Sulphur speciation in lazurite-type minerals (Na,Ca)8[Al6Si6O24](SO4,S)2 and their annealing products: a comparative XPS and XAS study
Tauson, Vladimir L.; Goettlicher, Joerg; Sapozhnikov, Anatoly N.; Mangold, Stefan; Lustenberg, Esfir E.
European Journal of Mineralogy Volume 24 Number 1 (2012), p. 133 - 152
published: Feb 1, 2012
ArtNo. ESP147052401011, Price: 29.00 €
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/SO4 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 O2 (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 S2O-, or by disproportionation into S° and S2. 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, SO2 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 haüyne-type), whose compositions and ratios depend on external (temperature, redox conditions) and internal factors (sulphur content, Na/Ca ratio).