Garnet and clinopyroxene solid solutions,and geothermometry based on Fe-Mg distribution coefficient

The available thermodynamic mixing data of aluminosilicate garnet and clinopyroxene have been critically reviewed, and integrated with the thermochemical and selected experimental data to express the Fe-Mg distribution coefficient (K_D) between these phases as function of pressure, temperature and composition. The predicted compositional dependence of K_D agrees with the available experimental and observational data. Owing to the lack of adequate data on the mixing properties of Jadeite with diopside and hedenbergite, the geothermometric application of the model has to be currently restricted to the Na-poor bulk compositions. The temperature of a variety of rocks that have equilibrated under a wide range of P-T conditions have been estimated, and found to agree, on the average within 25°C, with other reliable temperature estimates of these rocks. The latter are, however, often in sharp disagreement with the temperatures determined on the basis of Råheim and Green's (1974, Contrib. Mineral. Petrol.48, 179–203) experimental calibration of K_D as function of temperature and pressure on tholeiitic bulk composition.     

Fe-Mg cordierite stability in high-grade pelitic rocks based on experimental,theoretical, and natural observations

Stability relations of Fe-Mg cordierite with K feldspar have been determined for conditions of muscovite-quartz instability, applicable to highgrade metamorphism of pelitic rocks. Fe cordierite, K feldspar, and water break down to Fe biotite, sillimanite, and quartz at pressures above a line through 640 ° C, 2kbar and 710 ° C, 2.7 kbar. A P-X diagram for the Fe-Mg analogue of this reaction at 675 ° C is consistent with a naturally occuring cordierite-biotite K _D value of 0.53 if Al content of biotite and cordierite water of hydration are taken into account.At higher temperatures Fe cordierite breaks down alone to almandine, sillimanite, quartz and water at pressures above a line through 650 ° C, 3.41 kbar and 760 ° C, 2.9 kbar. For the Fe-Mg reaction, P-X data up to 4 kbar may be extrapolated with use of natural K _D values increasing toward one with increasing temperatures.Lines of constant cordierite composition for the two reactions intersect in an Fe-Mg univariant reaction of sillimanite-biotite-quartz to cordieritealmandine-K feldspar-water which is metastable relative to melt at = P _tot Reduced water pressure and impurities in the garnet and K feldspar greatly reduce the temperature of this reaction so that it becomes a reasonable reaction for upper amphibolite and granulite facies conditions.The results demonstrate that (1) cordierite may be used as a geobarometer if temperature and approximate can be estimated, (2) almandine low in Mn and Ca does not participate in cordierite reactions where muscovite is present, and (3) the reaction which forms cordierite, almandine, and K feldspar is a possible melt-forming reaction which, under reduced , occurs about 50 ° C above the muscovite melting reaction.     

Structural chemistry of synthetic cordierite: Evidence for solid solutions and disordered compositional domains in Bi-flux-grown Mg-cordierites

The structure, crystal chemistry, and microstructure of disordered and ordered Mg-cordierites synthesized in a bismuth oxide flux system have been studied by a combination of x-ray and neutron powder diffraction and quantitative x-ray microanalysis using analytical electron microscopy. Microchemical data obtained on Bi-flux cordierites using energy-dispersive x-ray analysis is interpreted through comparison with data collected on stoichiometric Mg₂Al₄Si₅O₁₈ glass and α- and β-cordierite samples synthesized by subsolidus crystallization of the glass. Bi-flux cordierites crystallize in both the hexagonal and orthorhombic polymorphs and contain 5 to 10 at% occupancy of bismuth on the C1 and C2 channel sites. The microstructure of Bi-flux α-cordierite is characterized by the existence of local domains of disordered cordierite solid solutions with variable composition and significant vacancy concentrations on the octahedral site. The β-cordierites have a more homogeneous microstructure but are still Al-deficient, Si-rich solid solutions.     

Thermodynamics of water in cordierite and some petrologic consequences of cordierite as a hydrous phase

Olivine melilitites from Namaqualand, South Africa are characterized by a broad range in olivine compositions on the scale of individual hand specimens. It is possible to distinguish four petrographically and chemically distinct olivine populations in both the northern and southern pipe clusters studied: (a) Scarce anhedral or subhedral olivines that display marked disequilibrium features with the surrounding matrix, and which are characterized by having high iron and extremely low nickel contents (referred to as HILN olivines) relative to the other olivines in the same rock, (b) A dominant population of euhedral and often skeletal (hopper) olivines that are richer in Mg and Ni than the HILN olivines in the same rock. There are in addition unusual hopper olivines that are petrographically similar to the skeletal olivines, but show aberrant zonation patterns. Hopper and HILN type olivines contain fluid and carbonate inclusions which apparently record the loss of a vapour phase and an immiscible carbonate liquid during magma ascent, (c) A third population consists of large rounded olivines (megacrysts), up to 40 mm in greatest diameter. Individuals are chemically homogeneous, but megacrysts from the same pipe collectively define a trend of decreasing Mg and Ni (Fo₉₂, 0.36% Ni to Fo₇₅, 0.17% Ni). The most fayalitic megacrysts are depleted in Mg and Ni relative to the hopper olivines in the same rock, (d) Scarce magnesium-rich (Fo₉₁) anhedral olivines which show strained extinction are believed to be xenocrysts.It is suggested that the HILN-type olivines crystallized from primitive carbonate-rich magmas under conditions of low oxygen fugacity, intermediate between the Ni-NiO and Fe-FeO buffers. Mineral-melt partition coefficients for the transition elements determined in basaltic systems are considered to be inappropriate to such carbonate-rich melts. Loss of volatiles and an immiscible carbonate liquid during magma ascent resulted in an increase in oxygen activity, a decrease in the Fe-Mg distribution coefficient (K _D ) for olivine and liquid and an increase in liquidus temperatures. These effects led to the rapid crystallization of Mg- and Ni-enriched skeletal hopper olivines. The unusual hoppers crystallized later than the HILN olivines but prior to the normal hoppers, under conditions chracterized by rapid and independent changes in oxygen activity and partition coefficients associated with the loss of volatiles and an immiscible carbonate liquid. The range in chemistry which characterizes the megacryst-olivine suite is believed to record physico-chemical changes to the magmas subsequent to separation from a mantle source area, but prior to crystallization of the HILN olivines. Most important of these changes was an increasing degree of polymerization of the liquid structure and a progressive decrease in oxygen activity as the molar ratio (CO ₃ ^2- /(CO ₃ ^2- + CO₂)) in the magma increased with decreasing pressure. Increasing polymerization of the liquid resulted in an increase in olivine-liquid partition coefficients for transition elements.Olivines in kimberlites show compositional characteristics and zonation patterns similar to those recognized in the olivine melilitites which, coupled with ilmenite compositions, suggests that the two magma types initially evolved along similar physico-chemical paths.     

Cation distribution and structure modelling of spinel solid solutions

 This paper presents an improved generalisation of cation distribution determination based on an accurate fit of all crystal-chemical parameters. Cations are assigned to the tetrahedral and octahedral sites of the structure according to their scattering power and a set of bond distances optimised for spinel structure. A database of 295 spinels was prepared from the literature and unpublished data. Selected compositions include the following cations: Mg²⁺, Al³⁺, Si⁴⁺, Ti⁴⁺, V³⁺, Cr³⁺, Mn²⁺, Mn³⁺, Fe²⁺, Fe³⁺, Co²⁺, Ni²⁺, Zn²⁺ and vacancies. Bond distance optimisation reveals a definite lengthening in tetrahedral distance when large amounts of Fe³⁺ or Ni²⁺ are present in the octahedral site. This means that these cations modify the octahedral angle and hence the shared octahedral edge, causing an increase in the tetrahedral distance with respect to the size of the cations entering it. Some applications to published data are discussed, showing the capacity and limitations of the method for calculating cation distribution, and for identifying inconsistencies and inaccuracies in experimental data. Received: 19 February 2001 / Accepted: 1 June 2001     

Dependence of the neutron and gamma-ray emission of Mercury on its surface composition and temperature

Results of computations of the energy spectrum of neutrons radiated by the planet Mercury, generated by the action of cosmic rays, are presented. The dependence of the neutron radiation on both the temperature and composition of the subsurface layers of the planetary regolith are investigated. The fluxes of γ-rays in lines of aluminum (²⁷Al, 7.7240 MeV), silicon (²⁸Si, 3.5390 MeV), calcium (⁴⁰Ca, 1.9427 MeV), and iron (⁵⁶Fe, 7.6312 and 7.6456 MeV) generated during neutron capture by these nuclei are also calculated. The intensity of the radiation in these lines depends on both the composition and temperature of the surface. This must be taken into account when interpreting measurements of γ-ray radiation from nuclear lines produced in neutron capture reactions on the substance of Mercury.     

Thermodynamics of minerals of variable composition: Andradite-grossularite and pistacite-clinozoisite solid solutions

Exchange-mineral equilibria with Al and Fe³⁺ aqueous chloride solutions (aq.), Andr + AlCl _aq ³ = FeCl _aq ³ + Gros, (1) Psc + AlCl _aq ³ = FeCl _aq ³ + Czo, (2) were studied under the following experimental conditions: 500°;C and 580°;C and 1 and 2 kbar, respectively, with an overall concentration of metals in the aqueous solutions of about 0.5 M and pH 3. The mixing functions of the components in garnet and epidote were calculated from the experimental data. Thermodynamic treatment of experimental evidence for reaction (1) led to the conclusion that, within the accuracy of experiment, garnet in the andradite-grossularite series was an ideal solid solution. However, epidote solid solution markedly departed from the ideal, as was shown by concentration and pressure-temperature (PT) dependencies of Gibbs's molar excess energies and by mixing-volume concentration dependence.     

Energetics of hydration of cordierite and water barometry in cordierite-granulites

The Gibbs free energy and volume changes attendant upon hydration of cordierites in the system magnesian cordierite-water have been extracted from the published high pressure experimental data at \(P_{{\text{H}}_{\text{2}} {\text{O}}} \) =P _total, assuming an ideal one site model for H₂O in cordierite. Incorporating the dependence of ΔG and ΔV on temperature, which was found to be linear within the experimental conditions of 500°–1,000°C and 1–10,000 bars, the relation between the water content of cordierite and P, T and \(f_{{\text{H}}_{\text{2}} {\text{O}}} \) has been formulated as $$\begin{gathered} X_{{\text{H}}_{\text{2}} {\text{O}}}^{{\text{crd}}} = \hfill \\ \frac{{f_{{\text{H}}_{\text{2}} {\text{O}}}^{{\text{P, T}}} }}{{\left[ {{\text{exp}}\frac{1}{{RT}}\left\{ {64,775 - 32.26T + G_{{\text{H}}_{\text{2}} {\text{O}}}^{{\text{1, }}T} - P\left( {9 \times 10^{ - 4} T - 0.5142} \right)} \right\}} \right] + f_{{\text{H}}_{\text{2}} {\text{O}}}^{{\text{P, T}}} }} \hfill \\ \end{gathered} $$ The equation can be used to compute H₂O in cordierites at \(P_{{\text{H}}_{\text{2}} {\text{O}}} \) <1. Our results at different P, T and partial pressure of water, assuming ideal mixing of H₂O and CO₂ in the vapour phase, are in very good agreement with the experimental data of Johannes and Schreyer (1977, 1981). Applying the formulation to determine \(X_{{\text{H}}_{\text{2}} {\text{O}}}^{{\text{crd}}} \) in the garnet-cordierite-sillimanite-plagioclase-quartz granulites of Finnish Lapland as a test case, good agreement with the gravimetrically determined water contents of cordierite was obtained. Pressure estimates, from a thermodynamic modelling of the Fe-cordierite — almandine — sillimanite — quartz equilibrium at \(P_{{\text{H}}_{\text{2}} {\text{O}}} = 0\) and \(P_{{\text{H}}_{\text{2}} {\text{O}}} \) =P_total, for assemblages from South India, Scottish Caledonides, Daly Bay and Hara Lake areas are compatible with those derived from the garnetplagioclase-sillimanite-quartz geobarometer.     

云的含水量及其水循环

通过分析云的含水量表明:①全球的云中含水量约为90×109t,与大气中气体状态的水汽比,不足其1%,在计算大气总含水量时可以忽略;②积雨云含水分丰富,但是计算显示它持有的水分也少于3mm。③由于云的含水量很小,而每年有1000mm的雨雪从云中降落,所以云中的水分循环很快,从成云到致雨(雪)不足2h。     

Fe4O5 and its solid solutions in several simple systems

Experiments at high pressures and temperatures reveal the stability of a Fe₄O₅-type structured phase in several simple chemical systems. On the one hand, the Fe₄O₅ end-member is stable in the presence of SiO₂-rich phases, including stishovite, but contains ≤0.01 Si cations per formula unit. This indicates that Si is essentially excluded from this phase. On the other hand, the Fe₄O₅ phase can form solid solutions with Mg and Cr and can coexist with silicate phases at the high P–T conditions expected in the transition zone of the mantle (i.e. >~9 GPa). It can coexist with both wadsleyite and Mg-rich ringwoodite and can contain at least 25 mol% Mg₂Fe₂O₅ component. The Fe₄O₅ phase always contains the least amount of Mg in any given mineral assemblage. Cr-bearing Fe₄O₅ has been synthesised with up to 46 mol% Fe₂Cr₂O₅ component and can coexist with spinel and/or hematite-eskolatite solid solutions. Substitution of Mg and Cr for Fe²⁺ and Fe³⁺, respectively, leads to variations in Fe³⁺/∑Fe from the ideal value of 0.5 for the Fe₄O₅ end-member composition, which can influence its redox stability. These cations also have contrasting effects on the unit-cell parameters, which indicate that they substitute into different sites. This initial study suggests that Fe₄O₅-type structured phases may be stable over a range of P–T–fO₂ conditions and bulk compositions, and can be important in understanding the post-spinel phase relations in a number of chemical systems relevant to the Earth’s transition zone. Thus, the presence of even small amounts of Fe³⁺ could alter the expected phase relations in peridotitic bulk compositions by stabilising this additional phase.     

石期河流域岩溶水系统及其水资源构成分析

在深入研究石期河流域岩溶水系统含水介质结构特征、水资源形成的基础上,通过对流域岩溶水总排泄口流量动态长观资料系列的分析,选择2005年丰水期内最后一次洪峰(年内最大洪峰)后的流量衰减过程,根据该过程的数据资料建立本流域岩溶水总排泄口的流量衰减方程,并利用该流量衰减方程对流域岩溶水资源的构成进行定量分析研究.结果表明,石期河流域岩溶水系统在2005年最大洪峰状态下,可自然排泄的岩溶水资源总量为69 469.95×104 m3, 其中:流域坡面产流占3.64%,赋存于大型溶洞或管道中的溶洞-管道水占4.53%,赋存于小型管道或强岩溶化裂隙中的管道-裂隙水占74.10%,赋存于细小溶蚀裂隙中的溶隙水占17.73%.从衰减期的初始时刻起,它们各自可供消耗(可作为有效利用岩溶水资源)的时效分别为2日、8日、35日和126日以上.     

Simulation of the nucleation and growth of binary solid solutions in aqueous solutions

We present a model which, for the first time, accounts for nucleation, growth and/or resorption of particles of variable composition in aqueous solutions (AS). Devised for describing the precipitation of binary solid solutions, it yields the time evolution of all ion activities in the AS, together with the particle population characteristics: number, size and composition profile of particles as a function of time and of their time of nucleation. We apply this numerical approach to the prototypical case of (Ba,Sr)CO₃ solid solution precipitation. We demonstrate the great sensitivity of the composition profiles and particle sizes to the initial conditions under which the AS is prepared, and thus illustrate the possibility of engineering the particle characteristics into a chosen state. Finally, by comparing the precipitation of two solid solutions (Ba,Sr)CO₃ and (Ba,Sr)SO₄, we evidence the sensitivity of the particle composition profiles to the ratio of the end-member solubility products, which leads to the formation of core-shell particles in the case of (Ba,Sr)SO₄.     

Thermodynamic properties of solid solutions in the Ag-Au-Cu system

Data on the Cu content in native gold and silver and the Ag and Au contents in native copper are summarized. The standard thermodynamic functions of solid solutions in the Au-Cu and Ag-Cu binary systems and the Ag-Au-Cu ternary system have been estimated. The corresponding calculation module is prepared for the Selektor software.     

Static elasticity of cordierite I: Effect of heavy ion irradiation on the compressibility of hydrous cordierite

The effect of ion beam irradiations on the elastic properties of hydrous cordierite was investigated by means of Raman and X-ray diffraction experiments. Oriented single crystals were exposed to swift heavy ions (Au, Bi) of various specific energies (10.0–11.1 MeV/u and 80 MeV/u), applying fluences up to 5 × 10¹³ ions/cm². The determination of unit-cell constants yields a volume strain of 3.4 × 10^?3 up to the maximum fluence, which corresponds to a compression of non-irradiated cordierite at ~480 ± 10 MPa. The unit-cell contraction is anisotropic (e ₁ = 1.4 ± 0.1 × 10^?3, e ₂ = 1.5 ± 0.1 × 10^?3, and e ₃ = 7 ± 1 × 10^?4) with the c-axis to shrink only half as much as the axes within the ab-plane. The lattice elasticity for irradiated cordierite (? = 1 × 10¹² ions/cm²) was determined from single-crystal XRD measurements in the diamond anvil cell. The fitted third-order Birch–Murnaghan equation-of-state parameters of irradiated cordierite (V ₀ = 1548.41 ± 0.16 Å³, K ₀ = 117.1 ± 1.1 GPa, ?K/?P = ?0.6 ± 0.3) reveal a 10–11 % higher compressibility compared to non-irradiated cordierite. While the higher compressibility is attributed to the previously reported irradiation-induced loss of extra-framework H₂O, the anomalous elasticity as expressed by elastic softening (β _a ^?1 , β _b ^?1 , β _c ^?1  = 397 ± 9, 395 ± 28, 308 ± 11 GPa, ?(β ^?1)/?P = ?4.5 ± 2.7, ?6.6 ± 8.4, ?5.4 ± 3.0) appears to be related to the framework stability and to be independent of the water content in the channels and thus of the ion beam exposure.     

Study of the mercury content in solid natural samples

This article reports the study of the mercury content in solid samples by two variants of atomic absorption spectrometry (AAS): (1) using direct thermal evacuation (desorption) and (2) microwave sample digestion in an aggressive acid medium, with further analysis by AAS. The merits and limitations of each method are considered. It is shown that, unlike express thermal evacuation, acid digestion allows one to analyze a broad range of solid phase samples with a high content of strongly bound mercury, despite the complexity of sample preparation. The two methods indicated above have been well correlated for soil profiles and sediment with a minor content of strongly bound mercury, which allows one to accurately compare the obtained results; the complex use of these methods makes it possible to expand the range of analyzed natural objects and obtain reliable information about mercury contents.     

Some constraints on the thermodynamic mixing properties of Fe-Mg orthopyroxenes and olivines

Existing data on the temperature and composition dependence of the Fe²⁺-Mg²⁺ distribution between Fe-Mg olivine and orthopyroxene, the intra-crystalline distribution of Fe²⁺ and Mg²⁺ between M1 and M2 sites in orthopyroxene, and macroscopic activity-composition relations in olivine and orthopyroxene are shown to be inconsistent with generally accepted thermodynamic formulations which assume that the non-configurational Gibbs energy of orthopyroxene is independent of the degree of long-range ordering of Fe²⁺ and Mg⁺ between M1 and M2 sites. These data are interpreted in terms of the constraints they provide on the size of Bragg-Williams type energy, entropy, and volume terms for olivine and orthopyroxene. The apparent equilibrium constant for Fe-Mg exchange between olivine and orthopyroxene is shown to be a potentially useful ‘geothermometer’ for olivine-orthopyroxene assemblages with olivines with mole fraction of Fe₂SiO₄ component less than 0.2 or greater than 0.6. A provisional calibration of this ‘geothermometer’ is presented.