Dark blue lazurite from the Malo-Bystrinskoe lazurite deposit, Baikal Lake area, Eastern Siberian region, Russia, was analyzed by electron microprobe and revealed an unusually high content of total sulfur corresponding to 8.3 wt% S. The relative content of sulfur in sulfate and sulfur in sulfide form was determined by wet chemical analysis. The H2O content was measured by means of differential thermal analysis in combination with mass spectrometry and infrared (IR) spectroscopy. The charge-balanced empirical formula of lazurite calculated on the basis of 12 (Al+Si) atoms per formula unit was (Na6.97Ca0.88K0.10)Σ7.96[(Al5.96Si6.04)Σ12O24](SO4)1.092−(S3−)0.55S0.052− Cl0.04·0.72H2O. The presence of H2O molecules and (S3)– and (SO4)2– groups was confirmed by the combination of IR, Raman, electron paramagnetic resonance (EPR), and X-ray photoelectron spectroscopy (XPS) methods. The idealized formula of lazurite is Na7Ca[Al6Si6O24](SO4)2–(S3)–·H2O, and it is believed that extra-framework cations and anions are grouped into clusters of [Na3Ca·SO4]3+ and [Na4(S3)–]3+. The types of isomorphous substitutions in nosean and haüyne are discussed. Lazurite is a clathrate-type mineral, which may be an effective (S3)– sensor due to the stability of the trisulfur radical anion in isolated cages of the crystal structure. This specific feature makes it possible to study the behavior of this ubiquitous radical anion over larger T and P ranges as compared to free species. This kind of lazurite, with oxidized and reduced sulfur species, seems to be appropriate for the estimation of the fugacity of SO2 and O2 in metasomatic systems forming lazurite-containing rocks. The systematic presence of incommensurate modulations is a unique structural feature of Baikal lazurite and may be an important marker indicating provenance of the mineral.

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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, D str p and D sur p , 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 (D e) of REE (Ce, Eu, Er, Yb) and Fe in magnetite and hematite at 450 • C and 100 MPa. D sur e is elevated to two orders of magnitude as compared to D str e. 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.

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