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Layer Topologyo Stacking Variation, and Site Distortion in Melilite-Related Compounds in the System Cao-Zno-Geor-Sio, Trronr.Ts

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Layer Topologyo Stacking Variation, and Site Distortion in Melilite-Related Compounds in the System Cao-Zno-Geor-Sio, Trronr.Ts American Mineralogist, Volume 75, pages 847-858, 1990 Layer topologyostacking variation, and site distortion in melilite-related compoundsin the system CaO-ZnO-GeOr-SiO, Trronr.tsAnvrnnusrnn Laboratorium fiir chemischeund mineralogischeKristallographie, Universitiit Bern, Freiestrasse3, CH-3012 Bern, Switzerland FuNqors Rdrrrr,rsnnRcER,FnrBnnrcrr Snrrrnr BayerischesForschungsinstitut fiir experimentelleGeochemie und Geophysik, Universitiit Bayreuth, Postfach10 l2 51, D-8580 Bayreuth,F.R.G. Ansrnlcr Single crystals of melilite-related tetrahedral sheet structures in the system CaO-ZnO- GeOr-SiO, were grown by hydrothermal methods, sinter experiments,flux, and topotacti- cal techniques. Their crystal structures were determined and refined using X-ray single- crystal data. The high-temperature polymorph, high-CarZnGerOr, possessesan incommensurate structureat room temperature.The averagestructure with spacegroup P42rm, a -- 7.950(l), c : 5.186(l) A is of melilite type with layers formedby ZnOo and GeOntetrahedra. Anisotropic displacement parametersindicate that the modulation may be interpreted in terms of librations of tetrahedral units about the c axis. Low-CarZnGerOr, grown from a NarWOo flux or by an epitaxial method, also pos- sessesan incommensurate fine structure. The refined averagestructure, spacegroup P2,, a: s.020(2),b:7.995(2), c: 15.506(3)A, B :89.47(2) displaysa srackingvariant of melilite-like layers with an ABA'ABA' stacking sequence.The stacking fragment A'A is essentially a melilite unit. Layer B is rotated 90'about c relative to layer A. Ca atoms between layers A and B have a disordered arrangement. A new tetrahedral sheetstructure with four-, five-, and six-memberedrings of tetrahedra was synthesizedat the lowest temperatures.The structure has space group P2,/n, a : 9.112(l),b:7.900(2), c: 9.380(llA, B: 114.03(l)'andis well orderedwithout any indication of modulation. IxrnooucrroN those studies, this phase transition has been recognized Melilite-type compounds of general stoichiometry to occur in all CarTlSirO, melilites (with Tl : Mg,Zn, X2T3A7(with X : large, monovalent to trivalent cation, Fe,Co,Mn and mixtures thereof) but not in CarBeSirO, T : small divalent to quadrivalent cation, A : anion) (Rdthlisberger, 1989). This is in line with the argument are widespread among sulfides, oxysulfides, oxides, and proposed independently by Hemingway et al. (1985) and silicates, and the latter also form an important group of Seifert et al. (1987) that the formation of the incommen- natural minerals. Their structure (e.g., Smith, 1953) can surate melilite phaseis due to a structural misfit between be describedin space group P42,m and consistsof TrA, the dimensions of the tetrahedral sheet on the one hand tetrahedral dimers (T2 sites) connectedby means of ad- and the sizeof the interlayer (X : Ca) cation on the other. ditional tetrahedrally coordinated cations (Tl sites) to Only at elevated temperaturesdoes dynamic disorder of form tetrahedral sheets,with the large X cations in dis- the X ions lead to a stabilization of the unmodulated torted eight-coordinatedsites between the sheets(Fig. l). melilite structure. Meliphanite, Ca(Ca,Na)BeSi,OuF(Dal Negro et al., 1967) The theory for modulated phasespredicts that they are and leucophanite,CaNaBeSirO6F (Grice and Hawthorne, intermediate between a commensuratehigh-temperature 1989) also possessa meliliteJike structure. However, be- phase (in this case the unmodulated melilite structure) causeof cation- and anion-(O,F) ordering, the symmetry and a (commensurate) low-temperature superstructure. is lowered to 14 and P22,2t, respectively. Except for CarCoSirOr,where such a superstructuremight It has been recogrized by Hemingway et al. (1985) and occur metastably in the vicinity of 77 K (Riithlisberger, Seifertet al. (1987)that CarMgSirO,and solid solutions 1989), no experimentalproof of the existenceof low- with component CarFeSirO, undergo a reversible phase temperature superstructuresof the melilite type has been transition to a low-temperature incommensurate phase obtained so far. at ca. 358 to 523 K, depending on composition. Since Basedon the above concept, it was expectedthat max- 0003-o04x/90/0708-o847$02.00 847 848 ARMBRUSTER ET AL.: MELILITE-RELATED COMPOUNDS Low CarZnGerOr. Two methods were used. For epi- taxial growth, the outer portions of the high CarZtGe'O, sample were partly melted in a Pt crucible and then quenchedas above. At this point the sample consistedof ahigh-CarZnGerO, core with a glassrim. The glasswas subsequently recrystallized by slow continuous heating from 1023 Kto l2l3 K and maintained 12 h at the final temperature. Crystals up to 0.5 mm in diameter formed aggregateson the rim. The original high phasein the core acted as a nucleus and tumed milky white, probably in- dicating that a phasetransition had occurred. Crystals were also successfullygrown in a disodium tungstate flux (dehydrated Na,WOo'2HrO). This melt is soluble in water, which promotes separationof the crys- tals. The starting material was a powder of composition CarZnGe.Or, which was mixed with the flux in an ap- proximate ratio of l:10. The mixture was continuously (8 K/min) heated in a Pt crucible to 1293 K until the melt was homogeneous. Subsequently, additional CarZnGerO, was added until no more dissolved;the tem- perature was raised by 50 K until all aggregateshad dis- Fig. l. Coordinationpolyhedron diagram of the melilite appearedand maintained constant for 30 min; then the structureviewed along the c axis.In the averagehigh-CarZnGe.O, temperature was lowered (between I and 3 K/h) to 1023 structureand the average CarZnSirO, structure, ZnOo (Tl) tetra- K. This method yielded platy crystalsup to 3 mm in their hedraare at 0,0,0and t/z,t/2,0 and connect GerO, and SirO, groups maximum dimension. Often the crystalswere twinned or : (T2).Open circles represent Ca atomsat z 0.5. milky white. CarZnGer.rrSio.rrOr.Single crystals of this compound were synthesizedfrom high-purity erade Ca(OH)', ZnO, imizing the structural misfit by even further expansionof SiOr, and GeO, in the molar proportions2:1:l:1. Ho- the dimensions of the tetrahedral sheet would lead to a mogenized mixtures were dried at 353 K and used as stabilization of low-temperature structural variants of the starting material for hydrothermal syntheses.The sam- melilite type. Therefore, we have replaced the Si in ples were placed in a small Au capsulewith 30o/oNaOH CarZnSirO, by larger Ge atoms and studied both the Ge solution as solvent and sealed.Conventional hydrother- end-member and a phasewith intermediate composition. mal pressurevessels were used for 34 days at 828 K and A second reason for the selection of the system CaO- 2kbar. The platy experimental products consistedof the ZnO-GeOr-SiO, as an example of melilite-related struc- monoclinic phaseCarZnGe, ,rSio rrO, (composition from tures is the specific crystal-chemical behavior of Zn and crystal-structure site-occupancyrefinements) and Si-rich Ge in oxides. At temperaturesbelow 970 K akermanite melilite. This monoclinic phase could also be grown as CarMgSirO, decomposesinto a chain silicate (wollaston- the pure Ge end-member. Hydrothermal experiments ite) and a nesosilicatestructure (monticellite); thus stable conducted for 17 d at 890 K and 6200 bars with only low-temperature melilite-type compounds cannot be ex- Ca(OH),, ZnO, and GeO, (2:l:2) and HrO yielded an pected in this system. In contrast, Zn prefers tetrahedral X-ray powder pattern (Guinier camera CuKa,-radiation) coordination and Geo* tends to form extended poly- corresponding to that of the compound CarZnGerrr- anions to a much smaller degreethan Si. Sio7sO7.The same pattern was obtained when a melilite- type powder sample with the composition CarZnGerO, ExpnnrunNTAL PRoCEDURE was sinteredat 1345K or 1270K. Unfortunately, suitable Crystal growth single crystals of this end-member phasewere not found High CarZnGeror. Stoichiometric mixtures of analy- in either batch. Thus the structure was refined for the sis-gradeCaCO., ZnO, and GeO, were slowly (8 K/min) (Ge,Si) solid-solution member. heated to 1373 K, in order to decarbonizethe mixtures. The reground powder was placed in a Pt crucible and Crystal-structure analysis slowly heated until the sample melted. For 15 minutes Several crystals of each type were studied using a the melting point (1483-1493K) was exceededby 20 to precessioncamera (MoKa X-radiation) for space-group 30 K. The temperaturewas then continuously lowered (3 determination and crystal quality control. These inves- K/min) to 1463 K, where all CarZnGerO, had crystal- tigations indicated that both low and high Ca,ZnGe,O, lized. After quenching the crucible onto a steel plate, it exhibit incommensurate satellite reflections owing to a was immersed in water. The sample contained colorless, modulation within the (001) layers. These satellites transparent, platy crystals up to I mm in diameter. are much stronger for high CarZnGerO, than for low ARMBRUSTER ET AL.: MELILITE-RELATED COMPOUNDS 849 Trsle 1. Cell dimensionsand exoerimentaldata (Louisnathan, 1969)and the program Prometheus(Zucker factors and real as Average high Averagelow Ca2ZnGe.2s- et al., 1983). Neutral-atom scattering CaZnGerO, CaZnGe"O, Sio,5O, well as imaginary anomalous dispersion corrections were Compound tetragonal monoclinic monoclinic applied. Structure factors with a cutoff {* > 6 o (F"0.) Spacegroup P42,m Pt F2,tn were weightedwith the value l/o2.In the caseof low- a (A) 7.950(1) 8.020(2) 9.11 2(1) CarZnGerOr, the averagestructure was refined with unit b (A) 7.9s0(1) 7.ee5(2) 7.900(2) not whether reflection inten- c(A) 5.186(1 ) 15.506(3) 9.380(1) weights becauseit was clear q (') 90 90 90 sities were influenced by neighboring satellites. Bf) 90 89.47(21 114.03(1) "t,(') 90 90 90 dJimit (') 40 30 30 Rnsur,rs No. Refl. 635 2422 1680 Average high-CarZnGerO, structure No. Obs. >6"(F.*) 560 2334 1420 No. param. 34 351 101 Observed and calculated structure factors are summa- R" (./") 7.2 3.41 2.47 rized in Table 2.' Final atomic coordinatesand displace- B(%) 5.4 2.57 1.68 ment parametersare given in Table 3.
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