Lazurite: Validation as a Mineral Species with the Formula Na7Ca(Al6Si6O24)(SO4) $${\text{S}}_{3}^{{\bullet - }}$$ ⋅H2O and New Data

Geology of Ore Deposits - The status of lazurite as a valid mineral species has been confirmed. The neotype specimen from the Malaya Bystraya gem lazurite deposit, Baikal Lake area has been studied...     

Surface Typochemistry of Native Gold

Using the methods of electron spectroscopy of the surface and SEM–EDS, it is shown that native gold of the deposit related to the epithermal Au–Ag ore formation contains oxidized gold with an oxidation degree of Au (I) or higher on the surface. A thin layer (~15 nm) with high concentrations of Ag and S and an underlying SiO₂-bearing layer with a thickness of ~30–60 nm play a protective role providing preservation of Ag and Au sulfides in the surface parts of the Au–Ag grains under the oxidizing conditions. S-rich marginal parts of native gold particles may be represented by solid solutions Ag_2–xAu _x S or (with a lack of S) by agglomerates of Ag _n Au _m S clusters. The formation of surface zoning in the nanoscale on the surface of native Au is abundant in nature and may be applied in prospecting.     

Distribution and segregation of trace elements during the growth of ore mineral crystals in hydrothermal systems: geochemical and mineralogical implications

The interfacial crystal layer of poorly soluble mineral grown under hydrothermal conditions is modified chemically into a surficial nonautonomous phase (SNAP) and, in this capacity, takes part in growth process, doing several important functions. This paper considers some of them related to geochemistry and mineralogy. The new interpretation is given to the following phenomena: (1) selection of components during crystal growth in multiphase associations; (2) stability of multiphase parageneses having a common chemical component; (3) dual character of the distribution coefficients due to different properties of the crystal volume and SNAP; (4) formation of nano- and microinclusions of unusual composition different from the basic mineral phase; (5) spatial ordering of nano- and microparticles during their directed aggregation at the growing crystal face; (6) accumulation of valuable components (primarily noble metals), incompatible in most of mineral matrixes, in the surficial layer; and (7) effect of “hidden” metal content, associated with the presence of noble metals in the SNAP or of nano- and microinclusions formed during the SNAP evolution.     

Surface typochemistry of hydrothermal pyrite: Electron spectroscopic and scanning probe microscopic data. I. Synthetic pyrite

Techniques of X-ray photoelectron and Auger electron spectroscopy, scanning probe microscopy were used to demonstrate that the natural surface of hydrothermally synthesized pyrite, as well as vacuum fractures, contain a number of sulfide-sulfur species: disulfide, monosulfide, and, more rarely, polysulfide. The natural surface of hydrothermal pyrite is chemically modified compared to the inner volume into a nonautonomous phase film up to ～500 nm thick, which has a variable composition resembling that of pyrrhotite but with broader variations toward FeS₂. Its principal distinctive feature is the presence of a peak at ～710 eV in the XPS Fe 2p_3/2 spectrum, which is often higher than the main peak of bivalent low-spin Fe(II) in the pyrite structure (707 eV). The “basic” structure of the nonautonomous phase is a layer of variable composition Fe²⁺[S, S₂, S _n ]^2?, whose S/S₂ ratio varies from ～0.5 to ～2.0, averaging at ～1.1. This layer may include admixtures of minor elements, as follows from the appearance of a nonautonomous phase in the presence of As, which does not, however, form an individual phase. The polymerization of S at the surface is thereby more significant. The major oxisulfide components of this phase may be the sulfite and thiosulfate ions at a subordinate concentration of sulfate because of the instability of coexisting sulfate and disulfide ions, which results, in the presence of oxygen, in sulfite (thiosulfate) and sulfide ions in the nonautonomous phase. In line with XPS, scanning probe microscopic (SPM) data show that, at a high S activity in the “pure” system, the surface of the crystals contains practically no nanometer-sized phases and is characterized by low roughness (14–17 nm). At a low S fugacity in equilibrium with pyrrhotite and sphalerite, the average roughness of the surface increases to 25–65 nm, with the maximum height of the surface features of ～100–500 nm. This is consistent with Auger spectroscopic data, obtained after the etching (ion milling) of the surface with Ar⁺, on the thickness of the nonstoichiometric surface layer. Comparison with analogous data on other sulfides shows that crystals growing in hydrothermal environments have surface layers up to ～500 nm thick, which are different from the main volume of the crystal in chemistry, stoichiometry, and, possibly, also structure. This is scale of the surface heterogeneity at which the typochemistry of mineral surfaces may be manifested. The typochemistry of pyrite stems from the ability of the nonautonomous phase to “record” the growth conditions of crystals in terms of two major factors: the purity of the system (the occurrence of other phases, including virtual ones, i.e., potentially possible phases of admixture elements) and S fugacity (which influences the S/S₂ ratio at the surface). The geochemical role of the surface nonautonomous phase in pyrite may be very significant, particularly when minor elements are captured that are incompatible with the pyrite structure but can be easily accommodated in the less rigid structure of the nonautonomous phase.     

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.