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

Abstract

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.