Charge-balanced Substitution Research Articles

New constraints on the formation of main group pallasites derived from in situ trace element analysis and 2D mapping of olivine and phosphate

Pallasites are stony‑iron meteorites consisting mainly of olivine and Fe-Ni metal with a formation history that remains widely debated. Despite their simple mineralogy, relatively limited data are available on their geochemistry and the lateral elemental distribution in individual pallasite olivine crystals. In this work, laser ablation - inductively coupled plasma - mass spectrometry (LA-ICP-MS) was used for the elemental analysis of both pallasite olivine and phosphate phases, including 2D trace element mapping of olivine crystals. While the results obtained are in good agreement with literature values, important differences are observed for Al and Ni concentrations compared with bulk analytical methods. The element distribution maps reveal complex zoning in pallasite olivine, which can be explained based on i) diffusion gradients formed during olivine cooling, ii) the crystal chemistry of element substitution due to charge-balancing, and iii) inherited features of olivine before the metal-olivine mixing. Oscillatory zoning in an olivine crystal from Imilac provides strong evidence for the olivine not being a restite of partial melting, but rather having crystallized from a melt. Concentrations of Cr and Al are correlated both within single olivine crystals and between olivine crystals of different pallasites, forming a 1:1 linear trend. This likely results from a spinel-type charge-balancing substitution mechanism in pallasite olivine. Additionally, principle component analysis of laterally resolved multi-element concentration data of pallasite olivine provides an independent measure of the genetic relationships between the different main group pallasites within their parent body (−ies). Rare earth element (REE) contents of pallasite phosphate grains range from concentrations typically measured for chondritic or primitive achondrite-like phosphates to more (light) REE-depleted signatures, revealing the overall primitive nature of the melts from which the phosphate minerals crystallized. Phosphate in pallasites may thus have formed through the consumption of apatites during pro-grade metamorphism, melting, and melt extraction. Combined, the trace element signatures of olivine and phosphate in pallasites of the main group (PMG) suggest that these meteorites are common products of planetary processes on chondritic or primitive achondrite-like precursor bodies, with the different subgroups highlighting distinct formation and evolution histories.

Trace element partitioning between clinopyroxene and trachy-phonolitic melts: A case study from the Campanian Ignimbrite (Campi Flegrei, Italy)

The partitioning of trace elements between crystals and melts provides an important petrogenetic tool for understanding magmatic processes. We present trace element partition coefficients measured between clinopyroxene phenocrysts and trachy-phonolitic magmas at the Campi Flegrei (Italy), whose late Quaternary volcanism has been characterized by two major caldera-forming events (Campanian Ignimbrite at ~39ka, and Neapolitan Yellow Tuff at ~15ka). Our data indicate that the increase of trivalent rare earth elements and yttrium into the crystal lattice M2 site is facilitated by the charge-balancing substitution of Si4+ with Al3+ on the tetrahedral site. Higher concentrations of tetravalent and pentavalent high field strength elements on the M1 site are also measured when the average charge on this site is increased by the substitution of divalent cations by Alvi. In contrast, due to these charge balance requirements, divalent transitional elements become less compatible within the crystal lattice. On the basis of the lattice strain theory, we document that the incorporation of rare earth elements and yttrium in clinopyroxene is influenced by both compositional and physical parameters. Data from this study allow to update existing partitioning equations for rare earth elements in order to construct a self-consistent model for trachy-phonolitic magmas based on the lattice strain theory. The application of this model to natural products from the Campanian Ignimbrite, the largest caldera-forming eruption at the Campi Flegrei, reveals that the complex rare earth element pattern recorded by the eruptive products can be successfully described by the stepwise fractional crystallization of clinopyroxene and feldspar where the clinopyroxene–melt partition coefficient changes progressively as a function of the physicochemical conditions of the system.

Heftetjernite, a new scandium mineral from the Heftetjern pegmatite, Tordal, Norway

Heftetjernite, ideally ScTaO 4 , is a new scandium mineral from the Heftetjern pegmatite, Tordal, Telemark, Norway. In the type specimen, it occurs as minute, elongate tabular, very dark brown crystals in a single small void in albite. Other associated minerals are fluorite, muscovite, altered milarite, a metamict, dark greyish brown mineral of the pyrochlore-microlite group, and an unidentified, orange-brown, tabular, nearly X-ray amorphous Ti-Y-Ta-Nb-mineral. Electron-microprobe analysis yielded the empirical formula (Sc 0.64 Sn 0.13 Mn 0.12 Fe 0.08 Ti 0.06 ) ∑1.03 (Ta 0.69 Nb 0.30 ) ∑0.99 O 4 which clearly demonstrates the charge-balanced substitution scheme 2Sc 3+ = (Sn,Ti) 4+ + (Mn,Fe) 2+ . The mineral crystallises in the wolframite structure type, with space group P 2/ c and a = 4.784(1), b = 5.693(1), c = 5.120(1) A, β = 91.15(3)°, V = 139.42(5) A 3 ( Z = 2). A synthetic equivalent is known. Strongest lines in the calculated X-ray powder diffraction pattern of heftetjernite are [ d in A( I ) ( hkl )]: 3.000 (100) (11–1), 2.9570 (97) (111), 3.662 (53) (110), 2.4877 (34) (02–1), 4.783 (33) (100), 3.807 (32) (01–1). The crystal structure was refined to R ( F ) = 1.39 % from single-crystal X-ray diffraction data (293 K). It is based on two types of edge-sharing, distorted octahedra occupied predominantly by Sc and Ta, respectively. Heftetjernite is translucent to transparent, with a dark brownish (with a reddish hue) streak and adamantine lustre. It is brittle, has a perfect {010} cleavage, irregular fracture and a Mohs hardness estimated to be around 4.5 by comparison to ferberite; D x = 6.44 g/cm 3 (from crystal-structure analysis). Optically, the mineral is biaxial with an unknown optical sign, weakly pleochroic (yellowish brown with a reddish tint to reddish brown), with no observable dispersion. A mean refractive index of 2.23 was calculated from the Gladstone-Dale relationship using the X-ray density. Heftetjernite is named after its type locality. The mineral is compared with synthetic ScTaO 4 , ScNbO 4 , iwashiroite-(Y) and formanite-(Y) (both nominally YTaO 4 ), and some comments are made on the relation to Sc-bearing ixiolite.

High temperature calorimetric studies of heat of solution of NiO, CuO, La2O3, TiO2, HfO2 in sodium silicate liquids

The enthalpies of solution of La2O3, TiO2, HfO2, NiO and CuO were measured in sodium silicate melts at high temperature. When the heat of fusion was available, we derived the corresponding liquid–liquid enthalpies of mixing. These data, combined with previously published work, provide insight into the speciation reactions in sodium silicate melts. The heat of solution of La2O3 in these silicate solvents is strongly exothermic and varies little with La2O3 concentration. The variation of heat of solution with composition of the liquid reflects the ability of La(III) to perturb the transient silicate framework and compete with other cations for oxygen. The enthalpy of solution of TiO2 is temperature-dependent and indicates that the formation of Na–O–Si species is favored over Na–O–Ti at low temperature. The speciation reactions can be interpreted in terms of recent spectroscopic studies of titanium-bearing melts which identify a dual role of Ti4+ as both a network-former end network-modifier. The heats of solution of oxides of transition elements (Ni and Cu) are endothermic, concentration-dependent and reach a maximum with concentration. These indicate a charge balanced substitution which diminishes the network modifying role of Na+ by addition of Ni2+ or Cu2+. The transition metal is believed to be in tetrahedral coordination, charge balanced by the sodium cation in the melts.

Np Incorporation into Uranyl Alteration Phases: A Quantum Mechanical Approach

AbstractNeptunium is a major contributor to the long-term radioactivity in a geologic repository for spent nuclear fuel (SNF) due to its long half-life (2.1 million years). The mobility of Np may be decreased by its incorporation into the U6+ phases that form during the corrosion of SNF. The ionic radii of Np5+ (0.089nm) and U6+ (0.087nm) are similar, as is their chemistry. Experimental studies have shown that Np can be incorporated into uranyl phases up to concentrations on the order of 100 ppm. The low concentration of Np in the uranyl phases complicates experimental detection and presents a significant challenge for determining the incorporation mechanism. Therefore, we have used quantum mechanical calculations to investigate incorporation mechanisms and evaluate the energetics of Np substituting for U.CASTEP, a density functional theory based code that uses plane waves to model the behavior of the valence electrons and pseudo-potentials to model the behavior of the core and inner valence electrons, was used to calculate optimal H positions, relaxed geometry, and energy of different uranyl phases. The incorporation energy for Np in uranyl alteration phases was calculated for studtite, [(UO2)O2(H2O)2](H2O)2, and boltwoodite, HK(UO2)(SiO4)1.5(H2O). Studtite is the rare case of a stable, naturally-occurring peroxide mineral, in this case a uranyl hydroxyl peroxide that forms in the presence of H2O2 from the radiolysis of H2O. For studtite, two incorporation mechanisms were evaluated: (1) charge-balanced substitution of Np5+ and H+ for one U6+, and (2) direct substitution of Np6+ for U6+. For boltwoodite, the H atomic positions prior to Np incorporation were determined, as well as the Np incorporation mechanisms and the corresponding substitution energies. The preferential incorporation of Np into different structure types of U6+ minerals was also investigated. Quantum mechanical substitution energies have to be derived at Np concentrations higher than the ones found in experiments or expected in a repository in order to avoid the calculation of unit cells that are too large to be handled quantum mechanically. However, the quantum mechanical results are crucial for subsequent empirical force-field and Monte-Carlo simulations to determine the thermodynamically stable limit of Np incorporation into these uranyl phases.

Trace element incorporation into pyrope-grossular solid solutions: an atomistic simulation study

We have performed atomistic computer simulations on trace element incorporation into the divalent dodecahedral X-sites of pyrope (Py — Mg3Al2Si3O12) – grossular (Gr — Ca3Al2Si3O12) solid solutions. An ionic model and the Mott–Littleton two-region approach to defect energies were used to calculate the energetics of substitution by a range of divalent trace-elements and of charge-balanced substitution by trivalent ions in the static limit. Results are compared with experimental high-temperature, high-pressure garnet-melt trace element partitioning data obtained for the same garnet solid solution to refine our understanding of the factors controlling element partitioning into solid solutions. Defect energies (Udef,f), relaxation (lattice strain) energies (Urel), and solution energies (Usol) were derived using two different approaches. One approach assumes the presence of one type of hybrid X-site with properties intermediate between pure Mg and Ca sites, and the other assumes discrete Mg and Ca X-sites, and thus two distinct cation sublattices. The hybrid model is shown to be inadequate, since it averages out local distortions in the garnet structure. The discrete model results suggest trace elements are more soluble in Py50Gy50 than in either end-member compound. Physically this is due to small changes in size of the X-sites and the removal of unfavourable interactions between third nearest neighbours of the same size. Surprisingly, depending on the local order, large trace element cations may substitute for Mg2+ and small trace elements for Ca2+ in Py50Gr50. These solubilities provide an explanation for the anomalous trace-element partitioning behaviour along the pyrope–grossular join observed experimentally.

Compositional variation in minerals of the chevkinite group

AbstractThe composition of chevkinite and perrierite, the most common members of the chevkinite group, is closely expressed by the formulaA4BC2D2Si4O22, whereA= (La,Ce,Ca,Sr,Th),B= Fe2+,C= (Fe2+,Fe3+,Ti,Al,Zr,Nb) andD= Ti. TheAsite is dominated by a strong negative correlation between (Ca+Sr) and theREE. Chondrite-normalizedREEpatterns are very variable, e.g. inLREE/HREEand Eu/Eu*. TheCsite is dominated by Ti, Al and Fe2+, in very variable proportions. Most chevkinites and perrierites are close to stoichiometric, with cation sums between 12.9 and 13.5, compared to the theoretical 13. There is no single, generally applicable charge balancing substitution scheme in the group; however, the general relationshipdefines a linear array with r2= 0.91. Chevkinite and perrierite are shown to be compositionally distinct on the basis of CaO, FeO*, Al2O3and Ce2O3abundances. Chevkinite forms mainly in chemically evolved parageneses, such as syenites, rhyolites and fenites associated with carbonatite complexes. Perrierite is more commonly recorded from igneous rocks of mafic to intermediate composition. The compositional characteristics and possible structural formulae of other members of the chevkinite group are reviewed briefly.

Chevkinite-group minerals from salic volcanic rocks of the East African Rift

AbstractElectron microprobe analyses are presented of chevkinite-group minerals occurring as microphenocrysts in peralkaline rhyolites of the Greater Olkaria Volcanic Complex (Kenya) and as a groundmass phase in a peralkaline quartz trachyte lava from the Tarosero volcano (Tanzania), both in the East African Rift Valley. Their compositions conform closely to the formula: (REE, Ca, Th)4 Fe2+ (Fe2+, Al, Ti, Zr, Nb)2 Ti2 (Si4O22). Compared to published analyses of chevkinite-group minerals, the Olkaria phases are relatively enriched in Nb and the LREE; the Tarosero phase is more calcic and relatively Zr- and Nb-rich. The main substitution in the A site at Olkaria is Ca ⇌ Ce. The overall charge-balancing substitution seems to be (McDowell, 1979):Phenocryst/glass ratios are presented for Nb, REE, Sr, Th, U and Y in two, and Ba, Zr and Hf in one, Kenyan samples. Partition coefficients are lower in the more peralkaline rock, with the exception of Sr, which is higher. The lower values are consistent with a lower degree of polymerization of more peralkaline melts. The higher Sr value may be a function of Sr partitioning into phenocryst phases coexisting with chevkinite.

The crystal structure and cation ordering of Phase-X-(K1−x−n)2(Mg1−n[Al,Cr]n)2Si2O7H2x: A potential K- and H-bearing phase in the mantle

Phase-X, a potassium di-magnesium acid disilicate, is a high-pressure synthetic compound—a potential K-bearing silicate in the mantle—with space group P 6 3 cm (no. 185), a = b = 5.028(2) A, c = 13.216(3) A, V = 289.34 A 3 , Z = 2. The structure has been determined with 1521 CCD measured intensities and refined by the least-square method to R = 0.0187. The structure is built up of octahedral MgO sheets and layers containing disilicate groups, Si 2 O 7 , (with distinct Si1 and Si2 tetrahedra linked by the apical O2 atom) alternating along the c axis. The octahedral sheet is based on a hexagonal closest-packed array of two layers of non-equivalent O atoms, O1 and O3; two-thirds of all edge-sharing M octahedra are filled. Within the framework of the Si 2 O 7 groups are channel structures parallel to [100], [010], and [110] that contain K atoms disordered in the middle of a large trigonal cavity (the A site). The FTIR spectrum in the OH stretching region shows a sharp peak at 3602 cm −1 due to OH − ordered in one anion site; the position of hydrogen, which operates in a charge-balancing substitution for the partial occupancy of the A site (K 1− x □ x ) A ↔ H x □ 1−x ) H , is undetermined. Densification in phase-X is affected by the greater compression of the empty octahedra in the octahedral layer and by constraining the trigonal A cavity containing the K atom to the size of the Si 2 O 7 disilicate group. This dense packing contributes to the relatively high zero-pressure calculated density of 3.38 g/cm 3 .

Synthetic MgGa 2 O 4 -Mg 2 GeO 4 spinel solid solution and β-Mg 3 Ga 2 GeO 8 : chemistry, crystal structures, cation ordering, and comparison to Mg 2 GeO 4 olivine

Structural parameters and cation ordering are determined for four compositions in the synthetic MgGa2O4-Mg2GeO4 spinel solid solution (0, 8, 15 and 23 mol% Mg2GeO4; 1400 °C, 1 bar) and for spinelloid β-Mg3Ga2GeO8 (1350 °C, 1 bar), by Rietveld refinement of room-temperature neutron diffraction data. Sample chemistry is determined by XRF and EPMA. Addition of Mg2GeO4 causes the cation distribution of the MgGa2O4 component to change from a disordered inverse distribution in end member MgGa2O4, [4]Ga = x = 0.88(3), through the random distribution, toward a normal cation distribution, x = 0.37(3), at 23 mol% Mg2GeO4. An increase in ao with increasing Mg2GeO4 component is correlated with an increase in the amount of Mg on the tetrahedral site, through substitution of 2 Ga3+⇄ Mg2++Ge4+. The spinel exhibits high configurational entropy, reaching 20.2 J mol−1 (four oxygen basis) near the compositional upper limit of the solid solution. This stabilizes the spinel in spite of positive enthalpy of disordering over the solid solution, where ΔHD = αx + βx2, α = 22(3), β = −21(3) kJ mol−1. This model for the cation distribution across the join suggests that the empirically determined limit of the spinel solid solution is correlated with the limit of tetrahedral ordering of Mg, after which local charge-balanced substitution is no longer maintained.

XAFS characterization of the structural site of Yb in synthetic pyrope and grossular garnets. II: XANES full multiple scattering calculations at the Yb L I - and L III -edges

We present an X-ray absorption near-edge structure study performed at the Yb LI- and LIII-edges on synthetic pyrope (Mg3Al2Si3O12) and grossular (Ca3Al2Si3O12) garnets containing about 1% wt of Yb. For the first time Yb L-edge XANES spectra are analyzed by full multiple scattering theory using clusters of different sizes and different final-state potentials. A comparison between experimental spectra and model calculations indicates that Yb3+ enters the dodecahedral X-site in both pyrope and grossular, in agreement with the results of an EXAFS study. Based on the present results, the charge balancing substitution mechanism required by the replacement of divalent Mg and Ca cations with trivalent Yb3+ is discussed in terms of vacancies in dodecahedral sites surrounding the central Yb3+ absorber.

High temperature stability limit of phase egg, AlSiO 3 (OH)

The stability relations of phase egg, AlSiO3(OH), have been investigated at pressures from 7 to 20 GPa, and temperatures from 900 to 1700 °C in a multi-anvil apparatus. At the lower pressures phase egg breaks down according to the univariant reaction, phase egg = stishovite + topaz-OH, which extends from 1100 °C at 11 GPa to 1400 °C at 13 GPa where it terminates at an invariant point involving corundum. At pressures above the invariant point, the stability of phase egg is limited by the breakdown reaction, phase egg = stishovite + corundum + fluid, which extends from the invariant point to 1700 °C at 20 GPa. Stishovite crystallized in the Al2O3-SiO2-H2O system contains Al2O3, and the amount of Al2O3 increases with increasing temperature. It is inferred that the Al2O3 content is controlled by the charge-balanced substitution of Si4+ by Al3+ and H+. Aluminum-bearing stishovite coexisting with an H2O-rich fluid may contain a certain amount of water. Therefore, phase egg and stishovite in a subducting slab could transport some H2O into the deep Earth.

Physical and chemical characteristics of baddeleyite (monoclinic zirconia) in natural environments: an overview and case study

This report provides an overview of the natural occurrence, physical characteristics, and chemical composition of the mineral baddeleyite, ideally ZrO 2. A survey of the literature shows that baddeleyite is widespread in nature as a trace mineral, but has a rather limited composition of 87–99 wt% ZrO 2 with most of the remainder comprised of FeO, TiO 2 and HfO 2. Natural baddeleyite is invariably monoclinic due to the low concentrations of large cations such as Ca and Y. Although the concentrations of Th and U are low (generally <1500 ppm U and <100 ppm Th), the cumulative alpha-decay dose reaches 0.1–1.1 × 10 16 mg −1 for samples with ages of 1–2 × 10 9 yr. The available data also suggest that baddeleyite is highly durable in aqueous fluids. For nuclear waste form applications, both the radiation damage effects and durability of baddeleyite need to be examined in further detail. A case study of baddeleyite from the Jacupiranga carbonatite complex of southern Brazil provides some additional information on the crystal chemistry, durability and radiation damage effects of the mineral. This work shows that baddeleyite can incorporate up to 4.1 wt% Nb 2O 5 and 1.2 wt% Ta 2O 5. Incorporation of Nb 5+ and Ta 5+ is partially compensated by the incorporation of up to 0.4 wt% MgO and 0.3 wt% FeO in a charge balanced substitution of the form 3Zr↔2(Nb, Ta) + (Mg, Fe). Similar substitution mechanisms may enable the incorporation of Np 5+ in the ZrO 2 phases of fuel and waste matrices. The mineral is highly resistant to hydrothermal alteration which affected associated uranpyrochlore crystals. Many of the baddeleyite crystals are partially enclosed within the uranpyrochlore grains which contain 19–26 wt% UO 2 and 0.3–3.5 wt% ThO 2. These baddeleyite crystals received maximum alpha-particle doses of 3–4.5 × 10 16 mg −1 within 10 μm of the uranpyrochlore grain boundary, but the intense irradiation has not affected either the structural integrity or the aqueous durability of the mineral.

Oilwell Cement Clinkers

X-ray microanalysis have been made on a suite of nine oilwell cement clinkers. Elemental data were obtained on alite, belite, and ferrite phases and, for two clinkers only, also on aluminate. The alite and belite compositions are broadly similar to those previously reported for other portland cements. Guest ion concentrations suggest that several charge-balancing substitutions occur. bulk MgO and SO3 levels determine the Mg and S contents of both phases, but for Fe and Al the substitution levels are not strongly correlated with bulk composition. For ferrites the data are generally consistent with the results of Bergstrom et al. (Adv Cem Res 1992, 4, 141-147): the Mg content varies widely, is controlled by the bulk MgO, and is coupled with Si in a charge-balancing substitution for Fe. Estimates of the total phase assembly using the directly determined mineral compositions are compared with the predictions of a modified Bogue calculation (similar to that of Taylor, Adv Cem Res, 1989, 2, 73-77). The agreement is variable and some refinements to the method of calculation are indicated.

A thermochemical study of La 2−xA xCuO 4−y ( A=Ba, Sr, Ca, Pb)

The compounds La 2−x A x CuO 4−y ( A =Ba, Sr, Ca, Pb) were synthesized in their easily accessible single phase regions. High temperature solution calorimetry was performed to determine the enthalpies of formation at 704°C from oxide components. For small values of x (x <-0.1), the enthalpy of formation for Ba, Sr, and Ca substituted materials becomes progressively more exothermic with x . In contrast, Pb substituted materials show no change in the enthalpy of formation. For higher values of x (x >0.1), the enthalpy of formation of the Ba, Sr, and Ca substituted materials differs significantly. These enthalpy trends are interpreted as a balance of effects due to oxidation, oxygen vacancy formation, and oxygen vacancy ordering. The former depends strongly on the basicity of the divalent cation. For La 2−x Sr x CuO 4−y , the enthalpy of oxidation (formation of formal Cu 3+ ) is −129±20 kJ/(mole O 2 ), the enthalpy of the charge balanced substitution resulting in oxygen vacancy formation is 0±10, and the enthalpy of oxygen vacancy ordering is −40±10 kJ/(mole O 2− vacancy). For La 2−x Ba x CuO 4−y , the enthalpy of oxidation is −220±50 kJ/(mole O 2 ), while the enthalpy of the charge balanced substitution is 0±10 as in the strontium doped system. Apparent phase relationships in the La 2−x A x CuO 4−y ( A =Ba, Sr, Ca, Pb) pseudobinary system are interpreted in terms of these thermodynamic properties.

Experimental study of element partitioning between majorite, olivine, merwinite, diopside and silicate melts at 16GPa and 2,000.DEG.C..

We have determined experimentally the partition coefficients of elements between silicate minerals and coexisting melts at 16 GPa and 2, 000°C in chondritic and CaO-rich silicate systems. Majorite garnet + melt ± olivine forms in the MgO-rich chondritic system, while in the CaO-rich system, merwinite coexists with melt ± clinopyroxene. The partition coefficients (D-values) of La, Ce, Sm, Yb, Sc, Zr, Hf and other minor or major elements for majorite garnet, merwinite, and clinopyroxene/melt pairs have been determined through EPMA analysis. D-values of a limited number of elements were also determined for the olivine/melt pair with EPMA. The D(HREE and Sc) for majorite/melt are close to unity. These values are much lower than reported values for pyrope/melt pairs at 2–3 GPa and 1, 200–1, 500°C and for pyrope megacryst/host rock matrix pairs. The pressure-induced coordination change in Al3+ at higher pressures above 10 GPa is responsible for the enormous difference in the D values as well as the contrasting temperature conditions: The coupled substitution of (VIIIM2+, IVSi4+) = (VIIIHREE3+, IVAl3+), important for accommodating HREE3+ into garnets, is severely limited because IVAl3+ is unlikely to exist in the majorite/melt system. The mechanism to reduce D(REE and Sc) with increasing pressure may also be operative between the clinopyroxene/melt pair. Systematic comparison of Onuma diagrams for VIIIM2+ and VIIIM3+ in almandine, pyrope, and majorite garnet/melt pairs clearly indicates that high enrichments of HREE in almandine and pyrope garnets are related to charge-balanced substitution and the presence of more or less polymerizing silicate melts with IVAl3+. These Onuma diagrams also suggest that the partitioning behaviors of Fe2+ and Co2+ between garnet and melt are anomalous. This may reflect the tendency for Fe2+ and Co2+ to prefer six coordinated sites in melts to cubic sites in garnets due to the crystal field effect. The D(REE and Sc) value pattern for the merwinite/Ca-rich melt pair has been found to be quite similar to those for CaTiO3/melt and CaSiO3 (perovskite)/melt pairs. This is interesting in view of the substructure of merwinite that mixed Ca2+ and O2- layers comprise a pseudo-hexagonal dense packing analogous to the perovskite structure.

Charge balancing of trivalent trace elements in olivine and low-Ca pyroxene: A test using experimental partitioning data

Charge-balancing substitution mechanisms have been determined for the incorporation of the trivalent cations Al and Sc in low-Ca pyroxene and Al, Sc, Yb, and Cr in olivine. In low-Ca pyroxene, the substitution mechanism was determined by evaluating covariations of trivalent trace cations with Si, Mg, Fe, and Ca. In olivine, substitution mechanisms were determined by comparison of the observed compositional dependence of partitioning to the compositional dependence theoretically expected for each substitution reaction. In low-Ca pyroxene, both cations investigated (Al VI and Sc VI) are charge balanced by the coupled substitution of Al for Si in tetrahedral sites. In olivine, the smaller cations (Al VI and Cr VI +3) are charge balanced in part by this same substitution (Al ↔ Si) VI. However, the larger cations (Sc VI, Yb VI) are charge balanced by a vacancy substitution: 2( M +3) VI + vacancy VI ↔ 3 ( M +3) VI . The vacancy substitution is also important in charge balancing Al in olivine. These observations are important in establishing the mineral-melt reactions needed to construct the appropriate equilibrium constant for the partitioning reactions among melt, olivine, and low-Ca pyroxene. The correct equilibrium constant constrains the compositional dependence of trace element partitioning and thereby permits improved modeling of magma evolution.