Cocrystallization Coefficients Research Articles

Origin of Au-Ag Mineralization in Sphalerite Ores from Data on Sphalerite Co-Crystallization with Ag and Au in Model Hydrothermal Systems

Abstract ––Au-Ag mineralization occurrences in sphalerite ores of hydrothermal genesis are paradoxical in view of the incompatibility of these elements in sphalerite. The formation of sphalerite with Au and Ag impurities under hydrothermal crystallization of ZnS at 450 °C and 1 kbar pressure was studied experimentally. Sn impurity was taken as a source of point defects in crystals modelling the interaction of Au and Ag with vacancies. The Ag solubility in low-Fe sphalerite is estimated as 3.8 ± 0.7 μg/g, Au ̶ ≤ 0.6 μg/g. The main forms of Ag and Au occurrence in sphalerite are the inclusions of (Ag, Au)xS phases with x varies mainly from 1.8 to 2.0, and Au varies from 0.01 to 0.75 a.p.f.u. The primary forms of the elements in ores might be microinclusions (Ag, Au) 1.8-2.1S or close to (Ag, Au)S at higher fS2. In presence of Sn, solubilities of Au and Ag become higher. The behavior of Au corresponds to the substitution reaction Sn4+ + Au+ + v‒ ↔ 2Zn2+ in the presence of two types of vacancy defects (v–) ‒ the “inherent” vacancies dependent on the crystallization conditions and the vacancies accompanying Sn4+ incorporation. Ag entrance is seemingly more dependent on fS2 conditions and does not correlate with Sn. The extra vacancies arise because of metastable crystallization under the conditions of oversaturation of growth medium. This is supported by the spherulite morphology of growth products and the admixture of wurtzite ZnS form. The distribution and cocrystallization coefficients show an increasing trend for both precious metals (PM), due to which Au changes from incompatible to the category of highly compatible elements in sphalerite. The geochemical environments favorable for the formation of imperfect mineral crystals are considered. Such crystals are capable to uptake PMs and other incompatible in “ideal” crystal elements because of their interaction with vacancies, both constitutional (inherent to the substance) and non-equilibrium defects, and surficial nano-sized formations (nonautonomous phases). The evolution of these initially “invisible” forms of PM under metamorphic processes and remobilization of ore substance may result in Au and Ag escape and aggregation into microparticles.

Partitioning and speciation of lanthanides in the system magnetite (hematite) – hydrothermal solution at 450 oC and 100 MPa pressure

The initial results of the experimental study of a hydrothermal system including lanthanides (Ln) and Fe oxides (magnetite and hematite) are presented. Ln concentrations in solutions and crystals were determined by ICP-MS and LM-ICP-MS, accordingly. The Ln distribution and cocrystallization coefficients obtained are interpreted as the maximal estimates of “true” values corresponding to structurally bound admixture. It is shown that Ln (except for Eu) are the compatible elements in hydrothermal magnetite; heavy Ln (beginning with Tb) are compatible in hematite. The pronounced tendency of the elevation of both coefficients with the Ln atomic number beginning from Gd-Tb was established. This is significant for using the ratio of light and heavy Ln as a typochemical guide for localization of the source of ore elements. The high-Ln-containing phases in associations with magnetite and hematite were obtained. These phases have oxychloride (without Fe) and oxyhydroxide (with Fe) composition and demonstrate the example of co-locating light and heavy Ln in the common space of hydrothermal system due to mutual crystallization of the phases selectively accumulating light and heavy lanthanides.

Distribution and Cocrystallization Coefficients of a Wide Range of Typomorphic Elements in Magnetite, Hematite, and Sphalerite in Hydrothermal Systems

Abstract —Distribution of a wide range of elements in the systems with magnetite, hematite and sphalerite is studied by the method of thermogradient hydrothermal synthesis combined with internal fluid sampling at 450 °C temperature and 100 MPa pressure. The distribution and cocrystallization coefficients are determined; the literature and original data on these coefficients are summarized. The possibility of obtaining the reproducible data on elements distribution in the mineral − solution system in the occurrence of many typomorphic elements is substantiated. This considerably increases the experiment efficiency. A significant advantage of using cocrystallization coefficients rather than “conventional” distribution coefficients expressed by the ratio of the element concentrations in crystal and solution (fluid) is shown. The features of behavior and occurrence of elements in hydrothermal systems are provided with physico-chemical evidence, through application of cocrystallization coefficients. The examples of the behavior of typomorphic trace elements in sphalerite are considered, which support the theoretical analysis. The major (Fe, Mn, Zn and possibly Cu) and secondary (Ti, V, Al, and Co) components of ore-forming solutions are estimated according to the compositions of magnetite and hematite from hydrothermal ore deposits of various types. The similarity in compositions of magnetite and hematite does not prove their coformation from a single fluid, quite the reverse, and this fact indicates different compositions of fluids from which the minerals were deposited.

Gold Partitioning in a Model Multiphase Mineral-Hydrothermal Fluid System: Distribution Coefficients, Speciation and Segregation

The characteristics of Au partitioning in a multiphase, multicomponent hydrothermal system at 450 °C and 1 kbar pressure were obtained using experimental and computational physicochemical modelling and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis. Sphalerite and magnetite contained 0.1–0.16 ± 0.02 µg/g Au and coexisted with galena and bornite which contained up to 73 ± 5 and 42 ± 10 µg/g Au, respectively. Bornite and chalcopyrite were the most effective Au scavengers with cocrystallization coefficients Au/Fe and Au/Cu in mineral-fluid system n–n × 10−2. Sphalerite and magnetite were the weakest Au absorbers, although Fe impurity in sphalerite facilitated Au uptake. Using the phase composition correlation principle, Au solubility in minerals was estimated (µg/g Au): low-Fe sphalerite = 0.7, high-Fe sphalerite = 5, magnetite = 1, pyrite = 3, pyrite-Mn = 7, pyrite-Cu = 10, pyrrhotite = 21, chalcopyrite = 110, bornite = 140 and galena = 240. The sequence reflected increasing metallicity of chemical bonds. Gold segregation occurred at crystal defects, and on surfaces, and influenced Au distribution due to its segregation at crystal interblock boundaries enriched in Cu-containing submicron phases. The LA-ICP-MS analysis of bulk and surficial gold admixtures revealed elevated Au content in surficial crystal layers, especially for bornite and galena, indicating the presence of a superficial nonautonomous phase (NAP) and dualism in the distribution of gold. Thermodynamic calculations showed that changes in experimental conditions, primarily in sulfur regime, increased the content of the main gold species (AuCl2− and AuHS0) and decreased the content of FeCl20, the prevailing form of iron in the fluid phase. The elevation of S2 and H2S fugacity affected Au partitioning and cocrystallization coefficients. Using Au content in pyrite, chalcopyrite, magnetite and bornite from volcanic-sedimentary, skarn-hosted and magmatic-hydrothermal sulfide deposits, the ranges of metal ratios in fluids were estimated: Au/Fe = n × 10−4−n × 10−7 and Au/Cu = n × 10−4−n × 10−6. Pyrite and magnetite were crystallized from solutions enriched in Au compared to chalcopyrite and bornite. The presence of NAP, and associated dualism in distribution coefficients, strongly influenced Au partitioning, but this effect does not fully explain the high gold fractionation into mineral precipitates in low-temperature geothermal systems.

Crystal Growth through the Medium of Nonautonomous Phase: Implications for Element Partitioning in Ore Systems

The phenomena related to the crystal growth in close-to-natural multicomponent systems have been considered. It is shown that the distribution of rare-earth elements in magnetite and hematite and the distribution of noble metals (NMs) in pyrite and magnetite are controlled by surficial nonautonomous phases (SNAPs). The increase in the fractionation and cocrystallization coefficients of elements is related to the presence of these phases. The dependence of SNAPs on the physicochemical growth conditions suggests typomorphism of mineral surfaces. The SNAP evolution during crystal growth explains some specific features of mineral growth systems, in particular, the existence of highly determinate dependences of uniformly distributed incompatible element admixture on the specific surface area of crystal, as well as the formation of nano- and microinclusions and microzonality in crystals. The results obtained are important for the ore formation theory and the practical estimation of the economic potential of ore deposits in view of determining the “hidden” metal content and elaborating a rational technology to recover ore material resources.

Trace Element Partitioning Dualism under Mineral–Fluid Interaction: Origin and Geochemical Significance

Trace element (TE) partitioning in the system “mineral-hydrothermal solution” is studied by the method of thermo-gradient crystal growth coupled with internal sampling of a fluid phase. The analytical procedure used enables evaluating of structurally bound and superficially bound modes of TE in crystals and determining corresponding dual partition coefficients. The case of precious metals (PM—Au, Pt, Pd) at 450 and 500 °C and 100 MPa pressure is considered. The minerals are pyrite, As-pyrite, magnetite, Mn-magnetite and hematite and fluids are ammonium chloride-based hydrothermal solutions. The partition coefficients for structural and surficial modes, Dpstr and Dpsur, are found to be unexpectedly high (except for Au in pyrite). High concentrations of PM are attributed to superficial nonautonomous phases (NAPs), which can be considered as primary concentrators of PM. We also have studied the co-crystallization (exchange) coefficients (De) of REE (Ce, Eu, Er, Yb) and Fe in magnetite and hematite at 450 °C and 100 MPa. Desur is elevated to two orders of magnitude as compared to Destr. It is shown that not only physicochemical parameters affect REE distribution in hydrothermal systems, but also NAP presence and its composition. The crystal growth mechanism specified by the agency of NAP is suggested. The study of PM distribution in natural pyrite of gold-ore deposits supported the importance of differentiating between structurally and superficially bound TE modes for correct use of experimental D values to determining element concentrations in ore-forming fluids.

Cocrystallization coefficients of Cr, V, and Fe in hydrothermal ore systems (from experimental data)

The cocrystallization coefficients of Cr, V, and Fe (DMe/Fe) in magnetite and sulfide minerals (pyrite, chalcopyrite, and Fe-containing sphalerite) in multiphase associations are determined in hydrothermal-growth experiments with internal sampling at 450 °C and 100 MPa (1 kbar). The results are compared with previous data on DMn/Fe. Magnetite and pyrite are characterized by the highest DMe/Fe values for both Cr (1.2 and 2) and V (6.6 and 1.1). These minerals also show the highest mineral/solution distribution coefficients of Cr and V. For V and Cr in chalcopyrite, much lower DMe/Fe values (0.03 and 0.04, respectively) were obtained, which, however, are slightly higher than those for Mn in magnetite (0.01). Although the deposition of magnetite and iron sulfides has no significant effect on the evolution of Mn in solution and Mn-Fe partitioning, crystallization of magnetite and pyrite favors a decrease in Cr and V contents relative to Fe content in solution.The data obtained can be used to reconstruct the chemical composition of paleofluids. Spinel minerals with close contents of Mn, V, and Cr can form through a hydrothermal process provided that the solutions are highly enriched in Mn relative to Fe and have V and Cr contents close to the Fe one. Such solutions seem to be exotic. Usually, a magnetite-forming hydrothermal fluid contains V and Cr as millionths of Fe, while the Mn content in it can be of the same order of magnitude as the Fe content. The data obtained may be of interest for reconstructing the evolution of the chemical composition of the World Ocean in different geologic periods.The study has shown that the bulk distribution coefficient of variable-valence elements between mineral and hydrothermal solution varies over a wide range of values even at constant pressure, temperature, and solution composition and can be used only for qualitative estimation of the element compatibility. In contrast, the bulk cocrystallization coefficient of chemically similar elements is less dependent on physicochemical conditions, has a nearly three times lower variation coefficient, and permits an element partitioning analysis in heterogeneous mineral-fluid systems.

COCRYSTALLIZATION COEFFICIENTS OF Cr, V, AND Fe IN HYDROTHERMAL ORE SYSTEMS (from experimental data)

COCRYSTALLIZATION COEFFICIENTS OF Cr, V, AND Fe IN HYDROTHERMAL ORE SYSTEMS (from experimental data)

Using cocrystallization coefficients of isomorphous admixtures for determination of element concentrations in ore-forming solutions (by the example of Mn/Fe ratio in magnetite)

The cocrystallization coefficient of Mn and Fe (DMn/Fe) in magnetite crystals is determined in hydrothermal-growth experiments with internal sampling at 450 and 500°C and 100MPa (1kbar). It is weakly dependent on temperature in the studied PT-region and is constant over a wide range of Mn/Fe values. This permits using the magnetite composition as an indicator of Mn/Fe in the fluid under equilibrium: (Mn/Fe)aq≈100 (Mn/Fe)mt. Since Mn is often a macrocomponent of the fluid and a microcomponent of magnetite, local analysis of fluid inclusions for Mn might help to determine Fe even in iron minerals. This will permit evaluation of the contents of other ore metals if the DMe/Fe values are known. For fine crystals (<0.1–0.2mm) with low contents of Mn (<0.01–0.02%), it is necessary to take into account the fractionation of Mn into the surficial nonautonomous phase, in which its content can reach several percent. Comparison of these data with earlier data on the distribution of Mn in the system magnetite–pyrite–pyrrhotite–greenockite–hydrothermal solution shows that DMn/Fe remains constant in the presence of sulfur and sulfides. Precipitation of magnetite, in which Mn is a compatible admixture, cannot affect radically Mn/Fe in the solution because of the low DMn/Fe value. This effect is still more unlikely for pyrrhotite and pyrite, in which Mn is an incompatible admixture. The most probable mechanism of Mn fractionation into the solid phase is crystallization of FeOOH at lower temperatures. This is indirectly supported by the strong fractionation of Mn into the nonautonomous oxyhydroxide phase on the surface of magnetite crystals. The necessity of a more rigorous validation of “the new Fe/Mn geothermometer for hydrothermal systems” is substantiated.

ОБ ИСПОЛЬЗОВАНИИ КОЭФФИЦИЕНТОВ СОКРИСТАЛЛИЗАЦИИ ИЗОМОРФНЫХ ПРИМЕСЕЙ ДЛЯ ОПРЕДЕЛЕНИЯ КОНЦЕНТРАЦИЙ ЭЛЕМЕНТОВ В РУДОНОСНЫХ РАСТВОРАХ (на примере Mn/Fe-отношения в магнетите)

ОБ ИСПОЛЬЗОВАНИИ КОЭФФИЦИЕНТОВ СОКРИСТАЛЛИЗАЦИИ ИЗОМОРФНЫХ ПРИМЕСЕЙ ДЛЯ ОПРЕДЕЛЕНИЯ КОНЦЕНТРАЦИЙ ЭЛЕМЕНТОВ В РУДОНОСНЫХ РАСТВОРАХ (на примере Mn/Fe-отношения в магнетите)

Equilibrium coefficients of co‐crystallization of M2+ ions with MgSeO4·6H2O and their dependences on various physicochemical factors

AbstractEquilibrium co‐crystallization coefficients of low amounts of M2+ions (M2+ = {Ni2+, Cu2+, Co2+, Zn2+, Mn2+, Cd2+}) with MgSeO4·6H2O at 25 °C have been determined. Their values are comprised in the range: 0.058 (for Cd) &lt; keq &lt; 1.57 (for Co) and depend on some physicochemical and crystal chemical properties of both: co‐crystallizing salts (MSeO4·nH2O) and co‐crystallizing ions (M2+). These dependences are sometimes such strong, that they make it possible to derive simple formulae permitting estimation of keq coefficients at average error not exceeding 17%. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

A study of the behavior of cadmium and zinc microquantities in sorption and coprecipitation processes in tetrahydrofuran solutions

The sorption of 109Cd and 65Zn microquantities with NaX and NaA zeolites in the presence of divalent lanthanides (Ln2+) (Ln=Tm, Dy, and Nd) from tetrahydrofuran (THF) solutions was studied. It was found that in contrast to 137Cs+, 109Cd and 65Zn, as well as 85Sr2+ microquantities were sorbed by zeolites very poorly: ∼99% of the species remained in solution. The distribution coefficients (Kd) of 109Cd and 65Zn were ∼0.3 and ∼0.4 ml/g, respectively. When Tm2+ was used as a reducer, it partially oxidized in solution to Tm3+ to form precipitates of the composition TmI3·3THF. A study of the co-crystallization of 109Cd and 65Zn, as well as 85Sr2+ microquantities with the TmI3·3THF solid phase in the presence of Tm2+ from THF solutions showed that in contrast to 85Sr2+, 109Cd and 65Zn microquantities co-crystallized with the TmI3·3THF solid phase. The co-crystallization coefficients (D) for 109Cd and 65Zn depended on the ratio Tm3+/Tm2+ in solution and increased with its increasing. The assumption was made that in the presence of Tm2+, 109Cd2+ and 65Zn2+ form M+ ions, which quickly react with M2+ ions to form dimers of the composition M23+ (M=Cd and Zn).

Cocrystallization of microamounts of 137Cs, 90Sr, and 90Y from aqueous solutions with the solid phase of mixed potassium neodymium ferrocyanide

Cocrystallization of microamounts of 137Cs, 90Sr, and 90Y with the solid phase of mixed potassium neodymium ferrocyanide was studied in relation to the K4[Fe(CN)6]: Nd(NO3)3 ratio in aqueous solution. At the K4[Fe(CN)6]: Nd(NO3)3 ratio higher than 2, all the radionuclides studied virtually quantitatively (to 96–99%) cocrystallize with the KNd[Fe(CN)6]·4H2O solid matrix. The cocrystallization coefficients D calculated by the Henderson-Krechek equation exceed 103. Leaching of 137Cs and 152Eu from the K(137Cs)Nd(152Eu)· [Fe(CN)6]·4H2O solid matrix with water and with aqueous solutions of HNO3 (0.1 and 1.0 M) and KOH (4.0 M) was studied.

Coprecipitation of Trace Amounts of 137Cs and 85Sr with [Na(18-Crown-6]BPh4 from Neutral and Alkaline Solutions

Coprecipitation of 137Cs and 85Sr with [Na(18-crown-6]BPh4 solid phase from aqueous, aqueous-ethanolic, and alkaline solutions is studied. 137Cs and 85Sr cocrystallize with [Na(18-crown-6]BPh4 from aqueous and aqueous-ethanolic solutions. The cocrystallization coefficients D of 137Cs and 85Sr from aqueous solutions are 2.6±0.5 and 3.3±0.3, respectively. For aqueous-ethanolic solutions, the corresponding values are 4.4±0.5 and 3.4±0.4. In the alkaline solutions (0.1 and 1 M NaOH), 54–74% of 137Cs and 37–51% of 85Sr pass into the [Na(18-crown-6]BPh4 solid phase, depending on the crown ether concentration in the system.

Behavior of Trace Amounts of Cd and Zn in Sorption and Coprecipitation in Tetrahydrofuran Solutions

Sorption of trace amounts of 109Cd and 65Zn on zeolites NaX and NaA in the presence of divalent lanthanides Ln2+ (Ln = Tm, Dy, Nd) from tetrahydrofuran (THF) solutions is studied. In contrast to 137Cs+ and similar to 85Sr2+, trace amounts of 109Cd and 65Zn are not practically sorbed on zeolites (about 99% of these radionuclides remains in the solution). The distribution coefficients Kd of 109Cd and 65Zn are ∼0.3 and ∼0.4 ml g−1, respectively. In THF solutions, Tm2+ is oxidized to Tm3+, and TmI3 ⋅ 3THF is precipitated. Study of cocrystallization of trace amounts of 109Cd and 65Zn and also of 85Sr with this precipitate from THF solutions containing Tm2+ revealed that, in contrast to 85Sr2+, 109Cd and 65Zn traces cocrystallize with the solid solvate phase. The cocrystallization coefficients D of 109Cd and 65Zn increase with increasing Tm3+/Tm2+ ratio in the solution. The mechanism suggested is that, in the presence of Tm2+, 109Cd2+ and 65Zn2+ are reduced to single-charged cations M+, which then rapidly react with the double-charged cations M2+ with the formation of dimers M 2 3+ (M = Cd, Zn).

COCRYSTALLIZATION OF LOW AMMOUNTS OF M2+ IONS DURING CoSO(4)7H2O CRYSTALLIZATION

The equilibrium cocrystallization coefficients DM/CoSO4.7H2O of low amounts (10-3- 10-1%w/w) of M2+ ions (M2+ = {Ni2+, Mg2+, Cu2+, Zn2+, Fe2+, Mn2+, Cd2+, Ca2+}) with CoSO47H2O have been determined with the method of long-time stirring of crushed CoSO47H2O crystals in their saturated solution at 20oC and compared with coefficients determined by means of the method of isothermal decreasing of supersaturation during 3 ­ 360 hours of stirring. This enabled the time needed to reach equilibrium to be found. It is different for different microcomponents. The determined cocrystallization coefficients are diverse: from <0.008 for Ca2+ to 1.20 for Fe2+. Their dependence on some physicochemical and crystal-chemical properties of both sulfate hydrates (CoSO47H2O and MSO4nH2O) and metal M2+ ions has been discussed. They depend mainly on solubility in water and structure of corresponding sulfate hydrates as well as on radii of M2+ ions. It is possible to calculate cocrystallization coefficients with empirical formula based on determined relationships between some of the considered properties of macrocomponent and microcomponents and DM/CoSO4.7H2O coefficients at the average relative error not exceeding 10%

Inclusion of isomorphous impurities during crystallization from solutions

Developments in the field of inclusion of isomorphous impurities during crystallization from solutions are reviewed. The main features of distribution coefficients like effective, equilibrium and thermodynamic cocrystallization coefficients are presented and discussed. From an analysis of the data, some general characteristics of the influence of various factors, such as solubility of components, ionic radii, enthalpy of dehydration, electronegativity, complex formation of the ions and temperature, on the equilibrium cocrystallization coefficient are given.

Electrochemical Cocrystallization Experiments with Gd2Cl3

Gd2Cl3 is electrocrystallized from molten GdCl3 at 900 K with a current efficiency of 70% referring to the reaction 2GdCl3+3e⁻=Gd2Cl3+3Cl-. The reaction is used to determine cocrystallization coefficients for trivalent rare earths and actinoids (Y, Nd, Cm, Pu), divalent lanthanoids (Eu, Yb), and Sr. The oxidation potential for the above reaction is determined as 2.65 ≤ E° ≤ 2.68 V.

Solubility equations for the ethylsulphates of trivalent Y, Pm, Pu, Am and Cm as determined by co-crystallization

Assuming that the correlation between the cocrystallization coefficients and solubilities of the co-crystallizing ethylsulphates in the [(Ln,M) (H2O)9] (C2H5SO4)3−H2O system is valid when M is changed from lanthanides into the title elements, the solubilities of the ethylsulphates of trivalent Y, Pm, Pu, Am and Cm in water at 288–318 K have been determined “from the matrix”. The solubilities of Y, Pm, Pu, Am and Cm ethylsulphates and of all the lanthanide ethylsulphates are given in the form of smoothing equations of the lg molality=A+B/T type. From the B parameters of the solubility equations the enthalpies of solution have been estimated. The crystallization behaviour of yttrium in the ethylsulphate system is between that of holmium and that of erbium.

Relationship between the solubilities and separation factors for the co-crystallization of lanthanide ethylsulphates

Co-crystallization coefficients of lanthanide ethylsulphates [Ln(H2O)9] (C2H5SO4)3 were determined and compared with the solubility of these salts. In contrast to the common opinion denying any simple correlation between co-crystallization coefficients and solubilities it was established that a simple relationship does exist for ethylsulphates of light lanthanides. It was concluded that in the case of light lanthanides an almost complete compesation of activity coefficients in the RATNER equation is due to the similar structure of the hydrated cations [Ln(H2O)9]3+ in both the solid and the aqueous phase. For ethylsulphates of heavy lanthanides such a compensation was not observed, since in this case the cations in the aqueous phase are probably hydrated by 8 water molecules, while in the solid phase the coordination number is still 9. The solubility of Pm, Pu, Am and Cm ethylsulphates was determined by the method “from matrix”.