Electron paramagnetic resonance study of the site preferences of Gd3+ and Eu2+ in polycrystalline silicate and aluminate minerals

Electron paramagnetic resonance (EPR) measurements were made on Gd³⁺ and Eu²⁺ ions in polycrystalline samples to determine the nature of the sites occupied by those ions in mineral structures. Both Gd³⁺ and Eu²⁺ ions were incorporated at Ca²⁺ structural sites in β-Ca₂SiO₄, pseudo-CaSiO₃, CaMgSiO₄, CaMgSi₂O₆, hex-CaAl₂Si₂O₈, CaAl₂O₄, and Ca₃Al₂O₆. For tri-CaAl₂Si₂O₈, Eu²⁺ was incorporated at a Ca²⁺ site and Gd³⁺ was incorporated at a site where the crystalline electric field was disordered. That difference in behavior may contribute to the anomalous behavior of Eu in plagioclase feldspar. Both Gd³⁺ and Eu²⁺ were incorporated as aggregates or clusters of those ions in Mg₂SiO₄ and clino-MgSiO₃.     

EPR measurement of the effect of glass composition on the oxidation states of europium

The techniques of electron paramagnetic resonance (EPR) were used to measure the concentration ratio of Eu²⁺ to Eu³⁺ in quenched silicate liquids as a function of their compositions. The compositional end members were CaAl₂Si₂O₈ and either MSiO₃ or M₂Si0₄, M = Mg, (Ca_0.5, Mg_0.5), and Ca. All of the liquids were quenched from 1650 ± 25°C, 10^?6.9±0.6 atm of oxygen, and 10^?6.1±0.6 atm total pressure. For a particular choice of M, the ratio of Eu²⁺ to Eu³⁺ increased as much as a factor of 24 with increasing atomic ratio (Al + Si)/(O); for a constant value of (Al + Si)/(O), the ratio of Eu²⁺ to Eu³⁺ increased in the order Mg > (Ca_0.5,Mg_0.5) >Ca. In order to interpret the compositional dependence of the redox equilibrium of Eu in a systematic manner, the concept of a solvent coefficient was introduced.     

Thermodynamics of multicomponent pyroxenes: II. Phase relations in the quadrilateral

The model for the thermodynamic properties of multicomponent pyroxenes (Part I) is calibrated for ortho- and clinopyroxenes in the quadrilateral subsystem defined by the end-member components Mg₂Si₂O₆, CaMgSi₂O₆, CaFeSi₂O₆, and Fe₂Si₂O₆. This calibration accounts for: (1) Fe-Mg partitioning relations between orthopyroxenes and augites, and between pigeonites and augites, (2) miscibility gap features along the constituent binary joins CaMgSi₂O₆-Mg₂Si₂O₆ and CaFeSi₂O₆-Fe₂Si₂O₆, (3) calorimetric data for CaMgSi₂O₆-Mg₂Si₂O₆ pyroxenes, and (4) the P-T-X systematics of both the reaction pigeonite=orthopyroxene+augite, and miscibility gap featurs, over the temperature and pressure ranges 800–1500°C and 0–30 kbar. The calibration is achieved with the simplifying assumption that all regular-solution-type parameters are constants independent of temperature. It is predicated on the assumptions that: (1) the Ca-Mg substitution is more nonideal in Pbca pyroxenes than in C2/c pyroxenes, and (2) entropies of about 3 and 6.5 J/K-mol are associated with the change of Ca from 6- to 8-fold coordination in the M2 site in magnesian and iron C2/c pyroxenes, respectively. The model predicts that Fe²⁺-Mg²⁺ M1-M2 site preferences in C2/c pyroxenes are highly dependent on Ca and Mg contents, with Fe²⁺ more strongly preferring M2 sites both in Ca-rich C2/c pyroxenes with a given Fe/(Fe+Mg) ratio, and in magnesian C2/c pyroxenes with intermediate Ca/(Ca+Fe+Mg) ratios.The proposed model is internally consistent with our previous analyses of the solution properties of spinels, rhombohedral oxides, and Fe-Mg olivines and orthpyroxenes. Results of our calibration extend an existing database to include estimates for the thermodynamic properties of the C2/c and Pbca pyroxene end-members clinoenstatite, clinoferrosilite, hedenbergite, orthodiopside, and orthohedenbergite. Phase relations within the quadrilateral and its constitutent subsystems are calculated for temperatures and pressures over the range 800–1700°C and 0–50 kbar and compare favorably with experimental constraints.     

A multi-analytical study of the crystal structure of unusual Ti–Zr–Cr-rich Andradite from the Maronia skarn, Rhodope massif, western Thrace, Greece

Unusual Ti–Cr–Zr-rich garnet crystals from high-temperature melilitic skarn of the Maronia area, western Thrace, Greece, were investigated by electron-microprobe analysis, powder and single-crystal X-ray diffraction, IR, Raman and Mössbauer spectroscopy. Chemical data showed that the garnets contain up to 8 wt.% TiO₂, 8 wt.% Cr₂O₃ and 4 wt.% ZrO₂, representing a solid solution of andradite (Ca₃Fe³⁺ ₂Si₃O₁₂ ≈46 mol%), uvarovite (Ca₃Cr₂Si₃O₁₂ ≈23 mol%), grossular (Ca₃Al₂Si₃O₁₂ ≈10 mol%), schorlomite (Ca₃Ti₂[Si,(Fe³⁺,Al³⁺)₂]O₁₂ ≈15 mol%), and kimzeyite (Ca₃Zr₂[Si,Al₂]₃O₁₂ ≈6 mol%). The Mössbauer analysis showed that the total Fe is ferric, preferentially located at the octahedral site and to a smaller extent at the tetrahedral site. Single-crystal XRD analysis, Raman and IR spectroscopy verified substitution of Si mainly by Al³⁺, Fe³⁺ and Ti⁴⁺. Cr³⁺ and Zr⁴⁺ are found at the octahedral site along with Fe³⁺, Al³⁺ and Ti⁴⁺. The measured H₂O content is 0.20 wt.%. The analytical data suggest that the structural formula of the Maronia garnet can be given as: (Ca_2.99Mg_0.03)_Σ=3.02(Fe³⁺ _0.67Cr_0.54Al_0.33Ti_0.29Zr_0.15)_Σ=1.98(Si_2.42Ti_0.24Fe_0.18Al_0.14)_Σ=2.98O₁₂OH_0.11. Ti-rich garnets are not common and their crystal chemistry is still under investigation. The present work presents new evidence that will enable the elucidation of the structural chemistry of Ti- and Cr-rich garnets.     

Mayenite-supergroup minerals from burned dump of the Chelyabinsk Coal Basin

Three minerals of the mayenite supergroup have been found in fluorellestadite-bearing metacarbonate rock (former fragment of petrified wood of ankeritic composition) from the dump at the Baturinskaya-Vostochnaya-1-2 mine. These are eltyubyuite Ca₁₂Fe1°Si₄O3₂Cl6, its fluorine analog Ca₁₂Fe₁₀³⁺Si₄O₃₀F₁₀, and chlormayenite-wadalite Ca₁₂(Al,Fe)₁₄O₃₂Cl₂-Ca₁₂(Al,Fe)₁₀Si₄O₃₂Cl₂. The first two phases occur in the reaction mantle around hematite, magnesioferrite, and Ca-ferrite aggregates (“calciohexaferrite” CaFe₁₂O₁₉, “grandiferrite” CaFe₄O₇, and “dorrite phase” Ca₂(Fe₅^3 +Mn0^0.5Mg_0.5)(Si_0.5Fe_5.5³⁺)O₂₀) and, rarely, as individuals in grained aggregates of fluorellestadite-cuspidine (± lar- nite ± rusinovite Ca₁₀(Si₂O₇)₃Cl₂). Assemblages of zoned chlormayenite-wadalite crystals are found in grained aggregates of fluorellestadite- cuspidine, which lack Ca-ferrite. Also, harmunite CaFe₂O₄, chlorellestadite, fluorapatite, anhydrite, rondorfite CasMg(SiO₄)₄Cl₂, fluorine analog of rondorfite CasMg(SiO₄)₄F₂, “Mg-cuspidine” Ca_3.5(Mg,Fe)_0.5(Si₂O₇)F₂, fluorite, barioferrite BaFe₁₂O₁9, zhangpeishanite BaFCl, and other rare phases are identified in this rock. Data on the chemical composition and Raman spectroscopy of the mayenite-supergroup minerals are given. The genesis of metacarbonate rock is considered in detail: “oxidizing calcination” of Ca-Fe-carbonates with the formation of hematite and lime; reaction between hematite and lime with the formation of different Ca-ferrites; formation of larnite as a result of reaction between SiO₂ and lime or CaCO₃; and reactionary impact of hot Cl-F-S-bearing gases on early assemblages. Eltyubyuite and its fluorine analog crystallized at the stages of gas impact. It is presumed that the maximum temperature during the formation of rock reached 1200–1230 °C. © 2015, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.     

Dielectric constants of crystalline and amorphous spodumene,anorthite and diopside and the oxide additivity rule

The dielectric constants and dissipation factors of LiAlSi₂O₆, CaAl₂Si₂O₈ and CaMgSi₂O₆ in both the crystalline (α-spodumene, anorthite, and diopside) and amorphous forms were determined at 1 MHz using a two-terminal method and empirically determined edge corrections. The results are: spodumene κ′ ₁₁=7.30 tan δ= 0.0007 κ′₂₂=8.463 tan δ= 0.0002 κ′₃₃ =11.12 tan δ= 0.0007 anorthite κ′ _a ^*=5.47 tan δ= 0.0009 κ′_b ^*=8.76 tan δ= 0.0010 κ′_c ^*=7.19 tan δ= 0.0013 diopside κ′₁₁=9.69 tan δ= 0.0016 κ′₂₂ = 7.31 tan δ= 0.0007 κ′₃₃=7.29 tan δ= 0.00019 LiAlSi₂O₆ κ′=8.07 tan δ= 0.047 amorphous CaAl₂Si₂O₈ κ′=7.50 tan δ= 0.0024 amorphous CaMgSi₂O₆ κ′=8.89 tan δ= 0.0021 amorphous The dielectric properties of a spodumene glass, progressively crystallized at different conditions, were also determined. As the crystallization temperature was increased from 720 to 920° C, κ′ increased from 6.22 to 6.44. The dissipation factor, tan δ, remained constant at 0.020. Similarly, as the crystallization time at 750° C increased from 0.5 hr to 6.0 hr, κ′ increased from 6.28 to 6.35. The deviations of the measured dielectric polarizabilities as determined from the Clausius-Mosotti equation from those calculated from the sum of oxide polarizabilities according to α _D(mineral, glass) = σ α _D(oxides) are +7.4% for α-spodumene, +1.2% for diopside, and +28.0, +19.6 and +15.9% for amorphous spodumene, anorthitie and diopside compositions, respectively. Positive deviations in α-spodumene and anorthite are consistent with lower than normal apparent cation bond valence sums and are believed to be evidence for loosely bonded “rattling” Li and Ca ions. Diopside, with Ca and Mg ions having normal bond valence sums, exhibits no abnormal deviation from additivity. Larger positive deviations in amorphous SiO₂, LiAlSi₂O₆, CaAl₂Si₂O₈ and CaMgSi₂O₆ are postulated to arise from a combination of loosely bonded cations and disordered O⁼ ions where the oxygen dielectric polarizability increased from its normal value of 2.0 ų in well-behaved oxides to 2.2–3.0 Å₃ in the amorphous phases.     

Thermochemical study of glasses in the system CaMgSi2O6-CaAl2SiO6

High temperature solution calorimetry of glasses in the system CaMgSi₂O₆ (Di)-CaAl₂SiO₆ (CaTs) show them to have negative enthalpies of mixing with a regular enthalpy parameter, W_H, of -11.4 ± 0.7 kcal. Negative heats of mixing between alumina-rich and alumina-poor glasses seem to be a general phenomenon in aluminosilicates and are not confined only to glassy systems containing anorthite as a component. The thermodynamic behavior of glasses in the system SiO₂-Ca_0.5;AlO₂-CaMgO₂ appears to vary in a smooth fashion, with small positive heats of mixing near SiO₂ and substantial negative heats of mixing for other compositions. The exothermic behavior with increasing $\text{A1}\text{(Al + Si)}$ may be related to local charge balance of M²⁺ and Al³⁺. The negative heats of mixing in MgCaSi₂O₆-CaAl₂SiO₆, MgCaSi₂O₆-CaAl₂Si₂O₈ and NaAlSi₃O₈-CaAl₂Si₂O₈ glasses are in contrast to the positive heats of mixing found in MgCaSi₂O₆-CaAl₂SiO₆ (pyroxene) and NaAlSi₃O₈-CaAl₂Si₂O₈ (high plagioclase) crystalline solid solutions.     

再论花岗岩按照Sr-Yb的分类:标志

2006年作者曾经按照Sr=400×10~(-6)和Yb=2×10~(-6)作为标志将花岗岩分为埃达克岩、喜马拉雅型花岗岩、浙闽型花岗岩和广西型花岗岩,在浙闽型中又分出南岭型(Sr100×10~(-6)和Yb2×10~(-6)),于是花岗岩被分为5类。Sr=400×10~(-6)和Yb=2×10~(-6)是根据阿留申群岛中的Adak岛的资料得出来的。本文统计了全球花岗岩6000多个数据(其中,埃达克型花岗岩为2810个,喜马拉雅型花岗岩636个,浙闽型花岗岩1183个,南岭型花岗岩1518个,广西型花岗岩142个,总共6289个),统计的结果,各类花岗岩的地球化学特征大致如下:(1)埃达克型花岗岩富Al_2O_3和Sr,贫Y和Yb,具较高和变化的铕异常,绝大多数样品的Sr300×10~(-6),Yb2.5×10~(-6)(当Sr=400×10~(-6)～600×10~(-6)时Yb值最大,Sr超过600×10~(-6),Yb降低至2×10~(-6)),Al_2O_3在14%～18%之间,Eu/Eu~*大多在0.6～1.2范围;(2)喜马拉雅型花岗岩贫Sr和Yb,具中等的Al_2O_3和变化的Eu/Eu~*,Sr300×10~(-6)和Yb2×10~(-6)(少数Sr300×10~(-6)),Al_2O_3为13%～17%,Eu/Eu~*为0.2～1.0;(3)浙闽型花岗岩贫Sr富Yb,Sr在40×10~(-6)～400×10~(-6)之间,Yb1.5×10~(-6),Al_2O_3和Eu/Eu~*的变化类似喜马拉雅型花岗岩,Al_2O_3为12%～17%,Eu/Eu~*为0.4～1.0;(4)南岭型花岗岩以很低的Sr、Al_2O_3和Eu/Eu~*以及很高的Yb而不同于上述各类花岗岩,通常Yb1.5×10~(-6),Sr100×10~(-6)(Yb变化大,绝大多数2×10~(-6);当Yb在2×10~(-6)～8×10~(-6)时,部分样品Sr可100×10~(-6),但很少200×10~(-6));Al_2O_314%,集中在11%～13%之间,Eu/Eu~*0.7,大多0.4;Yb越大,Sr越低,负铕异常越明显。文中讨论了花岗岩Sr-Yb分类的意义,指出本分类适用于产于大陆和海洋的绝大多数中酸性岩浆岩(可能不适用于一部分特别富铁和钾的花岗岩,如具有高Sr和Yb特征的广西型花岗岩)。不同类型的花岗岩主要反映了源区压力的不同,而源区成分、温度、部分熔融程度、水和挥发分的加入以及岩浆混合等的影响可能是次要的。文中指出,该分类的依据、其实质,是熔体与残留相平衡的理论。与浙闽型花岗岩平衡的残留相是斜长石,与喜马拉雅型花岗岩平衡的是斜长石+石榴石,与埃达克型花岗岩平衡的是石榴石,与南岭型花岗岩平衡的是富钙的斜长石。文中指出,加强实验岩石学研究,将年代学和地球化学研究密切结合起来是深化花岗岩研究的关键。     

Laser-induced time-resolved luminescence of tugtupite,sodalite and hackmanite

We have interpreted a number of luminescence centers in natural tugtupite Na₈Al₂Be₂Si₈O₂₄Cl₂, sodalite Na₈Al₆Si₆O₂₄C₂ and hackmanite Na₈Al₆Si₆O₂₄(Cl₂,S) by use of laser-induced time-resolved luminescence spectroscopy. The main new results are the following: Fe³⁺, Mn²⁺, Eu²⁺, Ce³⁺, mercury type (potentially Pb²⁺, Tl⁺, Sn²⁺ and/or Sb³⁺), radiation induced luminescence centers; several types of S₂ ⁻ centers. Spectral shift connected with the presence of luminescence centers, which are detected together with S₂ ⁻ centers and impossible to resolve with continuous wave luminescence spectroscopy, is the possible reason for spectral diversity of S₂ ⁻ luminescence centers presented in different publications.     

REE constraints on fractionation processes of massive-type anorthosites on the Lofoten Islands, Norway

Summary Rare Earth Element (REE) data of 34 samples of magmatic rocks from the Lofoten Islands in Norway lend support to the derivation of anorthosites, ferrodiorites and jotunites by fractionation and cumulus processes from typical basaltic magma. Both REE concentration and Eu anomalies (expressed as Eu/Eu^*) form continuous linear trends from anorthosite towards gabbro, ferrodiorite and jotunite in discrimination diagrams against molar CaO/Al₂O₃ ratios indicating the predominant accumulation of plagioclase. Eu/Eu^* decreases from about 4 in the cumulates (anorthosites) to around 1 in the fine-grained gabbroic dikes and to below 1 in some ferrodiorites and the jotunite. The various types of ferrodiorites and the jotunite are regarded as residual liquids, in some cases with variable amounts of cumulus plagioclase. The whole fractionation series from gabbro towards anorthosites and ferrodiorites can be observed in a single intrusion. With increasing fractionation, the REE patterns generally change from flat, slightly LREE-enriched or LREE-depleted to steep and strongly LREE-enriched. These changes and the REE abundances are mainly controlled by the abundance of apatite. Temporally and spatially related mangerites and charnockites form a trend from low-SiO₂ mangerites with Eu/Eu^* > 1 to intermediate-SiO₂ acidic mangerites with Eu/Eu^* ≈ 1 and charnockites with Eu/Eu^* < 1. Accordingly, the low-SiO₂ mangerites are interpreted as alkali feldspar-rich cumulates and the charnockites as residual liquids derived from the acidic mangerites. The mangerites with Eu/Eu^* around 1 have patterns similar to those of some highly evolved ferrodiorites possibly indicating a genetic link. Received December 12, 1999; revised version accepted November 15, 2000     

Influence of liquid structure on diffusive isotope separation in molten silicates and aqueous solutions

Molecular diffusion in natural volcanic liquids discriminates between isotopes of major ions (e.g., Fe, Mg, Ca, and Li). Although isotope separation by diffusion is expected on theoretical grounds, the dependence on mass is highly variable for different elements and in different media. Silicate liquid diffusion experiments using simple liquid compositions were carried out to further probe the compositional dependence of diffusive isotopic discrimination and its relationship to liquid structure. Two diffusion couples consisting of the mineral constituents anorthite (CaAl₂Si₂O₈; denoted AN), albite (NaAlSi₃O₈; denoted AB), and diopside (CaMgSi₂O₆; denoted DI) were held at 1450 °C for 2 h and then quenched to ambient pressure and temperature. Major-element as well as Ca and Mg isotope profiles were measured on the recovered quenched glasses. In both experiments, Ca diffuses rapidly with respect to Si. In the AB-AN experiment, D_Ca/D_Si ≈ 20 and the efficiency of isotope separation for Ca is much greater than in natural liquid experiments where D_Ca/D_Si ≈ 1. In the AB-DI experiment, D_Ca/D_Si ≈ 6 and the efficiency of isotope separation is between that of the natural liquid experiments and the AB-AN experiment. In the AB-DI experiment, D_Mg/D_Si ≈ 1 and the efficiency of isotope separation for Mg is smaller than it is for Ca yet similar to that observed for Mg in natural liquids.The results from the experiments reported here, in combination with results from natural volcanic liquids, show clearly that the efficiency of diffusive separation of Ca isotopes is systematically related to the solvent-normalized diffusivity - the ratio of the diffusivity of the cation (D_Ca) to the diffusivity of silicon (D_Si). The results on Ca isotopes are consistent with available data on Fe, Li, and Mg isotopes in silicate liquids, when considered in terms of the parameter D_cation/D_Si. Cations diffusing in aqueous solutions display a similar relationship between isotopic separation efficiency and D_cation/D_H2O, although the efficiencies are smaller than in silicate liquids. Our empirical relationship provides a tool for predicting the magnitude of diffusive isotopic effects in many geologic environments and a basis for a more comprehensive theory of isotope separation in liquid solutions. We present a conceptual model for the relationship between diffusivity and liquid structure that is consistent with available data.     

Schorlomite: a discussion of the crystal chemistry, formula, and inter-species boundaries

Examination of schorlomite from ijolite at Magnet Cove (USA) and silicocarbonatite at Afrikanda (Russia), using electron-microprobe and hydrogen analyses, X-ray diffraction and Mössbauer spectroscopy, shows the complexity of substitution mechanisms operating in Ti-rich garnets. These substitutions involve incorporation of Na in the eightfold-coordinated X site, Fe²⁺ and Mg in the octahedrally coordinated Y site, and Fe³⁺, Al and Fe²⁺ in the tetrahedrally coordinated Z site. Substitutions Ti⁴⁺Fe³⁺Fe³⁺_–1Si_–1 and Ti⁴⁺Al³⁺Fe³⁺_–1Si_–1 are of major significance to the crystal chemistry of schorlomite, whereas Fe²⁺ enters the Z site in relatively minor quantities (<3% Fe). There is no evidence (either structural or indirect, such as discrepancies between the measured and calculated Fe²⁺ contents) for the presence of ^[6]Ti³⁺ or ^[4]Ti⁴⁺ in schorlomite. The simplified general formula of schorlomite can be written as Ca₃Ti⁴⁺₂[Si_3-x(Fe³⁺,Al,Fe²⁺)_xO₁₂], keeping in mind that the notion of end-member composition is inapplicable to this mineral. In the published analyses of schorlomite with low to moderate Zr contents, x ranges from 0.6 to 1.0, i.e. Ti⁴⁺ in the Y site is <2 and accompanied by appreciable amounts of lower-charged cations (in particular, Fe³⁺, Fe²⁺ and Mg). For classification purposes, the mole percentage of schorlomite can be determined as the amount of ^[6]Ti⁴⁺, balanced by substitutions in the Z site, relative to the total occupancy in the Y site: (^[6]Ti⁴⁺–^[6]Fe²⁺–^[6]Mg²⁺– ^[8]Na⁺)/2. In addition to the predominant schorlomite component, the crystals examined in this work contain significant (>15 mol.%) proportions of andradite (Ca₃Fe³⁺₂Si₃O₁₂), morimotoite (Ca₃Fe²⁺TiSi₃O₁₂), and Ca₃MgTiSi₃O₁₂. The importance of accurate quantitative determination and assignment of Fe, Ti and other cations to the crystallographic sites for petrogenetic studies is discussed.

Cation distribution in amphiboles synthesized along the Ga analogue of the tremolite–aluminotschermakite join

 Amphiboles were synthesized from bulk compositions prepared along the join Ca_1.8Mg_5.2Si₈O₂₂(OH)₂–Ca_1.8Mg₃Ga₄Si₆O₂₂(OH)₂ hydrothermally at 750–850 °C and 1.0–1.8 GPa, and along the join Ca₂Mg₅Si₈O₂₂F₂–Ca₂Mg₃Ga₄Si₆O₂₂F₂, anhydrously at 1000 °C and 0.7 GPa to document how closely the tschermak-type substitution is obeyed in these analogues of aluminous amphiboles. Electron-microprobe analyses and Rietveld X-ray diffraction structure refinements were performed to determine cation site occupancies. The extent of Ga substitution was found to be limited in both joins, but with the fluorine series having about twice the Ga content (0.6 atoms per formula unit, apfu) of the hydroxyl-series amphiboles (0.3 apfu). The tschermak-type substitution was followed very closely in the hydroxyl series with essentially equal partitioning of Ga between tetrahedral and octahedral sites. The fluorine-series amphiboles deviated significantly from the tschermak-type substitution and, instead, appeared to follow a substitution that is close to a Ca-pargasite substitution of the type: ^[6]Ga³⁺+2^[4]Ga³⁺+1/2^[A] Ca²⁺ = ^[6]Mg²⁺+2^[4]Si⁴⁺+1/2^[A]□. Infrared spectroscopy revealed an inverse correlation between the intensity of the OH-stretching bands and the Ga content for the hydroxyl- and fluorine-series amphiboles. The direct correlation between the Ga and F content and inverse relationship between the Ga and OH content may be a general phenomenon present in other minerals and suggests, for example, that high F contents in titanite are controlled by the Al content of the host rock and that there may be similar direct Al–F correlations in tschermakitic amphiboles. Evidence for the possibility that Al (Ga) might substitute onto only a subset of the tetrahedral sites in tschermakitic amphiboles was sought but not observed in this study. Received: 5 March 2001 / Accepted: 31 July 2001     

Laser Ablation ICPMS study of trace element partitioning between plagioclase and basaltic melts: an experimental approach

Plagioclase-melt partition coefficients (D) for 34 trace elements at natural concentration levels were determined experimentally in a natural MORB composition at atmospheric pressure using thin Pt-wire loops. Experiments were carried out at three temperatures (1,220, 1,200, and 1,180°C), and at three different oxygen fugacities (fO₂ = IW, QFM, air) in order to assess the effect of fO₂ on the partitioning of elements with multiple valence (Fe, Eu, Cr). Run products were analyzed by laser-ablation ICP-MS. Most trace element Ds increase slightly as temperature decreases, except for D _Zr, D _Fe, D _Eu and D _Cr that vary systematically with fO₂. Applying the Lattice Strain Model to our data suggests the presence of Fe²⁺ entirely in the octahedral site at highly to moderate reducing conditions, while Fe³⁺ was assigned wholly to the tetrahedral site of the plagioclase structure. Furthermore, we provide a new quantitative framework for understanding the partitioning behaviour of Eu, which occurs as both 2+ and 3+ cations, depending on fO₂and confirm the greater compatibility of Eu²⁺, which has an ionic radius similar to Sr, relative to Eu³⁺ in plagioclase and the higher Eu²⁺/ Eu³⁺ under reducing conditions. For petrogenetic basaltic processes, a combined fractionation of Eu²⁺–Sr and Fe–Mg by plagioclase has considerable potential as an oxybarometer for natural magmatic rocks.     

Effect of pressure on the diffusivity of network-forming cations in melts of jadeitic compositions

The diffusivities of network-forming cations (Si⁴⁺, Al³⁺, Ge⁴⁺ and Ga³⁺) in melts of the jadeitic composition NaAl(Si, Ge)₂O₆ and Na(Al, Ga)Si₂O₆ have been measured at pressures between 6 and 20 kbar at 1400°C. The rates of interdiffusion of Si⁴⁺-Ge⁴⁺ and Al³⁺-Ge³⁺ increase with increasing pressure at constant temperature. The results are consistent with the ion-dynamics computer simulations of Jadeite melt by Angellet al. (1982, 1983). The coefficient measured for the Si⁴⁺-Ge⁴⁺ interdiffusion is between 8 × 10^?10 and 2.5 × 10^?8$\text{cm}{}^2\text{sec}$ at 6 kbar, depending on the composition of the melt, whereas at 20 kbar it is between 7 × 10^?9 and 2 × 10^?7$\text{cm}{}^2\text{sec}$. The effect of pressure is greater for more Si-rich compositions (i.e., closer to NaAlSi₂O₆ composition). The coefficient measured for the Al³⁺-Ga³⁺ inter- diffusion is between 9 × 10^?10 and 3 × 10^?9 cm²/sec at 6 kbar and between 3 × 10^?9 and 1 × 10^?8$\text{cm}{}^2\text{sec}$ at 20 kbar. The rate of increase in diffusivity with pressure of Al³⁺-Ga³⁺ (a factor of 3–4) is smaller than that of Si⁴⁺-Ge⁴⁺ (a factor of 7–17).The Si⁴⁺-Ge⁴⁺ interdiffusion in melts of Na₂O · 4(Si, Ge)O₂ composition has also been measured at 8 and 15 kbar for comparison. The effect of pressure on the diffusivity in this melt is significantly smaller than that for the jadeitic melts. The increase in diffusivity of the network-forming cations in jadeitic melts with increasing pressure may be related to the decrease in viscosity of the same melt. The present results, as well as the ion-dynamics simulations, suggest that the homogenization of partial melts and mixing of magmas would be more efficient at greater depths.     

Uvarovite: Stability of uvarovite-andradite solid solutions at low pressure

Uvarovite (Ca₃Cr₂Si₃O₁₂) forms a complete solid solution series with andradite (Ca₃Fe ₂ ⁺³ Si₃O₁₂) below 1,137±5 ° C at a total pressure of 1 atm. Pure uvarovite decomposes to pseudowollastonite (CaSiO₃)+eskolaite (Cr₂O₃) at 1,385 ± 10 ° C. The incorporation of Ca₃Fe ₂ ⁺³ Si₃O₁₂ component in the uvarovite structure lowers the thermal stability of the garnet. The breakdown assemblage is garnet_ss (Ca₃(Cr,Fe⁺³ ₂)Si₃O₁₂)+pseudowollastonite (CaSiO₃)+hemeskolaite_ss(Cr,Fe⁺³O₃). Pure andradite decomposes to pseudowollastonite (CaSiO₃)+hematite (Fe₂O₃) at 1,137±5 °C. Andradite thermal stability is increased by incorporation of Ca₃Cr₂Si₃O₁₂ component by 248 °C.At 1,264±5 °C pseudowollastonite+hematite react to liquid defining a thermal minimum of the CaSiO₃-Cr₂O₃-Fe₂O₃ ternary system. This minimum is located at about 64.5 wt.-% CaSiO₃, 0.5 wt.-% Cr₂O₃, and 35.0 wt.-% Fe₂O₃. Uvarovite and andradite bulk compositions start to melt at 1,420 °C and 1,265 ±5 °C, respectively.The unit-cell parameter for uvarovite is 11.999 (2) Å, the refractive index 1.866 (2). The substitution of Cr⁺³ by Fe⁺³ increases a and n almost linearly toward the andradite end member which displays a unit-cell parameter of 12.059 (3) Å and a refractive index of 1.887 (2).     

Luminescence of Eu<Superscript>2+</Superscript> in feldspars from muscovite pegmatites

The data on photoluminescence (PL) that precisely detects Eu²⁺ centers and X-ray luminescence (XL) were compared for plagioclases and potassium feldspars in 21 samples from muscovite pegmatites of the Mama region. The Eu contents determined in 10 samples vary from 10^?4 to 10^?6 wt %. Europium occurs mainly as bivalent species that replaces Sr²⁺, Ca²⁺, and Ba²⁺. Eu is gained in the products of early crystallization, and its relative amounts decrease by an order of magnitude in the course of pegmatite formation down to complete disappearance in late generations of feldspars. It is shown that Eu²⁺ can be detected in XL spectra, and the Eu²⁺ content can be determined in qualitative terms, for instance, by the intensity of radiation band 400–420 nm in plagioclase.     

Transport properties of high albite crystals,near-endmember feldspar and pyroxene glasses,and their melts to high temperature

Thermal diffusivity (D) was measured using laser-flash analysis (LFA) from oriented single-crystal albite and glasses near LiAlSi₃O₈, NaAlSi₃O₈, CaAl₂Si₂O₈, LiAlSi₂O₆ and CaMgSi₂O₆ compositions. Viscosity measurements of the supercooled liquids, over 2.6 × 10⁸ to 8.9 × 10¹² Pa s, confirm strongly non-Arrhenian behavior for CaAl₂Si₂O₈, and CaMgSi₂O₆, and near-Arrhenian behavior for the others. As T increases, D _glass decreases, approaching a constant near 1,000 K. Upon crossing the glass transition, D decreases rapidly. For feldspars, D for the melt is ~15% below D of the bulk crystal, whereas for pyroxenes, this difference is ~40%. Thermal conductivity (k _lat = ρC _P D) of crystals decreases with increasing T, but k _lat of glasses increases with T because heat capacity (C _P ) increases with T more strongly than density (ρ) and D decrease. For feldspars, k _lat for the melt is ~10% below that of the bulk crystal or glass, whereas this decrease for pyroxene is ~50%. Therefore, melting substantially impedes heat transport, providing positive thermal feedback that could promote further melting.     

Uvarovite: Stability of uvarovite-grossularite solid solution at low pressure

Uvarovite (Ca₃Cr₂Si₃O₁₂) forms a complete solid solution series with grossularite (Ca₃Al₂Si₃O₁₂) below 855 ± 5 ° C at a total pressure of 1 atm. Pure uvarovite decomposes to pwo (CaSiO₃) + esk (Cr₂O₃) at 1385 ± 10 ° C. The incorporation of about 5 wt-% of Ca₃Al₂ Si₃O₁₂ component in the uvarovite structure raises the thermal stability of the garnet_ss to 1410 ± 5 ° C, and uvarovite₉₅ grossularite₀₅ melts incongruently to pwo (CaSiO₃) + coresk_ss ((Al, Cr)₂O₃) + L. Pure grossularite decomposes to wo (CaSi0₃) + geh (Ca₂Al₂SiO₇) + and (CaAl₂Si₂O₈) at 855 ± 5 ° C, grossularite thermal stability is increased by incorporation of Ca₃Cr₂Si₃O₁₂ component by 530 ° C. At 1280±5 ° C coresk_ss + L react to gar_ss + geh + an defining an invariant tequilibrium of the CaO-Cr₂O₃-Al₂O₃-SiO₂ quaternary system. Liquid reacts to gar_ss + pwo + geh + an at 1263 ±5 ° C terminating univariant and divariant liquid relations occurring along the join Ca₃Cr₂Si₃O₁₂-Ca₃Al₂Si₃O₁₂. The unit-cell parameter for uvarovite is 11.996(2) Å, the refractive index 1.865(3). The substitution of Cr by Al decreases a and n almost linearly toward the grossularite end member which displays a unit-cell parameter of 11.848(2) Å and a refractive index of 1.732 (1).     

Garnet-pyroxene equilibria in the system CaO-MgO-Al2O3-SiO2 (CMAS): prospects for simplified (‘T-independent’) lherzolite barometry and an eclogite-barometer

New experimental data on compositions of garnets in two-pyroxene — garnet assemblages in the system CaO –MgO –Al₂O₃ –SiO₂ (CMAS) are presented for conditions between 1,100 and 1,570° C and 30 to 50 kb. Garnets in these assemblages become less calcic with increasing pressure. Garnet-orthopyroxene barometry (Al-solubility-barometry) pertinent to geobarometry for garnet lherzolites has been evaluated with a set of experimental data covering the range 900 to 1,570° C and 15 to 100 kb. Various formulations of this barometer work well to 75 kb. Phase equilibria are not sufficient to positively verify the thermodynamic validity of any of such models. Empirical garnet-orthopyroxene barometry at least in the system CMAS can be formulated to obtain a pressure estimate without previous temperature estimation (P(kb)=34.4-19.175 1n X _Al ^M1 +17.702 1n X _Ca ^M2 ). The potential application of an analogous garnetclinopyroxene equilibrium is limited because the amount of Ca-Tschermaks in natural clinopyroxenes is usually quite small in garnet lherzolites and many eclogites. The Ca-Mg exchange between garnet and clinopyroxene appears however sufficiently sensitive to pressure to allow calibration of a CMAS barometer. The reaction 3CaMgSi₂O₆+Mg₃Al₂Si₃O₁₂=3Mg₂Si₂O₆+Ca₃Al₂Si₃O₁₂ has a V ^o of 3.5 cm³. The total pressure dependency of this reaction is however closer to a theoretical V ^o of about 5 cm³ when excess volume properties of the phases involved are taken into account. We have calibrated such a barometer (mean error of estimate 2.8 kb) for assemblages with pyrope-rich (py>80) garnets and orthopyroxenes. This may provide the basis for a geobarometer for eclogites from kimberlites.Abbreviations Used in the Text CaTs Ca-tschermak's molecule, CaAl₂SiO₆ - cpx clinopyroxene - di diopside, CaMgSi₂O₆ - en enstatite, Mg₂Si₂O₆ - gr grossular, Ca₃Al₂Si₃O₁₂ - gt garnet - MgTs Mg-Tschermak's molecule, MgAl₂SiO₆ - opx orthopyroxene - px pyroxene - py pyrope, Mg₃Al₂Si₃O₁₂ - a _i ^j activity of component i in phase j - activity coefficient - G(I) molar Gibbs free energy difference of reaction (I) at standard state unless specified otherwise - H(I), (H _I) molar enthalpy (difference) of phase (reaction) (I) at standard state unless specified otherwise - S (I), (S _I) molar entropy (difference) of phase (reaction) (I) at standard state unless specified otherwise - V ^o, (V _I ^o) molar volume (difference) of phase (reaction) (I) at standard state - X _i ^j mole fraction of component i in phase j