Crystallography Reports, Vol. 50, Suppl. 1, 2005, pp. S1–S9. Original Russian Text Copyright © 2005 by Tauson, Sapozhnikov. STRUCTURE OF INORGANIC COMPOUNDS Stability of the Modulated Structure of BaÏkal Lazurite and Its Recrystallization at a Temperature of 600°C over a Wide Range of Sulfur Dioxide Fugacities V. L. Tauson and A. N. Sapozhnikov Vinogradov Institute of Geochemistry, Siberian Division, Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033 Russia e-mail: [email protected] Received February 8, 2005 Abstract—The stability of three-dimensional incommensurate modulation in cubic lazurite from the BaÏkal region is experimentally investigated at T = 600°C. It is found that the X-ray photoelectron spectra of the annealed samples exhibit a peak corresponding to sulfite and a split peak associated with the (Na,Ca)SO4 sul- fate. It is assumed that the splitting is caused by the ordering of the complexes not involved in the framework. This assumption is confirmed by the presence of a similar split peak in the X-ray photoelectron spectra of tri- clinic lazurite. It is demonstrated that the initial modulation is retained at the fugacity f = 8 × 10–3 bar. The SO2 decisive factors responsible for the retention of the three-dimensional incommensurate modulation are the tem- perature and the fugacity of sulfur dioxide. The latter quantity should be close to the stability boundary of the basic lazurite structure. The growth and transformation mechanisms of the modulation formation are consid- ered. © 2005 Pleiades Publishing, Inc. INTRODUCTION mum concentrations of sulfide sulfur and potassium are typical of the monoclinic modification. Transforma- It is well known that lazurite is stable to external tions, in particular, ordering of sulfide and sulfate clus- actions. This mineral has been used to prepare an ters during lazurite recrystallization, occur as a result of extremely resistant blue (sky blue) dye since antiquity. a change in the oxidation state of sulfur, which serves According to geothermometric data, lazurite is formed as a center of formation of intraframework clusters and ° in the range ~600–500 C [1]. A decrease in the temper- is sensitive to external conditions, primarily, to SO2 ature is accompanied by a sequential series of phase activity (fugacity) f [2]. transformations in lazurite due to the change in its SO2 chemical composition. In the BaÏkal region, three struc- The three-dimensional incommensurate modulation tural (cubic, triclinic, monoclinic) modifications of of the structure of cubic lazurites from Southern BaÏkal lazurite are formed in the aforementioned temperature deposits has attracted particular interest of researchers range. Cubic lazurite has a three-dimensional incom- dealing with aperiodic structures [3, 4]. The point is mensurate structure in which the modulation period is that the sinusoidal three-dimensional modulation is not a multiple of the subcell parameter. Triclinic and unstable in terms of the continuum theory of elasticity, monoclinic lazurites have commensurately modulated because the conditions for equilibrium of an elastic structures, which are responsible for the appearance of medium are not satisfied for this modulation. Note that additional superstructure (satellite) reflections around the sodalite-like structure of lazurite belongs to loose the reflections of the cubic subcell in the X-ray diffrac- structures with large holes, which are occupied by 3+ 3+ tion patterns. The high-temperature cubic structure [Na3CaSO4] sulfate and [Na3CaS] sulfide clusters with a three-dimensional incommensurate modulation [5], whose displacements are weakly affected by the is formed at temperatures from ~600 to ~500°C. The aluminosilicate framework and do not necessarily give composition of this modification is characterized by rise to considerable internal stresses. In this case, the high ratios between sulfate and sulfide sulfur species character of deformation is similar to plastic flow, for and virtually does not contain potassium. Afganite, which the conditions of mechanical equilibrium are sat- which is a facial analogue of lazurite, is formed at lower isfied by itself [2]. The three-dimensional incommen- temperatures. When forming afganite-bearing metaso- surately modulated crystal structure of cubic lazurite matites, triclinic lazurite is predominantly formed in from the BaÏkal region is not determined, even though the temperature range ~560–520°C and the monoclinic Bolotina et al. [3, 6] attempted to solve this structure variety is formed at ~500°C at the front of substitution within a twin model consisting of three orthorhombic of phlogopite for lazurite. Compositions with maxi- components related by the [111] axis of the cubic cell. 1063-7745/05/5001-S0001 $26.00 © 2005 Pleiades Publishing, Inc. S2 TAUSON, SAPOZHNIKOV Rastsvetaeva et al. [7] determined the average structure EXPERIMENTAL TECHNIQUE of this mineral without regard for satellite reflections. Such a structure corresponds to a superposition of all The experiments were performed in evacuated cells in which the positions of individual atoms are sealed silica glass (optical quality) tubes. A gold tube determined. The cation sites in the average structure are 3 mm in diameter with an oxygen buffer (500 mg) and split into three subsites, which can be explained by the two platinum tubes 1.5 mm in diameter, one of which contained initial lazurite (as a rule, 25 mg) and the other inadvertent errors arising when the parameters of the tube involved a sulfur fugacity indicator (pyrrhotite, modulated structure are calculated within the average 100 mg), were placed in the silica tube with an inner approximation. In the commensurately modulated diameter 6 mm and ~80 mm in length. The tubes were structure of triclinic lazurite, Na and Ca cations occupy placed in pockets of a massive steel holder mounted in different positions and the unit cell involves two a shaft furnace and held for a required time at 3+ 4.5+ [Na3CaSO4] and four [Na5.5Ca0.5SO4] clusters 600 ± 3°C, followed by cooling in air. The fugacity f of arranged in an ordered manner [8]. a component in a gaseous phase characterizes its partial pressure in a nonideal gaseous medium. This quantity The factors responsible for the formation of the can be provided by buffer mixtures (for example, a three-dimensional incommensurate modulation in metal–oxide mixture for oxygen and a metal–sulfide cubic lazurite, its frequent occurrence, and retention in mixture for sulfur) or be determined from an indicator metasomatites of lazurite deposits in the Southern phase of variable composition. At a given temperature BaÏkal region remain unclear. Sapozhnikov et al. [9] and total pressure, the composition of such a phase (for experimentally revealed that the modulation can be eas- example, wüstite Fe1 – xO or pyrrhotite Fe1 – xS) depends ily released by annealing the mineral in air at moderate only on the fugacity of the corresponding gas compo- temperatures (for 24 h at 600°C). It is not known nent (oxygen, sulfur). In this work, the sulfur fugacity whether the three-dimensional incommensurate modu- was determined from the pyrrhotite composition and lation is formed via the growth or transformation mech- the SO2 fugacity was calculated from the thermody- anism and whether the modulation is reproduced upon namic data on the reaction with the participation of pyr- growth of lazurite crystals. In our earlier work [2], it rhotite and S2 (see below). was demonstrated that, in lazurites rich with sulfate When using the Ni–NiO oxygen buffer, the oxygen ions, short-period modulations (involving from four to activity in the system is maintained at a constant level six unit cells) can arise, whose stability is provided by and the SO fugacity is calculated from the reaction the balance between the elastic energy of sinusoidal 2 deformations and the energy of cluster ordering in the 1 O +SO---S = . (1) mineral structure. This corresponds to the conditions of 2 2 2 2 metastable forced equilibrium of type II (according to the classification proposed in [10]). However, a similar Within ~100 h after the beginning of the experiment, equilibrium is possible only over a narrow range of SO2 the magnetite phase is formed in the pyrrhotite indica- –2 –5 fugacities, namely, from ~10 to ~10 bar at T < tor. This allows us to independently calculate the SO2 600°C [2]. Geochemically, it is very difficult to con- fugacity according to the reaction ceive factors responsible for the variation of the fugac- 3 3 ity over the aforementioned narrow range taking into Fe O + ------1+ S = ---Fe S2SO+ . (2) account that the fugacity in the stability region of lazur- 3 4 2x 2 x x 2 ite can vary by ten orders of magnitude and that natural cubic lazurite almost always has a modulated structure. As was shown in our previous work [11], the dis- In order to gain an insight into the nature of this unusual crepancy between the results of calculations from two phenomenon, it is necessary to analyze more accurately reactions does no exceed 0.2logf SO . Higher fugacities modulation transformations at fixed values of tempera- 2 f were achieved using the metal–oxide buffer of tures and activities of volatile components. This paper O2 reports on the first results of such investigations. The variable composition. This buffer was prepared by release of the modulation in natural three-dimensional annealing Ni + Cu and Cu2O + NiO mixtures in sealed incommensurately modulated lazurite was studied over silica tubes. Then, the metal and oxide components of the buffer were mixed in a ratio of 3 : 2. The heat treat- a wide range of SO2 activities. It was found that, at ° ment conditions were as follows: annealing at 800°C 600 C (the upper limit of the temperature range of ° lazurite formation in the BaÏkal region), the modulation for two days and then annealing at 600 C for one day. Since the oxide component of the system is nonideal, is released at all fugacities f varied within five SO2 the oxygen fugacity cannot be determined from the orders of magnitude in the range where the modulation composition of the buffer mixture and the SO2 fugacity exists.