A. N. Sapozhnikov

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).

The nature of the stability of an incommensurate 3D structural modulation in Baikal lazurite: Experimental data at 550°C

The nature of the stability of an incommensurate 3D modulation (ITM) in the structure of Baikal lazurite was evaluated using the methods of experimental geochemistry and X-ray photoelectron spectroscopy. It was shown that ITM with a period of 4.6a is preserved in the lazurite structure at 550°C almost without changes within the time interval from t = 100 h to at least 2000 h, although its initial (t = 0) development was not restored. In contrast to higher temperatures (≥ 600°C), the activities of gas species have no significant influence on the process of modulation release, except for the region of low O2, S2, and SO2 fugacities, where the type of modulation changes, and the monosulfide ion appears in the lazurite composition. At T = 550°C and probably at lower temperatures, SO2 fugacity ceases to be the critical parameter of ITM existence. The ordered state of polysulfide and sulfate clusters corresponding to the ITM period of Baikal cubic lazurite is stable at T = 550°C and is an example of forced equilibrium. It develops in response to a crystal chemical event occurring at a temperature of Tx within 600–550°C and is related to the thermal compression of the structure resulting in the isolation of structural cages containing clusters with different states of sulfur. Their mutual interaction, which leads to the rapid release of the modulation at higher temperatures owing to the equalizing of cluster sizes in the cages, ceases. As a result, the proportions of reduced (S22−, and Sx2−) and oxidized (SO42−, So32−, and S2O32− sulfur species show negligible variations, and there is only partial reduction of sulfate to sulfite and thiosulfate. Lazurite samples with disulfide and polysulfide ions behave similarly, which suggests that an important condition for the preservation of ITM is the presence of sulfur-bearing anions with different sizes rather than particular sulfur species in structural cages. The degree of ordering in the distribution of clusters attained at Tx remains unchanged owing to the development of forced equilibrium maintained by the energy balance between framework deformation and cluster ordering. Natural lazurite with an ITM structure could not form at temperatures higher than Tx, i.e., above 550–600°C

Average structure of incommensurately modulated monoclinic lazurite

The average structure of the monoclinic modification of lazurite Ca1.26Na6.63K0.04[Al6Si6O24](SO4)1.53S0.99Cl0.05 (discovered in the Lake Baikal region) incommensurately modulated along the c axis is solved by the single-crystal X-ray diffraction method. The unit-cell parameters are a = 9.069(1) Å, b = 12.868(1) Å, c = 12.872(1) Å, γ = 90.19(1)°, sp. gr. Pa, R = 6.9%, 2057 reflections. The AlO4 and SiO4 tetrahedra form a partially ordered framework. The positions in the cavities of the framework are split and randomly occupied by Na and Ca atoms and the SO4, S2, S3, and SO2 anionic groups. The structure of the mineral is compared with the superstructure of triclinic lazurite. Conclusions are drawn about the causes of the incommensurate modulation in monoclinic and other lazurites.

Onset Temperatures and Kinetics of Quartz Glass Crystallization

A complex analysis of the crystallization of quartz glass from quartzites of the Bural-Sardyk deposit (the Eastern Sayan) has been performed. A mineralogical and petrographic characterization of the quartzites of this deposit is presented, and techniques of quartz grit preparation and quartz glass formation are described. Quartz glass samples prepared from grits of two types have been thermally tested, and crystallization onset temperatures and character of their crystallization were determined for them.

Sulfhydrylbystrite, Na5K2Ca(Al6Si6O24)(S5)(SH), a new mineral with the LOS framework, and re-interpretation of bystrite: cancrinite-group minerals with novel extra-framework anions

Abstract Sulfhydrylbystrite, Na5K2Ca(Al6Si6O24)(S5)(SH), cell parameters a = 12.9567(6) Å, c = 10.7711(5) Å, space group P31c, is a new mineral belonging to the cancrinite group. It was found at Malaya Bystraya lazurite deposit, Lake Baikal area, Eastern Siberian Region, Russia, associated with lazurite, calcite, diopside, phlogopite and pyrite. The mineral develops at the margins of masses of lazurite, replacing it in some areas with the formation of nonequilibrium lazurite-diopside-sulfhydrylbystrite association. It is translucent, yellow to orange, with vitreous lustre, yellow streak and Mohs hardness of 4.5-5. The empirical formula, based on 12 (Si + Al), is Na5.17K1.87Ca0.99[Al6.01Si5.99O24](S5)2-0.86(SH0.86)Cl0.07, Z = 2. The crystal structure of sulfhydrylbystrite may be described as an ABAC stacking of six-membered rings of SiO4 and AlO4 tetrahedra and extra-framework cations and anions located within structural cages. There are two type of cages, cancrinite and losod, stacked into chains at (0, 0, z) and (⅔, ⅓, z), respectively. The cancrinite cage hosts Ca2+ and (SH)- ions, whereas the (S5)2- polyanion is in the losod cage associated with Na+ and K+ cations. In addition, (SH)- and (S5)2- anions are detected in the structure of a mineral for the first time. For comparison, a structural and compositional study of a bystrite sample from the same deposit was carried out. Bystrite is confirmed to contain pentasulfide anions in the losod cages, similar to those of sulfhydrylbystrite, in contrast to previous studies. However, bystrite has chloride in cancrinite cages, whereas sulfhydrylbystrite has hydrosulfide in that position. The unit-cell parameters are distinctly different: bystrite has a = 12.8527(6) Å, c = 10.6907(5) Å in the same P31c space group.

Crystal structure of chlorinated mineral of cancrinite group with bystrite-type framework

The formula of chlorinated bystrite (rare mineral of cancrinite group) has been refined to Na7.22Ca0.92[Al6Si6O24](S5)0.94Cl1.01. The X-ray diffraction data on the contents of chlorine and sulfur in this mineral are confirmed by the results of chemical analysis. The bystrite cage is found to contain a (S5)2–cluster, which is elongated along the c axis and has a chain configuration. Different ways of aggregation of sulfur atoms lead to cis- and trans-configurations of chains. Columns of cancrinite cages contain–Ca–Cl–Ca–Cl–chains, which were observed previously in the structures of minerals of davyne subgroup, afghanite, and tounkite.