American Mineralogist, Volume 74, pages 394-410,1989 Incommensurate-modulated structure of nosean, a sodalite-group mineral ISHMAEL HASSAN, * PETER R. BUSECK Departments of Geology and Chemistry, Arizona State University, Tempe, Arizona 85287, U.S.A. ABSTRACT High-resolution transmission electron microscopy has provided evidence for ordering of [Na.. SO.]H and [Na.' H20]4+ clusters in nosean, ideally Na8[AI6Si602.]SO..H20. Re- flections of type hhl, 1= 2n + 1, indicate that space group P43n is an average for nosean. Space group P23, a subgroup of P43n, applies to one type of domain that occurs in nosean and allows for a good model structure for nosean. These domains are based on ordering of the above clusters, and the domains are separated by antiphase domain boundaries. Domains that are based on positional modulations of the framework oxygen atoms also occur; some parts of the crystal show varying superstructure fringes that are absent in other parts. The complex satellite reflections arise from incommensurate modulations of the framework oxygen~atom positions in several directions and are prominent along (001), (110), (112), and (412); their periodicities differ with direction. The different sizes of the [Na.' SO.]H and [Na.' H20]4+ clusters and their degree of ordering influence the positional modulations of the framework oxygen atoms. The cluster ordering, which is enhanced by the net charge differences between the clusters, accounts for the two well-defined frame- work oxygen-atom positions in nosean. INTRODUCTION specimens from different localities. In addition to differ- Many materials have the sodalite structure (Table 1). ent chemistries, the satellite reflections are related to the It is characterized by a framework of corner-linked TO. temperature of formation of the minerals and the order- ing processes that are associated with decreasing temper- tetrahedra (T = Si4+, AP+, BeH, BH, GaH, and Ge4+) and intervening large cages. Such cages are used as build- ature. ing blocks for many zeolites. The minerals of interest for The origin of the complex satellite reflections in soda- this paper include the sulfate-rich sodalites (hauyne, lazu- lite-group minerals is not known. In the first phase of this rite, and, in particular, nosean). The ideal formula for project, the structures ofa number of soda lite-group min- nosean is Na8[AI6Si602.]SO.' H20. erals have been refined using single-crystal X-ray diffrac- The presence of complex satellite reflections from sul- tion techniques (Table 1). One of the aims was to provide fate-rich sodalites is well established (Saalfeld, 1959, 1961; accurate structural data to form a basis for evaluating and Schulz and Saalfeld, 1965; Taylor, 1967; Lohn and Schulz, proposing superstructure models. Although the structures 1968; Hassan et aI., 1985). Saalfeld (1961) suggested that were refined from measured intensities of sublattice re- the superstructure in hauyne is related to interframework flections, the resulting structures include the averaged ef- atomic order, and Schulz (1970) indicated that in the no- fects of positional and compositional modulations, and sean structure all the atoms contribute to the superstruc- thus they include information on the superstructures. The ture reflections. Positional disorder has been reported for data from these structures, therefore, form a necessary lazurite (Hassan et aI., 1985). Saalfeld (1961) suggested starting point for understanding the superstructures of so- that the nosean superstructure is commensurate, having dalite-group minerals. a supercell six times that of the subcell, but Ito and Sa- Although electron microscopy has not previously been danaga (1966) interpreted the same superstructure in terms done on nosean, such studies were performed on the re- of multiple twinning of an orthorhombic cell. Taylor lated mineral hauyne. A multidimensional modulation of (1967) measured superstructure periodicities of nine the structure was suggested to explain the satellite reflec- specimens of sulfate-rich sodalites and found that their tions (Morimoto, 1978). Tsuchiya and Takeuchi (1985) superstructures are incommensurate. The ratios of the showed selected-area electron diffraction (SAED)patterns published superstructure to subcell spacings range from for [110] and [111] zones of a hauyne from Laacher See, 4.65 to 8.04. Details of satellite reflections from sulfate- Germany. They interpreted a dark-field image from the rich sodalites differ for each mineral and differ among [111] zone as domains with dimensions of the order of 500 to 1500 A. They concluded that hauyne consists of incoherent domains in six orientations, each having a su- Present address: Department of Geology, University of New perstructure modulated along one of the (110) directions * Mexico, Albuquerque, New Mexico 87131, U.S.A. and fringe periodicities of about 50 A. They proposed a 0003-004 X/89/0304-0394$02.00 394 HASSAN AND BUSECK: NOSEAN INCOMMENSURATE-MODULATED STRUCTURE 395 TABLE 1. Materials with sodalite-type structures Space Cell (A) Phase Chemical formula Cage clusters S§ group a R Reference Sodalite Na,[ AI,Si,02,]CI2 100% [Na,-CIJ3+ N P43n 8.882 0.017 Hassan and Grundy (1984a) Hackmanite Na,[AI.Si,02,]CI2 100% [Na,' CIJ3+ N P43n 8.877 0.035 Peterson (1983) Basic sodalite Na,[ AI.Si,02,](OH)2 -2H2O 100% [Na,.OH.H2O]3+ N P43n 8.890 0.028 Hassan and Grundy (1983a) Nosean Na,[ AI,Si,0,.](SO')2' H2O 50% [Na.' SO,]2+ y P43n* 9,084 0.057 Hassan and Grundy (1989a) 50% [Na" H2O]4+ Hauyne Na,..Ca2K, [AI,Si.02,](SO ,),..(OH)... 75% [Na3Ca.SO,]3+ N, Y P43n* 9.166 0.036 Hassan (1983) 25% [K2Ca' OH]3+ Lazurite Na.Ca2[ AI.Si,02'](SO ,),.,5.., 71 % [Na3Ca' SO,]3+ Y P43n* 9.105 0.070 Hassan et al. (1985) 29% [Na3Ca -S]3+ Helvite Mn,[Be.Si.0,,]S2 100% [Mn"S].+ N P43n 8.291 0.024 Hassan and Grundy (1985) Danalite Fe,[Be.Si.02,]S2 100% [Fe"S].+ N P43n 8.232 0.024 Hassan and Grundy (1985) Genthelvite Zn,[Be.Si,02,]S2 100% [Zn,'S]6+ N P43n 8.109 0.028 Hassan and Grundy (1985) Bicchulite Ca.[AI,Si,02,](OH), 100% [Ca..4OH]'+ N 143m 8.825 0.012 Sahl (1980) 5ynthetic Ca,[ AI'202,]02 100% [Ca.-O].+ N 143m 8.86 0.069 Ponomarev et al. (1971) Synthetic Ca,[AI,20,.](WO.)2 100% [Ca..WO.].+ Y Aba2 26.151t 0.041 Depmeier (1984) Synthetic Zn,[B,,02,]02 100% [Zn.' 0]'+ N 143m 7.480 0.044 Smith-Verdier and Garcia-Blanco (1980) Synthetic Na.[AI.Ge.0,.](OH)2 100% [Na,.OH]3+ N P43n 9.029 0.048 Belokoneva et al. (1982) Synthetic Na,[Ga,Si,O,.](OH). -6H2O 100% [Na.' OH ,3H2O]3+ N P43n 8.856 0.084 McCusker et al. (1986) Tugtupite Na,[ AI,Be2Si,02,]CI2 100% [Na..CIJ3+ N 14 8.640* 0.023 Hassan (1983); Dan0 (1966) § 5 = Satellite reflections: N = no, Y = yes. Average space groups because these minerals may contain antiphase domain boundaries; the space group for each domain is P23, * t b = 13.075, c = 9.319, c = 8.873. * model based on a structure analysis of intensities ob- incommensurate and arise from the positional modula- tained from domains that have a common orientation. tions of the framework atoms, particularly the two in- Their model consists of intergrowths of a hauyne-like slab dependent framework oxygen-atom positions. The posi- of five subcells and a nosean-like slab of three subcells. tional modulations are related to the degree of ordering This model resembles that of e-plagioclase, which con- of clusters of different sizes, which is enhanced by the net sists of slabs of anorthite-like and albite-like structures; charge differences of two valence units between the clus- the boundaries between the slabs are in antiphase rela- ters. tion. Consequently, the present study has analogies to and perhaps implications for the plagioclase feldspars. STRUCTURES OF SODALITE-GROUP MINERALS This project entails the use of high-resolution trans- The cations in the TO. tetrahedra are fully ordered in mission electron microscopy (HRTEM)to image the struc- the different sodalite frameworks, and the structures are tures of sodalite materials showing complex satellite re- characterized by sixfold rings of TO. tetrahedra parallel flections. HRTEMimaging allows further modeling of the to {Ill} planes. These rings are stacked in a cubic AB- superstructures because both satellite and substructure CABC . sequence. The structures are further charac- reflections are used to form the image. In addition, in terized by fourfold rings of TO. tetrahedra parallel to suitable cases, ordering can be determined, and if it is {lOO}planes. The linkage of the framework TO. tetra- associated with antiphase domain boundaries (APBs), the hedra results in a cubo-octahedral cage, called a ~-cage in latter can also be imaged. Moreover, electron diffraction zeolite chemistry (Fig. la; also Fig. 1 of Hassan and allows distinction to be made between modulations re- Grundy, 1984a). sulting from atomic positions (= displacive modulations) The net negative charge on the sodalite framework units vs. atomic substitution (= density modulations); sodalite- can be neutralized by a large variety of interframework group and their polymorphic cancrinite-group minerals ions. The substitution is limited by the cage sizes and by serve as ideal examples of these modulations. A prelim- the charge requirements of the framework units. The flex- inary report on the sulfate-rich sodalites and an alumi- ible sodalite cage adapts itself to different sizes of cage nate, Cas[AI1202.](CrO.b was given by Hassan and Bu- ions by cooperative rotation of the TO. tetrahedra (Beag- seck (1987). ley et aI., 1982; Depmeier, 1984; Hassan and Grundy, Here we present HRTEMresults on the ordering of [Na.. 1984a). The two cages per unit cell in sodalite materials SO.]2+and [Na.' H20]4+ clusters in nosean; these clusters can order if the two types of clusters differ in either net form domains that are separated by APBs.