. The organic content of MCC results in high concentrations of Fe–DOM complexes that not only act as a reservoir contributing to daily changes in [Fe_tot] as measured by our methods, but whose photochemistry may contribute highly oxidizing reactive oxygen species to the stream. In situ studies suggest that photochemical reduction of organically bound Fe³⁺ occurs, followed by thermal release of Fe²⁺ to the water column and subsequent rapid re-oxidation.     

Alteration patterns of chromian spinels from La Cabaña peridotite,south-central Chile

La Cabaña peridotite is part of a dismembered ophiolite complex located within the metamorphic basement of the Coastal Cordillera of south-central Chile, and is the only location in Chile were Cr-spinels have been described so far. The La Cabaña peridotite is part of the Western Series unit, which comprises meta-sedimentary rocks, metabasites, and serpentinized ultramafic rocks. This unit has been affected by greenschist-facies metamorphism with reported peak PT conditions of 7.0–9.3 kbar and 380°–420 °C. Within La Cabaña peridotite Cr-spinels are present in two localities: Lavanderos and Centinela Bajo. In Lavanderos, Cr-spinel occurs in small chromitite pods and as accessory/disseminated grains with a porous or spongy texture in serpentinite, whereas in Centinela Bajo Cr-spinel is present as accessory zoned grains in partly serpentinized dunites, and in chromitite blocks. All Cr-spinels display variable degrees of alteration to Fe²⁺-rich chromite with a variation trend of major elements from chromite to Fe²⁺-rich chromite similar to those observed in other locations, i.e., an increase in Fe₂O₃ and FeO, a decrease in Al₂O₃ and MgO. Cr₂O₃ content increases from chromite to Fe²⁺-rich chromite in chromitite pods from Lavanderos and chromitite blocks from Centinela Bajo, but decreases in ferrian chromite zones in accessory grains from Centinela Bajo. The minor element (Ti, V, Zn, Ni) content is mostly low and does not exceed 0.4 wt.%, with the exception of MnO (<0.9 wt.%), which shows a correspondence with increasing degree of alteration. Cr# (Cr/Cr?+?Al) versus Mg# (Mg/Mg?+?Fe²⁺) and Fe³⁺/Fe³⁺+Fe²⁺ versus Mg# plots are used to illustrate the Cr-spinel alteration process. Overall, the Cr-spinels from Lavanderos (chromitite pods and disseminated grains) exhibit Cr# values ranging from 0.6 to 1.0, Mg# (Mg/Mg?+?Fe²⁺) below 0.5, and (Fe³⁺/Fe³⁺+Fe²⁺) <0.4. Cr-spinels from chromitites in Centinela Bajo have Cr# and Mg# values that range from 0.65 to 1.0, and 0.7-0.3, respectively, and (Fe³⁺/Fe³⁺+Fe²⁺)?3+/Fe³⁺+Fe²⁺) ratio is less than 0.4 in chromite cores and Fe²⁺-rich chromite, and >0.5 in ferrian chromite and Cr-magnetite. Interpretation of the data obtained and Cr-spinel textures indicate that the alteration of Cr-spinel is a progressive process that involves in its initial stages the reaction of chromite with olivine under water-saturated conditions to produce clinochlore and Fe²⁺-rich chromite. During this stage the chromite can also incorporate Ni, Mn, and/or Zn from the serpentinization fluids. As alteration progresses, Fe²⁺-rich chromite loses mass resulting in the development of a spongy texture. In a later stage and under more oxidizing conditions Fe³⁺ is incorporated in chromite/Fe²⁺-rich chromite shifting its composition to an Fe³⁺-rich chromite (i.e., ferrian chromite). Depending on the fluid/rock and Cr-spinel/silicate ratios, Cr-magnetite can also form over Fe²⁺-rich chromite and/or ferrian chromite as a secondary overgrowth. The compositional changes observed in Cr-spinels from La Cabaña reflect the initial stages of alteration under serpentinization conditions. Results from this study show that the alteration of Cr-spinels is dependent on temperature. The degree and extent of alteration (formation of Fe²⁺-rich and/or ferrian chromite) are controlled by the redox nature of the fluids, the Cr-spinel/silicate and the fluid/rock ratios.     

绿柱石中铁离子的晶格占位及致色作用

绿柱石族宝石颜色丰富,一直是市场上重要的宝石品种。几十年来,国内外学者对不同颜色绿柱石的致色机理及改色工艺开展了广泛的研究。铁离子作为重要的致色过渡金属离子之一,其价态调控是绿柱石改色工艺的关键,所以铁离子的价态、晶格占位和对应的致色作用也一直是绿柱石研究的焦点。铁离子被认为可能存在于绿柱石中的铝氧八面体Al³⁺格位、硅氧四面体Si⁴⁺格位、铍氧四面体Be²⁺格位、结构通道2a或2b位和晶格间隙6g位等位置,是蓝、绿或黄等颜色绿柱石的主要致色元素。本文通过对绿柱石的晶体结构、铁离子的核外电子排布和晶格占位等方面文献资料的梳理及综合分析,认为Fe²⁺和Fe³⁺分别对蓝色和黄色绿柱石的呈色起着主导作用。当Fe²⁺和Fe³⁺处于晶格中不同位置时致色作用的具体差异,仍有待于进一步研究。     

Some petrological aspects of the Prairie Creek diamond-bearing kimberlite diatreme,Arkansas

Based on modal and chemical composition, the rocks of the Prairie Creek diatreme situated 4 km SSE of Murfreesboro, Pike County, Arkansas, are classified as micaceous kimberlite. The K-Ar isotopic analysis of phlogopite from this diatreme yielded an age of 106 ± 3 m.y. (Albian) which is in agreement with stratigraphic relations. Electron beam probe data on minerals from kimberlite breccia, one of the three textural types, are presented. The breccia is considered as the potential source of the diamonds that have been mined at the diatreme. It contains phenocrysts of olivine (Fo_90–92) and serpentine pseudomorphs after olivine embedded in a groundmass of serpentine, minor calcite, chrome-diopside, phlogopite (Mg/Mg+Fe = 84.15%), perovskite, spinels, and pentlandite. Xenoliths of shales, sandstones, and mantle-derived ultramafic material are also present. Spinels are rich in Cr, Ti, and Fe and generally low in Al. Zoned spinels show enrichments in Ti and Fe towards their rims. A positive correlation between 100(Fe³⁺+Ti)/(Cr+Al+Fe³⁺+Ti) and 100 Mg/(Mg+Fe²⁺) ratios exists in these spinels and probably reflects an oxygen fugacity increase during magma crystallization. Occluded gases in diamonds and kimberlites corroborate the hypothesis that the parent magma of the Prairie Creek kimberlite was derived by partial melting of upper-mantle garnet lherzolite under volatile-rich conditions, primarily enriched in H₂O and CO₂.     

Titanian andradite in a metapyroxenite layer from the Malenco ultramafics (Italy): implications for Ti-mobility and low oxygen fugacity

Ti-andradite (melanite) has been found in a metapyroxenite layer in the upper part of the Malenco ultramafics(Italy), coexisting with clinochlore, diopside and magnetite. Field observations, as well as major and trace elementbulk-rock composition, strongly suggest a cumulate origin for the layer. Textural relationships indicate thatTi-andradite formed during two different metamorphic stages. Under peak metamorphic conditions (400–450°C, 5±2 kbar)Ti-andradite grew in an assemblage of diopside, clinochlore, magnetite and rare ilmenite and perovskite. Later, retrograde brittle deformationinduced formation of veins containing the paragenesis Ti-andradite, vesuvianite, diopside, chlinochlore, magnetite and accessory perovskite.The Ti-andradite varies considerably in TiO₂ (0.11–9.62 wt%), Fe₂O₃(14.3–30.5 wt%), Al₂O₃ (0.65–3.90 wt%), Cr₂O₃(>0.18–0.98 wt%) and SiO₂ (32.1–36.1 wt%); this is mostly, but not entirely, due to distinct zoning.Ti-andradite contains 0.32 to 0.66 wt% H₂O as determined by infrared spectroscopy and 0.83 to 1.76 wt% FeO. The CaO shows almost no variation (34.1±0.7 wt%) and Ca completely fills the dodecahedral site. Single crystal site refinements indicate that no tetrahedral Ti or Fe replaces Si. Titanium incorporation is attributed to similar degrees of substitution along the exchange vectors Ti³⁺ Fe³⁺, Ti⁴⁺ Al^IV Al _-1 ^VI Si_-1 and (Fe²⁺, Mn²⁺, Mg²⁺)Ti⁴⁺ 2Fe _-1 ³⁺ . The presence of mixed valence states of both Fe and Ti suggests a low oxygen fugacity during crystallization of Ti-andradite. Mass balance calculations indicate an isochemical origin of the first generation of Ti-andradite in the clinopyroxenite layer. Its occurrence is restricted to antigorite-free mineral assemblages containing clinochlore of 0.95X _Al>1.1. The hydrothermal crystallization of Ti-rich andradite in veins demonstrates Ti mobility in aqueous fluids under moderate P-T conditions. The zonation patterns indicate disequilibrium conditions during vein crystallization. As no fluorine-, carbonate- and phosphate-bearing minerals were found, OH^- is most probably the ligand complexing Ti.