Partitioning data pertaining to Fe-Mg ordering around trace cations in olivine and low-Ca pyroxene

We have used trace element partitioning data available in the literature to investigate nonideality of the cations of Yb, Sm, Gd, Ca, Mn, Sc, Ni, and Al in silicate melt, olivine, and low-Ca pyroxene. Results are consistent with ordering of Mg and Fe around trace cations in olivine and pyroxene. On the basis of these data, we suggest there is an increasing tendency for Fe to congregate in the vincinity of the trace cation as the size of the trace cation increases. These results are important both in achieving a better understanding of trace element behavior in crystals and in constraining the temperature and compositional dependence of trace element partitioning.     

Mineral reactions participating in intragranular fracture propagation: implications for stress corrosion cracking

Two examples of mineral reactions accompanying intragranular fracturing of silicates are described from amphibolites at the Grenville front, Coniston, Ontario, and a granodiorite within the Miéville shear zone, Switzerland. At these localities coarse grained (> 1 mm) hornblende and biotite respectively have undergone initial deformation by intense transgranular fracturing. The ambient temperature of deformation is estimated at 400–500°C for the amphibolites, and 250–300°C for the granodiorite.Fracture intensity within hornblende increases with progressive deformation until fractures coalesce and grains lose cohesion. Fractures are occupied by hornblende in close optical continuity with the parent grain, and typically contain median sutures decorated by arrays of solid inclusions. Relative to the parent grain, hornblende in fractures is depleted in Ti, Al, plus K, and enriched in Mg. Given the preferential partitioning of Mg, Al^VI and Ti into the M2 site of calcic amphiboles, the decrease of Ti and Al^VI in the host-to-crack transition is consistent with the corresponding increase of the Mg/(Mg + Fe²⁺) ratio.Fractures within biotite are bounded by an envelope of paler brown biotite which corresponds to a decrease of Ti and increase of Fe + Mg relative to regions unaffected by cracking. Fractures are occupied by secondary ilmenite, low-Ti biotite and high-Ti muscovite. Ti and Al do not vary significantly as a function of Mg/(Fe + Mg) in the host-to-crack transition, as anticipated from the approximately equal partitioning of these two cations into the M1 and M2 octahedral cation sites. The direct relationship of the mineral reactions to the fractures is taken as evidence for the participation of the reactions in crack propagation. These features may thus represent examples of natural stress corrosion cracking.     

黑云母的红外光谱研究

关于云母族矿物的红外光谱研究,国内外一些学者曾做过不少工作。笔者就近年来所收集到的一些产自宁芜地区、南岭地区花岗岩类岩石中27个黑云母样品,进行了红外光谱测定,确定了某些特征吸收峰的偏移及其与化学组分之间的关系,并探讨了黑云母矿物中主要金属阳离子的相互置换和     

漳州地区白垩至纪I型和A型花岗岩中黑云母的矿物学特征

漳州地区白垩纪花岗岩由Ｉ型和Ａ型花岗岩组成，本文对该地区两类花岗岩中黑云母的矿物化学、矿物物理特征进行了系统研究。发现在两类花岗岩中的黑云母矿物学特征存在重大差别，如Ａ型花岗岩中黑云母更富铝、铁、氟及水而贫镁、硅，形成温度及氧逸度较低等。在Ａ型花岗岩中的黑云母穆斯堡尔谱也与Ｉ型不同，四极分裂（Ｑ．Ｓ）较高，且Ｆｅ￣（２＋）在Ｍ＿２位，Ｆｅ（３＋）在Ｔ位占位率也较高。两类花岗岩中的黑云母化学成分随岩浆演化也呈现不同的变化规律。黑云母的矿物学特征可提供花岗岩成因类型的有益信息。     

Transient and quasi-steady-state dissolution of biotite at 22-25°C in high pH, sodium, nitrate, and aluminate solutions

Biotite dissolution under conditions of high pH and high aluminum, sodium, and nitrate concentrations analogous to those found in tank wastes at the Hanford Site was investigated using continuously stirred flow-through reactors at 22 to 25 °C. Experiments were designed to simulate tank leaks into the Hanford vadose zone where Fe(II) from biotite is the dominant reducing agent available to immobilize certain contaminants. Both non-steady-state and steady-state dissolution kinetics were quantified; interest in non-steady-state kinetics derives from the inherently transitory nature of tank leaks. Biotite was conditioned in pH 8 solutions to simulate the alkaline environment of the Hanford sediment, and then reacted in pH 10-14 solutions, some including 0.055 M Al(NO₃)₃ and/or 2 M or 6 M NaNO₃. Initial dissolution transients (intervals of rapid release rates that decay to slower steady-state rates) showed fast preferential release of K followed by near-stoichiometric release of Si, Al, and Mg, and slower release of Fe. Each increase in pH resulted in a second transient with the greatest amounts of Si, Al, and K released at pH 14, followed by pHs 13, 12, 11, and 10. Fe release also was highest at pH 14, but unchanging at pHs 10-13 within experimental error. Transient releases at high pH are attributed to dissolution of amphoteric secondary phases such as ferrihydrite that are inferred from saturation calculations and solid analyses to form during the conditioning interval. Transient release of Si was inhibited by the presence of 0.055 M Al(NO₃)₃; the effects of Al(NO₃)₃ and NaNO₃ on the release rates of Al, Fe, Mg, and K were variable and generally outweighed by the effect of pH. Quasi-steady-state release rates were slowest at pH 11-12 (10^−12.2 mol biotite m⁻² s⁻¹ for Si) and increased in either direction in pH away from this minimum (to 10^−11.5 at pHs 8 and 14 for Si). Fe release rates at high pH were sufficient to account for observed Cr(VI) reduction at Hanford. The net release rates of the major framework cations, from which the biotite dissolution rate is inferred, may reflect the precipitation of secondary phases or the alteration of biotite to vermiculite. The most extensive solid-phase alterations were observed in Na-enriched solutions.     

橄榄岩中尖晶石化学成分分带的机理及其地质意义

橄榄岩中尖晶石的化学成分分带显示，在由温度降低引起的亚固相平衡过程中，与橄榄石及斜方辉石相邻的尖晶石边缘贫Mg和Al、富Fe和Cr，而与单斜辉石相邻的尖晶石边线相反。尖晶石的Fe和Mg分带为其他矿物的粒间Fe－Mg离子扩散所致；尖晶石中的Cr和Al分带则是尖晶石晶体内部Cr－Al离子的交互扩散所致。本文从热力学理论角度说明了尖晶石晶体内部的Cr－Al交互扩散受其Fe－Mg分带的控制；从热力学计算证明了尖晶石内Al必须从富Fe处向富Mg处扩散，解释了尖晶石中Mg和Al及Fe和Cr分带的一致性。     

Crystal chemistry, structure analyses and phase transition experiment on an omphacite from eclogitic metagabbro from Syros island, Greece

Summary Pyroxene samples, from the Greek island of Syros, taken from a blueschist-eclogite facies Mg-rich metagabbro, were investigated by chemical and XRD analyses and M?ssbauer spectroscopy. Single-crystal XRD and microprobe analysis showed that the natural sample is a typical omphacite of intermediate composition in the Ca–Na pyroxene solid solutions. The space group P2/n was confirmed and the cations Mg, Al and Ca, Na were found to be ordered in the M1 and M2 positions, respectively. M?ssbauer spectroscopy showed that there is both ferrous and ferric iron in the structure, with the ratio 1.38:1. The M2 sites are fully occupied by Ca and Na, thus the iron (Fe²⁺ and Fe³⁺) can substitute only for Mg and Al in the M1 sites. Partial disorder was attained by annealing the sample at 850 °C, 20 kbar for 7 days, as confirmed by decrease of intensity of reflections affected by the C-type extinction.     

A normalizing procedure for comparing chemical variations in pyroxenes in mafic granulites

Data from analyses of three coexisting pairs of pyroxenes with a wide range of Fe content from each of two localities are used to show the large systematic variation and predictable correlation of Fe (or Mg) of a pyroxene with its content of Al, Mn and Na in mafic granulites. Comparisons of pyroxenes can then be made more meaningful by normalizing Al, or other elements, to an appropriate Mg value. As both P and T may affect the element distribution of the two pyroxenes differently (especially Al and Na) the factor used in normalizing is found to vary from region to region.     

Crystal chemistry of ferric iron in (Mg,Fe)(Si,Al)O<Subscript>3</Subscript> majorite with implications for the transition zone

Fifteen samples of (Mg,Fe)SiO₃ majorite with varying Fe/Mg composition and one sample of (Mg,Fe)(Si,Al)O₃ majorite were synthesized at high pressure and temperature under different conditions of oxygen fugacity using a multianvil press, and examined ex situ using X-ray diffraction and Mössbauer and optical absorption spectroscopy. The relative concentration of Fe³⁺ increases both with total iron content and increasing oxygen fugacity, but not with Al concentration. Optical absorption spectra indicate the presence of Fe²⁺–Fe³⁺ charge transfer, where band intensity increases with increasing Fe³⁺ concentration. Mössbauer data were used in conjunction with electron microprobe analyses to determine the site distribution of all cations. Both Al and Fe³⁺ substitute on the octahedral site, and charge balance occurs through the removal of Si. The degree of Mg/Si ordering on the octahedral sites in (Mg,Fe)SiO₃ majorite, which affects both the c/a ratio and the unit cell volume, is influenced by the thermal history of the sample. The Fe³⁺ concentration of (Mg,Fe)(Si,Al)O₃ majorite in the mantle will reflect prevailing redox conditions, which are believed to be relatively reducing in the transition zone. Exchange of material across the transition boundary to (Mg,Fe) (Si,Al)O₃ perovskite would then require a mechanism to oxidize sufficient iron to satisfy crystal-chemical requirements of the lower-mantle perovskite phase.     

袁复礼石的矿物学新资料及晶体结构

袁复礼石是一种Ｍｇ、Ｆｅ３＋、Ａｌ３＋和Ｔｉ的硼酸盐新矿物。除了与遂安石、硬石膏和磷灰石共生外，还与硼铝镁石、镁橄榄石和方解石等共生，形成两个矿物共生组合。单矿物化学分析表明，后一组合中的袁复礼石更富铁，特别是Ｆｅ３＋，相对贫镁。袁复礼石的晶体结构分析证明，袁复礼石与硼钛镁石的结构类型一致，阳离子的占位为Ｍ（１）＝Ｍｅ３＋，Ｍ（２）＝Ｍｅ２＋。袁复礼石是Ｍ（１）位置上为Ｆｅ端员组分，Ｍ（２）位置上为Ｍｇ端员组分的新矿物。     

Crystal-chemistry and cation ordering in the system diopside-jadeite: A detailed study by crystal structure refinement

The Nybö eclogite pod in Norway is characterized by a great variety of clinopyroxene compositions with Jd contents ranging from less than 5% up to nearly 80%, whilst Ac+Hd contents remain almost constant (mostly within 10±5%).Unconstrained X-ray structure refinement has been carried out on 16 pyroxene crystals (8 with C2/c and 8 with P2/n space group) from the Nybö eclogite, and also on one omphacite crystal (from Lago Mucrone in the Sesia-Lanzo Zone, Western Alps) which displays the highest degree of cation ordering yet described. The final discrepancy factors range from 0.014 to 0.029. The population of the sites has been determined on the basis of bond length considerations and of the results of the site occupancy refinement. Six of these crystals were subsequently analysed by electron microprobe.The tetrahedral sites are occupied by Si with negligible amounts of Al. Al, Mg, Fe³⁺ and Fe²⁺ occur at the octahedral sites; in the ordered P2/n crystals Al and Fe³⁺ are concentrated at the M11 site, whilst Mg and Fe²⁺ are concentrated and the M1 site. The eight-coordinated sites contain Ca and Na with negligible amounts of Fe and/or Mg. Ordering of Ca and Na takes place in the P2/n samples in such a way that in the most ordered crystal the M2 site contains almost exactly 0.75 Na+0.25 Ca and the M21 site 0.25 Na+0.75 Ca.Some geometrical features of the tetrahedra as well as of the octahedra (e.g. tetrahedral quadratic elongation and TILT angle) are not a simple linear function of composition, even when no change in space group occurs. The crystals evidently do not behave like a binary system of the two components, Di and Jd, but behave rather as if the composition Di_0.50 Jd_0.50 was a distinct end member.The boundaries between disordered and ordered phases in the Nybö pyroxenes fall at about 0.35 and 0.65 Jd/(Di+ Jd), in close agreement with the previous TEM investigations.The degree of order varies with composition following a bell-shaped curve: different coaxial bell-shaped curves can be drawn for crystals which have similar compositions but come from different metamorphic environments. The order vs composition diagrams may be useful for the interpretation of the P-T-t histories of the host rocks.     

Amphibolitization of metagabbros in the Scottish Highlands

Abstract Compositions of actinolite, hornblende and cummingtonite, together with pyroxene and plagioclase, are studied in basic intrusions in the Dalradian of north-east Scotland, and the Glen Scaddle complex in the West Moine. Amphibolitization is due to influx of water from the country rocks. Pyroxene compositions are found to have adjusted to the regional metamorphic environment. Owing to the difficulty of diffusion of Al and Si, calcic amphiboles are zoned and commonly contain quartz blebs. Discontinuities in zoning give rise to actinolite-hornblende pairs. Compared with north-east Scotland, disequilibrium is less strong in the Glen Scaddle area: in the latter, plagioclase compositions have been greatly changed, Na partition between hornblende and plagioclase is close to equilibrium, the maximum Al content of hornblende is lower and zoning patterns are more consistent. The Fe/Mg ratio in calcic amphiboles varies with Al content, while approaching equilibrium partition with other minerals. Both zoning patterns and Fe/Mg partition with cummingtonite suggest that Fe/Mg of the calcic amphiboles increases more strongly with increasing (Al^vi+Fe³⁺) than can be explained simply by substitution of Al,Fe³⁺ for Mg on M2. Model reactions for amphibole formation are constructed. Cummingtonite formed at lower chemical potential of CaO than actinolite: Ca was exchanged for Mg,Fe between orthopyroxene-derived and clinopyroxene-derived local systems. Both cummingtonite and actinolite were formed because of kinetic constraints, as intermediate reaction products: actinolite-hornblende pairs represent disequilibrium. This work suggests that many occurrences of actinolite with hornblende, where the minerals are zoned, may also be due to diffusion kinetics.     

An orthopyroxene–biotite geothermometer and its application in crustal granulites and mantle-derived rocks

Using the experimental data on Fe–Mg exchange between orthopyroxene and biotite of Fonarev & Konilov (1986), an orthopyroxene–biotite geothermometer is developed. The thermometer is corrected for mixing of Ti and Al in octahedral sites in biotite and also for non-ideal mixing of Fe and Mg in orthopyroxene. The thermometer is applied to several amphibolite–granulite transition facies and granulite facies rocks and also to mantle xenoliths. It yields consistent results in rocks of widely varying bulk composition, and highly magnesian mantle xenoliths. This thermometer removes the difficulty of estimating temperature in garnet-free rocks in high-grade terrains and also provides independent estimates of temperature in garnet-bearing assemblages.     

Structure of Cu-bearing orthopyroxene, Mg(Cu.56,Mg.44)Si2O6, and behavior of Cu2+ in the orthopyroxene structure

Cu-bearing pyroxene, Mg(Cu.₅₆,Mg.₄₄)Si₂O₆, has been synthesized by a flux method and crystal structure refinement has been performed by single crystal X-ray diffraction. It is found that the crystal structure is orthorhombic (space group Pbca) with unit cell dimensions of a=18.221(4), b=8.890(1), c=5.2260(7)Å and the cell volume of 846.5( )3ų. In the M2-site one of the M-O bonds(M-O3B) is extremely expanded from 2.444(2) in enstatite to 2.732(2), thus the coordination polyhedron around M2-site is regarded as square pyramidal rather than square planar or octahedral. It is also found that the M1-site in the pyroxene structure is occupied almost exclusively by Mg, while the M2-site is almost evenly occupied by Mg and Cu. The observed extreme site preference shown by Cu²⁺ is unusual among the divalent cations with similar ionic sizes.     

Application of scanning X-ray analytical microscope to the petrographic characterization of a ductile shear zone: an alternative method to image microstructures

A ductile shear zone within a metasomatic biotite band in the Ryoke granite, Teshima, SW Japan, has been studied using the scanning X-ray analytical microscope (SXAM). This enabled the quantitative distributions of major elements, such as Si, K, Fe, Al and Ca, to be determined within the shear zone. These element maps were processed to transform them into images showing the distribution of minerals such as quartz, biotite, plagioclase and K-feldspar, which form the major minerals within the biotite band and the granite protolith. Mineral profiles based on these mineral maps compared with the simple shear strain profile reveal that the shear zone is most intense where quartz and biotite have been substituted for the primary mineral assemblage of the granite protolith, suggesting that the stresses imposed on the granite caused the shear strain to localize along the biotite band to produce the observed shear zone. It appears that the rheological behavior changed around 50–60% of quartz modal composition.     

Coexisting pyroxenes — a multivariate statistical approach

A population of 117 coexisting nonalkaline pyroxene pairs has been studied statistically to evaluate compositional and thermal effects on the element distribution. K_DMg^opx−cpx is influenced by the Fe/Mg-ratio, by the Ca content—especially of clinopyroxene—and by the content of tetrahedral Al. Fe and tetrahedral Al are found to be negatively correlated. A principal component analysis based on the variation of Si, Al^IV, Al^IV, Fe, Mg, Mn, Ca is performed. Dropping of highly correlated variables does not affect the result significantly. The first principal component reflects the major chemical variation in Fe and Mg. When using Ferrous and ferric iron as separate entries of the analysis, either the second or the third component demonstrates a temperature dependence. It is, however, not possible to obtain pure temperature and chemical components due to the composition not being uncorrelated to temperature of formation. From these components a graph reflecting temperature of formation has been constructed.     

The distribution of certain elements among coexisting calcic pyroxenes, calcic amphiboles and biotites in skarns

Skarns composed predominantly of Ca pyroxene, Ca amphibole, and biotite occur locally in the Grenville metamorphic terrain of the Canadian pre-Cambrian Shield. The enclosing rocks are Grenville gneisses and limestones, gabbroic rocks, veined gneisses, and granitic rocks. The metamorphic grade of the skarns is identical to that of the enclosing rocks, namely the upper region of the amphibolite facies.

Twelve skarn specimens were selected for chemical study. An investigation is made of the concentrations of Al, Fe, Mg, Mn, Ti, Ca, Na, K, Ba, V, Cr, Zr, Y, and Sc in eleven Ca pyroxenes, ten Ca amphiboles, and eleven biotites.

Consideration is given to the distribution of elements among coexisting minerals. Regular relationships appear when

1. (1) the concentration of an element in a mineral is related to the concentration of the element in a coexisting mineral by a linear or non-linear function

2. (2) the linear or non-linear function is itself a function of the concentration of another element in one or both of the coexisting minerals. These relationships indicate that, for the most part, minerals within each skarn specimen closely approached a state of chemical equilibrium.

A preliminary attempt is made to explain the distribution relationships in terms of crystal chemistry.     

Rare earth and high field strength element partitioning between iron-rich clinopyroxenes and felsic liquids

Rare earth elements are commonly assumed to substitute only for Ca in clinopyroxene because of the similarity of ionic radii for REE³⁺ and Ca²⁺ in eightfold coordination. The assumption is valid for Mg-rich clinopyroxenes for which observed mineral/melt partition coefficients are readily predicted by the lattice strain model for substitution onto a single site (e.g., Wood and Blundy 1997). We show that natural Fe-rich pyroxenes in both silica-undersaturated and silica-oversaturated magmatic systems deviate from this behavior. Salites (Mg# 48–59) in phonolites from Tenerife, ferrohedenbergites (Mg# 14.2–16.2) from the rhyolitic Bandelier Tuff, and ferroaugites (Mg# 9.6–32) from the rhyolitic Rattlesnake Tuff have higher heavy REE contents than predicted by single-site substitution. The ionic radius of Fe²⁺ in sixfold coordination is substantially greater than that of Mg²⁺; hence, we propose that, in Fe-rich clinopyroxenes, heavy REE are significantly partitioned between eightfold Ca sites and sixfold Fe and Mg sites such that Yb and Lu exist dominantly in sixfold coordination. We also outline a REE-based method of identifying pyroxene/melt pairs in systems with multiple liquid and crystal populations, based upon the assumption that LREE and MREE reside exclusively in eightfold coordination in pyroxene. Contrary to expectations, interpolation of mineral/melt partition coefficient data for heavy REE does not predict the behavior of Y. We speculate that mass fractionation effects play a role in mineral/melt lithophile trace element partitioning that is detectable among pairs of isovalent elements with near-identical radii, such as Y and Ho, Zr and Hf, and Nb and Ta.     

The partitioning of trace elements between clinopyroxene and trachybasaltic melt during rapid cooling and crystal growth

We present the variation in trace element partition coefficients measured at the interface between rapidly cooled clinopyroxene crystals and co-existing melts. Results indicate that, as the cooling rate is increased, clinopyroxene crystals are progressively depleted in Si, Ca and Mg counterbalanced by enrichments in Al (mainly tetrahedral Al^iv), Na and Ti. Partition coefficients (Ds) for rare earth elements (REE), high field strength elements (HFSE) and transition elements (TE) increase with increasing cooling rate, in response to clinopyroxene compositional variations. The entry of REE into the M2 site is facilitated by a coupled substitution where either Na substitutes for Ca on the M2 site or Al^iv substitutes for Si in the tetrahedral site. The latter substitution reflects an increased ease of locally balancing the excess charge at M2 as the number of surrounding Al^iv atoms increases. Due to the lower concentration of Ca in rapidly cooled clinopyroxenes, divalent large ion lithophile elements (LILE) on M2 decrease at the expense of monovalent cations. Conversely, higher concentrations of HFSE and TE on the M1 site are facilitated as the average charge on this site increases with the replacement of divalent-charged cations by Al^vi. Although crystallization kinetics modify clinopyroxene composition, deviations from equilibrium partitioning are insufficient to change the tendency of a trace element to be compatible or incompatible. Consequently, there are regular relationships between ionic radius, valence of the trace element and D. At both equilibrium and cooling rate conditions, Ds for isovalent cations define parabola-like curves when plotted against ionic radius, consistent with the lattice strain model, demonstrating that the partitioning of trace elements is driven by charge balance mechanisms; cation substitution reactions can be treated in terms of the energetics of the various charge-imbalanced configurations.     

An experimental study of crystalline basalt dissolution from 2 ? pH ? 11 and temperatures from 5 to 75 °C

Steady-state element release rates from crystalline basalt dissolution at far-from-equilibrium were measured at pH from 2 to 11 and temperatures from 5 to 75 °C in mixed-flow reactors. Steady-state Si and Ca release rates exhibit a U-shaped variation with pH where rates decrease with increasing pH at acid condition but increase with increasing pH at alkaline conditions. Silicon release rates from crystalline basalt are comparable to Si release rates from basaltic glass of the same chemical composition at low pH and temperatures ?25 °C but slower at alkaline pH and temperatures ?50 °C. In contrast, Mg and Fe release rates decrease continuously with increasing pH at all temperatures. This behaviour is interpreted to stem from the contrasting dissolution behaviours of the three major minerals comprising the basalt: plagioclase, pyroxene, and olivine. Calcium is primarily present in plagioclase, which exhibits a U-shaped dissolution rate dependence on pH. In contrast, Mg and Fe are contained in pyroxene and olivine, minerals whose dissolution rates decrease monotonically with pH. As a result, crystalline basalt preferentially releases Mg and Fe relative to Ca at acidic conditions. The injection of acidic CO₂-charged fluids into crystalline basaltic terrain may, therefore, favour the formation of Mg and Fe carbonates rather than calcite. Element release rates estimated from the sum of the volume fraction normalized dissolution rates of plagioclase, pyroxene, and olivine are within one order of magnitude of those measured in this study.