V. L. Tauson

Sulphur speciation in lazurite-type minerals (Na,Ca)8[Al6Si6O24](SO4,S)2 and their annealing products: a comparative XPS and XAS study

Sulphur speciation in lazurites and products of their high-temperature annealing in air or under various buffers is studied using S 2p X-ray photoelectron spectroscopy (XPS) and S K -edge XANES spectroscopy. The XPS as a more surface sensitive technique gives for instance less information on sulphide in the samples with oxidized surfaces whereas XANES spectroscopy enables access to such species in the bulk structure. However, both methods clearly indicate sulphate and polysulphides as main constituents in the structural cages of lazurites. Additionally, the data show sulphur species, such as sulphite, which mainly occur in a near-surface form, thiosulphate, monosulphide, and elemental sulphur. The non-cubic (orthorhombic and triclinic) lazurites – rich in sulphide – are characterized by high contents of polysulphide anions, which make up more than 20 at% of the total S. The cubic variety with low to medium S/SO 4 ratios contains less polysulphide species. The lower limit of polysulphide (primarily S 3 − ) responsible for the blue colour of lazurite is ~0.4 at%. However, in some cases there is no explicit correlation between blue colour and polysulphide content. Two types of transformation of polysulphide are considered. With high access of O 2 (fine grained powders) polysulphide may interact with sulphate anions to form sulphite ions, and in presence of thiosulphate, sulphate and elemental sulphur result. It appears to be the most probable mechanism for non-cubic lazurites rich in polysulphide. In the absence of access to air oxygen (in coarse grains), S 3 − can be transformed by the interaction with sulphate to thiosulphate and possibly to S 2 O − , or by disproportionation into S° and S 2− . The formation of S° is rarely observable in natural lazurites. Additional anions can participate in the reactions of more abundant sulphate and polysulphide anions giving rise to the formation and degradation of colour centres. Structural modifications of lazurite show different anionic compositions in their cages and, hence cannot be considered polymorphs in the strict sense. They rather represent phases of variable compositions depending on temperature, SO 2 fugacity, ordering and ratios of clusters containing Na and Ca. Modulations of the lazurite structure are possibly controlled by three types of clusters that are polysulphide, thiosulphate, and sulphate (nosean-type and hauyne-type), whose compositions and ratios depend on external (temperature, redox conditions) and internal factors (sulphur content, Na/Ca ratio).

Cadmium and mercury uptake by galena crystals under hydrothermal growth A spectroscopic and element thermo-release atomic absorption study

Cadmium and mercury speciation in galena crystals grown under hydrothermal conditions at 400°C and 500 bar is studied using X-ray photoelectron and Auger electron spectroscopy, and atomic absorption spectrometry with temperature control of element release. The main binding forms of Cd and Hg are identified as chemically adsorbed compounds, structurally bound elements in solid solution in galena, and non-autonomous surface phases produced by interaction of a component with altered (oxidized) surface of the growing crystal. The sulfur fugacity ( f S2) has been found to be the most important parameter affecting Cd and Hg speciation and distribution. Specifically, adsorbed Cd species are abundant at low f S2, whereas the maximum of adsorbed Hg occurs at the highest f S2 in equilibrium with S. This suggests different mechanisms of Cd and Hg uptake by the surface of galena crystals. Under elevated P, T conditions, the element retention requires its interaction with surface defects (Pb vacancies in the case of mercury at high f S2), or with active centers containing oxidized sulfur species (cadmium at low f S2). The incorporation of minor components into non-autonomous phases is considered to be an important mechanism of trace-element uptake. It is proposed that these phases could be mixed phases of lead sulphate and chloride or lead thiosulphate and chloride (sulfoxychlorides). The crystal/solution distribution coefficients are much higher for Hg than for Cd. Mercury is nearly quantitatively extracted from the solution by the galena crystals. The bulk distribution coefficients are appreciably higher than the distribution coefficient of structurally bound impurities for both elements, demonstrating the necessity of evaluation of structural species even far below the region of mineral saturation with the trace element. It is shown that the element thermo-release atomic absorption spectrometry can be applied as a useful complement to surface spectroscopic methods.

INTRODUCTION TO THE THEORY OF FORCED EQUILIBRIA : GENERAL PRINCIPLES, BASIC CONCEPTS, AND DEFINITIONS

Abstract Until now, only a small amount of work has been done to verify the constraints of using fundamental regularities of the exact sciences in geochemistry and mineralogy. As for the chemical thermodynamics, the most important problem is the inadequate presentation of the thermodynamic state of real mineral systems. Our contention is that this state cannot be rigorously referred to any conventional type, if examined by the traditional chemical thermodynamics, and must be analyzed in terms of forced-equilibrium theory. The forced equilibrium is defined as a specific thermodynamic state resulting from the action of forcing factors, that is, the conditions or constraints which restrict possible variations of principal or internal thermodynamic system parameters. The advantage of this approach is that it proceeds from the operative forcing factor to the actual type of equilibrium of the real system, whereas the traditional analysis usually postulates the type of equilibrium state without proof of adequacy. The equilibrium conditions for thermoelastic solids with a coherent interphase boundary are a good example of forced equilibrium. The numerical modelling of forced equilibria in some real mineral systems and the comparison of the results with experimental and natural data show that the actual thermodynamic states of mineral systems more often represent stable or metastable forced equilibria than kinetically depressed or metastable states in their traditional understanding.

Dualistic distribution coefficients of elements in the system mineral-hydrothermal solution. I. Gold accumulation in pyrite

The use of trace elements (TE) as geochemical indicators is complicated by the dualism of their distribution coefficients D due to the additional (i.e., above the concentrations of an isomorphic component) incorporation of elements at structural defects of various nature (including the surface of the crystal). A pressing problem in this situation is to determine the true D values that pertain to the structural component of an admixture Dstr and evaluate effects of other modes of TE occurrence. Only upon distinguishing Dstr in the bulk coefficient Dbulk it is possible to evaluate the ore potential of fluid in terms of certain TE from the composition of a mineral containing the TE. Pyrite synthesized in solutions of variable pH at 450°C and 1 kbar (100 MPa) at fluid portions sampled in a trap is utilized to demonstrate the role of a surface nonautonomous phase (NP) in the incorporation of gold in this mineral. The distribution coefficient of gold between pyrite and hydrothermal solution is 0.14 for “pure” pyrite and 0.05 for As-bearing pyrite (containing 0.02–0.05 wt % As), and these coefficients for NP are 310 and 170, respectively. This increases the Dbulk for evenly distributed (“invisible”) gold by factors of four and nine. In contrast to the results of earlier studies conducted at room temperature and pressure or parameters close to them, our data demonstrate that the accumulation of “invisible” Au in pyrite is controlled not only by reducing adsorption with the development of Au(0) particles and films but also by Au incorporation in NP developing in the surface layer of the crystal approximately 500 nm thick as chemically bound Au [most likely as Au(I)]. The possible reason for the high absorption capacity of NP is the defect (pyrrhotite-like) structure, which is not saturated with bonds of excess S and sulfoxi onions.

Quantitative determination of modes of gold occurrence in minerals by the statistical analysis of analytical data samplings

The systematization of processes responsible for the incorporation of trace elements in real crystals of minerals led to understanding of the role of such fairly common geochemical phenomena as endocrypy, sorption, and the origin of nonautonomous phases [1]. An experimental study conducted with trace elements and with regard for these phenomena and processes requires the application not only of modern complicated analytical techniques but also new technologies of experimental‐analytical studies. One of the elements of this technology may be the analysis of statistical samplings of analytical data on single crystals. The proposed approach is underlain by the postulate that, at a large enough number of homogeneous determinations (i.e., analyses of single crystals of similar composition by the same method), the number of the modes in which an element can occur roughly corresponds to the number of modes in the distribution of the concentration of this element. The rigorous solution of this problem can be accomplished with the use of statistically representative material. Then one can construct a diagram for the distribution frequency of the concentrations and determine the number of the distribution modes. In a real situation, the researcher usually has to constrain himself to considering approximately 20 determinations, which enables him to justify the modes only with significant contrasts. There are good reasons to believe that gold and other noble metals are contained, in a general case, in at least three modes, which are contrasting enough to be distinguished by statistical methods. It is well known that Au, Pt, and other noble metals can be adsorbed from solutions by mineral adsorbents via sorption, chemosorption, and the reduction of ions to the elemental state [2]. Moreover, these elements may be contained (although in insignificant concentrations) in the structures of various minerals. The corresponding modes of occurrence are structural admixtures (on the scale of atoms or small defects in crystals, a homogeneous distribution), adsorbed admixtures (molecular complexes or nonautonomous phases, the distributions may be either homogeneous or heterogeneous), and inclusions of autonomous phases (predominantly particles of native metal of various size, all of them much larger than the sizes of atoms, heterogeneous distributions). In order to subdivide a set X i of analytical data according to the occurrence modes of a trace element, one should consider the properties of each of the modes. These properties predetermine (among other things) the conditions of X i transition between the levels, which are identified with a set of the concentrations of the element in a certain mode of its occurrence. Technologically, the essence of this approach is the successive “screening” of a large enough representative sampling of analytical data on single crystals (monocrystals) in order to determine the whole concentrations of the element corresponding to certain modes in which the element occurs in the crystals.

The State of Palladium in the Nanosized Hydrogenation Catalysts Modified with Elemental Phosphorus

The size, nature, and surface state of nanoparticles formed by reduction of Pd(acac)2 with hydrogen in the presence of P4 have been elucidated by means of X-ray photoelectron spectroscopy, X-ray powder diffraction analysis, and transmission electron microscopy. The nanoparticles (average diameter of 5.6 nm) consist of Pd6P and palladium nanoclusters (at initial ratio P/Pd = 0.3). Dimethylammonium dihydro- and hydrophosphates are found in the surface layer of the catalyst nanoparticles. The nanoparticles are stabilized by ammonium salts formed via dimethylformamide hydrolysis.

Dualistic Distribution Coefficients of Elements in the System Mineral-Hydrothermal Solution. II. Gold in Magnetite

The system magnetite-Au-hydrothermal solution was employed to continue studying the distribution coefficients of trace elements in system with real crystals. The role of surface nonautonomous phase (NP) is elucidated. The distribution coefficient of an Au structural admixture between magnetite and hydrothermal solution at the experimental conditions [450°C, 1 kbar (100 MPa), and fluid sampling by a trap] is, according to the most representative data, 1.0 ± 0.3, and Au is thus not an incompatible element in magnetite, in contrast to pyrite and arsenopyrite [1], minerals for which this coefficient is much lower than one. The NP is enriched in Au with respect to the rest of the crystal by a factor of more than 4000, and this results in an one order of magnitude increase in the bulk distribution coefficient. Similar to pyrite, the reason for the dualistic nature of the distribution coefficient is the presence of an NP, which contains ∼2000 ± 500 ppm Au. The NP occupies the approximately 330-nm surface layer of the crystal, and the chemically bound Au [Au(III), according to XPS data] admixture is evenly distributed with depth within the layer, which is the reason for the strongly determinate dependences of the concentrations of the evenly distributed Au admixture on the size and specific surface area of the crystal. The occurrence of an NP is controlled by the chemistry of the system. The partial substitution of Fe for Mn and the synthesis of a phase close to jacobsite MnFe2O4 results in the disappearance of both the NP itself and the size dependence of the Au concentration. The XPS spectra of O 1s and Fe 2p are used to analyze two models: (i) a single goethite-like (O2−/OH−∼ 1) phase of variable composition and Fe in more than one valence state and (ii) a heterogeneous structure of alternating domains of wuestite- and goethite-like NP. The reason for the “excess” admixture in the former instance can be vacancies at Fe sites, whereas that in the latter one is the interaction of the admixture with nanometer-in-size nanometer in-size strained domains on the surface of the crystal.

Effect of crystallite size on solid state miscibility: Applications to the pyrite-cattierite system

Abstract The miscibility gap in the FeS2-CoS2 system has been studied for various crystallite sizes. The pyrite-cattierite solvus for bulk phases is strongly asymmetric and has a critical point of ~740°C and ~ 13 mol% CoS2. As shown from previously published data the critical point for small particles (⩽0.05 μm) occurs at 650°C or lower where the binodal curve becomes more symmetrical. Theoretical consideration of equilibrium conditions of coexisting solid solutions in a binary small-phase system in terms of generalized chemical potentials permits derivation of a system of nonlinear equations from which the compositions of coexisting solid solutions can be determined for any crystallite size. The data obtained give evidence for a crystallite size effect; that is, phase relations (particularly, phase boundary position) are dependent upon the crystallite size in a given mineral system. These observations are important in establishing phase boundaries under high pressure-high temperature conditions when finely dispersed starting materials are used. It appears that the crystallite size effect may be responsible for certain anomalies of solid state miscibility in mineral systems, for example, the formation of supersaturated crystalline solutions.

Specific Features of Mandible Structure and Elemental Composition in the Polyphagous Amphipod Acanthogammarus grewingkii Endemic to Lake Baikal

Background In crustaceans, several mechanisms provide for the mechanical strength of the cuticular “tools” (dactyli, claws, jaws), which serve to catch and crush food objects. Studies on the mandibles of the endemic Baikal amphipod Acanthogammarus grewingkii by means of electron microscopy and elemental analysis have revealed specific structural features of these mouthparts. Methodology The fine structure of the mandible has been studied by means of SEM, TEM, and AFM; methods used to analyze its elemental and phase composition include XEPMA, XPS, SEM-EDS analysis, and XRD. Conclusion Functional adaptations of the mandible in A. grewingkii provide for the optimum combination of mechanical hardness and fracture resistance, which is achieved due to a complex structure and composition of its cutting parts. Teeth of the mandible are covered by a thin layer of silica (10–20 µm). Their epicuticle is characterized by a high density, consists of three layers, and increases in thickness toward the tooth apex. The epicuticle is enriched with Br, while the concentrations of Ca and P reach the peak values in the softer internal tissues of the teeth. These data broaden the view of the diversity of adaptation mechanisms providing for the strengthening of cuticular “tools” in crustaceans.

The principle of continuity of phase formation at mineral surfaces

One of the main principles of physicochemical analysis—the principle of continuity—says that “continuous change of parameters of the state of the system results in continuous change of properties of individual phases; the properties of the whole system also show continuous changes if no new phases appear and no existing phases disappear” [1]. This formulation does not specify which phases are implied: surface, nonautonomous, or traditional autonomous phases. This problem becomes significant for studying mineral surfaces. Nonautonomous phases (NAP) are nanosized (submicron) surface phases, which are the products of chemical modification and structural reconstruction of surface layers of mineral crystals [2, 3]. They are formed in the crystal surface layer by means of its interaction with components of the growth medium or contacting autonomous phases. Unlike the latter, the nonautonomous phases cannot be extracted from the system without a change in their composition and structures and characteristics of the associated phases. At the same time, their reaction to changes in the state parameters and system composition is different from the reaction of traditional phases, owing to which they can incorporate trace elements, which are incompatible with the structure of the corresponding bulk phase [2]. The aim of this work is to clarify whether the principle of continuity is applicable to the nonautonomous phases and which mineralogical sequences it causes. The study objects are sulfide minerals ascribed mainly to the systems Pb‐S and Fe‐Zn‐S: galena PbS, pyrite, FeS 2 , pyrrhotite Fe 1 – x S , and sphalerite (Zn,Fe)S. Minerals were synthesized by the conventional technique of hydrothermal thermal-gradient synthesis in Ti inserts [3] at temperatures of 400, 450, and 500°C and pressure of 0.5 and 1 kbar both individually and as phase associations containing up to three phases (Fesphalerite + pyrite + pyrrhotite). The mineralizers were either 10% NH 4 Cl, or those with added Na 2 S or HCl (1 wt %) (pyrite synthesis). In some experiments with pyrite, the starting material was enhanced with As (up to 5 wt %), while the starting material for the synthesis of sphalerite and galena contained Cd and Hg sulfides (1 wt % each). Inserts equipped with a self-sealing closure were placed in autoclaves of stainless steel and held for 3 days in an isothermal regime to homogenize the starting material, and then for 7, 9, or 12 days at 500, 450, and 400°C , respectively, at a temperature drop ( 15°C along the outer wall of the autoclave). The experiments were terminated by quenching the autoclave in cold running water. Data previously obtained on the system PbS‐(HgS,CdS) (Cd and Hg sulfides as admixtures) [4] were also used in this work.