American Mineralogist, Volume 75, pages 847-858, 1990

Layer topologyostacking variation, and site distortion in -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, (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. The strong aniso- - Note:B: >llF.""l lF"d"llDlF.,"l, tropy of severaldisplacement parameters (Fig. 2) reflects R. : (>w(lF.""1 - | F"*l)'DwlF.*l')''. the modulation within (001).It is evident that the major effectof the modulation is a coupled libration ofthe tetra- hedra around c combined with a translation of Ca along CarZnGerOr,where the additional reflectionscan only be (110). When the averagestructure of high CarZnGerO, observedin strongly overexposedphotographs. The aim is compared with the structure of natural CarZnSitO, of the present study is a of determination and discussion (Louisnathan, 1969), the orientation of the probability the averagestructures, whereasan analysis the mod- of displacementellipsoids is seento be very similar, though ulation will be left for a forthcoming study. less pronounced in the silicate (Fig. 2). Recent electron CarZnGe,2rsi.rjo, gave sharp reflections without any microscopic investigations (Rdthlisberger, 1989) on ma- satellites. terial from the same type locality indicated that natural All reflection intensities were measured at room tem- CarZnSirO, (hardystonite) also exhibits incommensurate perature graphite in the

TaeLe3. Atom coordinates and displacementparameters U,,(A") for the average high-CarZnGerOTstructure

Ca Zn Ge 02 o3 X T1 T2

X 0.3320(2) 0 0.14206(9) 0.5 0.1400(8) 0.088(2) v 0.1680 0 0.35794 0 0.3600 0.179(1 ) z 0.5038(6) 0 0.9531(2) 0.181(3) o.277(21 0.783(2) B* (A1 2.e6(3) 1.27(1) 1.08(1) 6.1(4) 2.5(1) 7.3(41 U,, 0.049(1) 0.0196(4) 0.0152(2) 0.11(2) 0.042(3) 0.20(1) U". 0.049 0.0196 0.0152 0.11 0.042 0.042(4) u* 0.0141(8) 0.0092(6) 0.0109(4) 0.006(4) 0.012(3) 0.031(4) -0.075(7) Ur. 0.038(1) 0 0.0002(3) -0.09(2) 0.02s(5) Ur" 0.0001(5) 0 0.0040(2) 0 -0.002(2) 0.061(6) U.. -0.0001 0 -0.0040 0 0.002 -0.026(4) Note.'Be: 8/3 r, )(),(U,, ai ai a,.q)), o(B*): Schomakerand Marsh (1983),displacement parameters are ot the form: expl-2rru"h2a'2 + U22k2A2 + UePci2 + 2u,2hkdv + 2ughla*d + 2uBkl,-d)|. 850 ARMBRUSTER ET AL.: MELILITE-RELATED COMPOUNDS

a Trele 4. Selectedinteratomic distances (A) for melilite-typeav- erageCa2ZnGerO, and CarZnSirO,structures

Ca2ZnGe2O, CarZnSi,O, Ca-O1 2.52(1) 2.484(5) Ca-O2 2.457(6) 2.472(41 Ca-O3' 2.42(21 2.412(4) Ca-O2'' 2.708(41 2.699(4) A Ca-O3'' 2.80(1) 2.68s(4) Zn-O3 1.95(1) 1.937(4) T2-O3' 1 73(1) 1.619(4) T2-O2 1.68(1) 1.583(5) T2-O1 1.742(6) 1.649(3) T2-O1-T2 133(1) 138.5(5) Zn-O3-r2 113.5(5) 117.33(3) Note: Distancesmarked - occur twice.

type unit. Layer B also displays the characteristicfeatures of a melilite-type layer (five-membered rings of tetrahe- dra). However, it is rotated by 90oaround the c axis with respectto layer A or A' and also slightly shifted along a Gie.a). A stack of layersABA' yields two channel systemspar- allel to c. There are channels (type l) formed by five- membered rings of tetrahedra of different orientation other. A second,more irregular chan- B stackedabove each nel type, (type 2) is formed by superimposedfive-mem- bered rings that overlap only in a narrow square(Fig. 3). Ca in type- I channelsoccupies the intermediate layers at z : 0, t/t, 2/t,as in melilite. Ca in type-2 channelsis also located on these intermediate layers but in a disordered manner. Difference-Fouriermaps yielded three strong re- sidual peaksfor the two available Ca atoms. Two of these peakswere highly asymmetric and could not be modeled by conventional anisotropic displacement parameters. They were modeled by split half-atoms. In the final re- finement Ca population factors for type-2 channel sites were allowed to vary. Unconstrained occupancy refine- ments yielded a total of 6.25(25) Ca, which is in good Fig.2. ORTEP plots of melilite-type structuresviewed along agreement(l o) with the 6.0 Ca required by the formula. the c axis. (A) averagehigh-CarZnGerOT structure: note the strong The disordered Ca atoms affect the positions of the O anisotropy of selectedO and Ca displacement ellipsoids. The atoms coordinating Ca, as evident from the anisotropy resulting pattern can be interpreted in terms of rotations of tet- of O displacement parameters.However, a split- rahedral units parallel to the c axis. (B) averageCarZnSirOr: the model was not used becauseofobvious correlation prob- orientation of the displacementellispoids is similar to A, but the lems. Such correlations required that U, parameters of anisotropy and size of the ellipsoids are less pronounced. This selectedO atoms had to be fixed to obtain positive-defi- observation is in agreementwith the fact that CarZnSirO, also exhibits incommensurate superstructure reflections. (Scale of nite displacementmatrices for all O atoms. The observed displacementellipsoids is 700/0.) disorder effects are probably related to the modulated character of the fine structure. Observed and calculated structure factors are summarized in Table 5 (seefootnote layers different and more complex (Fig. 3) than that of l). Final coordinates, anisotropic displacement parame- melilite. Whereas in melilite the layers are stacked di- ters, and interatomic distancesare given in Tables 6, 7, rectly one above the other (AA sequence, period c : 5. I 88 and 8, respectively. A), in average low CarZnGerO, the c axis is tripled (15.506 The following nomenclature is used for atomic posi- A), reflecting the stacking sequence ABA'ABA' (Fig. a). tions: The layer type A or B is indicated by subscripts. The melilite-related layers A and A' are identical, as they In addition, cations and anions are arbitrarily numbered. can be derived from each other by a 2, (along D) axis. Ca on the intermediate layer with z : 0 is indicated by The A'A fragment of the structure represents a melilite- Ca", and Ca on the layers with z - th,2/: is indicated by ARMBRUSTER ET AL.: MELILITE-RELATED COMPOUNDS 851

Fig.3. Coordinationpolyhedron diagram ofthe averagelow- CarZnGerO,structure viewed along the c axis.Melilite-type lay- ersare stackedin an ABA'ABA' sequence.Large open circles representCa atomsatz - 0, 1, filled smallcircles Ca atomsat z = t/t, andopen small circles Ca atomsat z = 4t. The peanut- shapedoverlapping circles are due to split Ca atomswith the samez coordinate.For a moredetailed descriotion of the struc- ture consultthe text.

Cao. Eight-coordinated Ca"l and Ca"2 positioned be- tween A and A' layersyield (Ca-O) distancesof 2.60 and 2.62 A, respectively.These values are almost identical to those in highCarZnGerOr. Caol and Cab2are positioned ilr a type-l channel.Ca"l is also eight-coordinated((Ca-O) : 2.59), and Cao2is seven-coordinated((Ca-O) : 2.58 A;. Cao3, i\ a type-2 channel, is only occupied to 610/o and has six O neighbors with distorted octahedral coor- dination ((Ca-O) : 2.52 A;. fne remaining disordered Fig. 4. ORTEPplot of the averagelow-CarZnGerO, struc- Ca atoms Car4a, Cao4b, Cao5a,and Cao5bin the type-2 ture viewedalong the c axis.Note the stronganisotropy ofdis- channel have between five and seven O neighbors with placementellipsoids, which is causedby the incommensurate distancesbetween 2.1 and 1.2 A. ttre anisotropy of prob- fine structure.(A) layer type A at z = '/u;(B) layer type B at z = ability displacementellipsoids (Table 7) indicates that the Yz.(Scale of displacementellipsoids is 50o/o') disorder within the tetrahedral layers may be best de- scribed by librations of individual tetrahedra around the c axis. Displacementparameters of atoms on the libration Atomic coordinatesare listed in Table l0 and anisotropic axis yield displacementellipsoids that are approximately displacementparameters are in Table I l. Selectedinter- isotropic. As in high CarZnGerOr, Ca ellipsoids are elon- atomic distancesare given in Table 12. The tetrahedral gatedalong (l l0). For split Ca positionsthis anisitropy layersare parallelto (l0l), and the stackingis alongIl0l] is defined by the orientation of the Cao4a-Cao4band (stackingperiod l0 A). The tetrahedral tayers(Fig. 5) are Cao5a-Cao5bvectors. different than those of melilite, as the tetrahedranot only form five-membered rings but also occur in four- and HLr/n C4ZnGe,.rrSior.O, structure strongly distorted six-membered units (Fig. 6). Site pop- This compound is the only one of the three studied ulation refinements with Ge and Si form factors for the structuresthat crystallizeswith a centric spacegroup. The tetrahedral atoms Ge2 and Ge3 indicate a partly ordered orientation of crystal axes is also different from that of (Ge,Si)distribution. The Ge2 position is occupiedby 73o/o the other two structures.Observed and calculated struc- Ge and 27o/oSi, whereasthe Ge3 site is occupiedby 52o/o ture factors are summarized in Table 9 (seefootnote l). Ge and 480/oSi. This difference is also reflected in mean 852 ARMBRUSTER ET AL.: MELILITE-RELATED COMPOIINDS

TreLe 6. Population factors, coordinates, and equivalent iso- tropic displacement factors for the average low- CarZnGerO,structure

Pop z B< (A1 Zn^1 1.0 -0.2615(2) -0.0646(2) 0.1543(1)1.11(2) Zno? 1.0 -0.7447(2',, o.4237(2) 0.1646(1) 1.28(21 Ge"3 1.0 -0.6072(2) 0.7837(3) 0.1464(1) 0.8s(2) Geo4 1.0 -0.3880(2) 0.2840(3) 0.1913(1)1.38(2) Geos 1.0 -0.1131(2) 0.5788(3) 0.1836(1) 1.07(2',) GeoO 1.0 -0.8972(2) 0.0643(3) 0.1508(1) 0.87(2) ZnE'l 1.0 -0.7332(2) o.7644(2) 0.5085(1)1.1q2) GeB2 1.0 -0.612s(2) 0.4o77(3) 0.4721(1) 1.04(2) Ge"3 1.0 -0.1070(2) 0.6183(3) 0.s16s(1) 0.86(2) Ca"1 1.0 0.4168(6) 0.0996{6) 0.0004(2) 2.8s(8) Ca"2 1.0 0.0814(5) 0.7646{6) -0.0052(2) 3.10(7) Cabl 1.0 0.4042(5) 0.0684(s) 0.3308(2) 1.82(s) CaC 1.0 0.6011(4) 0.6316(6) 0.3373(2) 2.00(6) Cao3 0.61(1) -0.002(2) 0.346(2) 0.32s7(71 5.q2) Cao4a 0.37(5) 0.125(2) -0.243(11 o.32s4(71 1.4\2) Cab4b 0.41(6) 0.172(4) 0.6e6(8) 0.331(1) 7.2(8) Caosa 0.52(6) 0.866(2) 0.936(1) 0.3437(7) 1.e(2) Cabsb 0.35(7) 0.834{6) 0.01(1) 0.332(2) 8.2(2) oo1 1.0 0.s62(2) 0.858(2) 0.08,+8(7) 3.q3) oo2 1.0 0.104(2) 0.061(2) 0.2612(6) 2:t(2) oo3 1.0 0.077(1) -0.45q2) 4.2297(61 2.4(21 Fig. 5. Coordination polyhedron diagram of the structure of -0.080(3) oo4 1.0 0.241(2) 0.1034(8) 4.8(4) CarZnGer25sio ?5O7 (space group P2r/ n). The structure is viewed oo5 1.0 0.3s3(3) 0.602(2) 0.0s6(1) 4.6(s) parallel to perpendicular to the tetrahedral layers. The 0^6 1.0 0.401(1) -0.2031.2) 0.2s78(6) 1.s(2) [101], oo7 1.0 0.420(2) 0.329(3) 0.23e3(9) 6.2(6) layers (l0l) are stacked in an ABAB sequence.Pairs of tetra- o"8 1.0 0.168(2) o.247(2) 0.0980(8) 5.0(4) hedra with verticespointing up or down are occupiedby Ge and on9 1.0 0.811(2) o.762(21 0.2343(7) 2.8(3) Si, distorted tetrahedra seen edgewise are occupied by Zn. Open oAto 1.0 0.74q2) 0.438(2) 0.2338(7) 3.2(2) circles representCa. Three types oftetrahedral rings are present: oA11 1.0 0.670(3) 0.100(2) 0.2435(9) 4.0(4) oo12 1.0 0.624(2) 0.281(2) 0.0802(7) 3.8(3) five-memberedrings, four-membered rings, and extremely elon- oA13 1.0 0.881(2) 0.s81(2) 0.0719(8) 2.5(21 gated six-memberedunits. o"14 1.0 0.931(2) 0.012(3) 0.0906(8) 5.1(5) o"1 1.0 0.11 3(2) 0.136(2) 0.5934(7) 1 8(2) o.2 1.0 -0.3ee(1) -0.092(2) 0.41s7(6) 1.7(21 -0.060(2) o"3 1.0 0.183(2) 0.4316(7) 2.8(3) DrscussroN o"4 1.0 0.344(2) -0.3s8(2) 0.423(1) 3.6(4) o"5 1.0 0.235(21 o.274121 0.4315(8) 4.1(3) Layer topology 0"6 1.0 -0.084(2) 0.162(2) 0.4345(7) 2.2(21 or7 1.0 0.567(2) 0.352(3) 0.4174(8) 4.s(5) Tetrahedral layer stnrctures that do not occur in ordi- nary sheetsilicates are composed ofvertex-sharing tetra- Arote:Bq : 81312 >,(Z(Uoai ai' A.a)), standard deviationsin parenthe- ses, o(B*): Schomaker and Marsh (1983). hedra centered by Be, P, Si, B, Al, Mg, Fe, Co, Zn, elc. Thesetetrahedral sheetsare primarily linked by "soft cat- ions" like Na, K, Ca, Sr, and Ba (Liebau, 1985)that have no pronouncedpreference for one or another kind ofwell- defined coordination polyhedron. In contrast to tetrahe- T-O distances((T-O)), which are l.72OBA for Ge2 and dral sheetsin micas, the sheetsdiscussed here have two 1.6943 A for Ge3. The layers are stacked in an ABAB identical surfaces,with large cations both above and be- sequence,where the sheetsA and B are symmetrically low. equivalent but shifted along [I0lJ/2 with respectto each The topology oftetrahedral sheetscan be described with other. The tetrahedral sheet structure is characterizedby Schliifli symbols (Liebau, 1985), where each tetrahedron two channel types running parallel to [01]. As in low is representedby a point and the resulting points are con- CarZnGerOr, the first channel type is formed by the nected to form a network ofpolygons (Fig. 7). Each poly- stacking of five-membered rings of tetrahedra that are gon is specifiedby the number ofcorners. A node ofthe rotated relative to one another (Fig. 5). Ca occupying this network is describedby its surounding polygons. Super- kind of channel is eight-coordinated (square antiprism) scripts denote the number of equal polygons neighboring with (Ca-O) :2.569 A. 1.tresecond narrow channeltype the node. A network is describedby listing the nodeswith is formed by an alternating sequencealong [01] of four- nonequivalent polygon surroundings and indicating the membered and elongated six-membered rings. Ca be- relative frequency ofsuch nodes by right subscripts. tween the rings is in distorted octahedral coordination According to this nomenclature, a melilite-type layer with (Ca-O) : 2.419. Also striking is the small Ge2-O- with five-memberedtetrahedral rings has the symbol (5a) Ge3 angle of 122. In addition, the tetrahedradisplay (5')r.A review of compounds with the melilite structure strongly irregular T-O distances(Ge2: 1.658-1.729A). type is given by Bakakin et al. (1970) and Rdthlisberger The largestdistance is observed for the bridging O atom (1989). The structureof P2,/n Ca, (Ge,Si)rO,described of Ge2-O-Ge3. Ca is well ordered. in this paper possessesfour-, five-, and six-membered ARMBRUSTER ET AL.: MELILITE-RELATED COMPOUNDS 853

Taele 7. Displacementparameters U,i(4,) for the average low-CarZnGerOTstructure

u,, ur. u* Urz U,o Ur. Zn^1 0.o147(7) 0.01s3(7) 0.0123(6) -0.0005(6) 0.0018(5) 0.0020(6) Zno2 0.0212(71 0.0153(7) 0.0122(6) 0.0001(6) 0.0023(5) -0.0007(6) Geo3 0.014s(6) 0.0097(5) 0.0081(s) 0.0001(5) 0.001e(4) - 0.0019(5) Geo4 0.0197(7) 0.0223(71 0.0103(s) - 0.0016(6) 0.0047(5) - 0.0021(6) Geo5 0.0205(8) 0.0150(7) 0.0052(5) 0.0017(5) 0.0011(5) - 0.0013(5) Geo6 0.0119(6) 0.0141(6) 0.0071(5) 0.0001(6) -0.0003(5) 0.0005(5) Znsl 0.0153(7) 0.0145(7) 0.0121(6) -0.0005(6) 0.0024(5) 0.0022(6) GeE2 0.0178(7) 0.0129(7) 0.0088(s) 0.0012(6) 0.0034(5) 0.000qs) Ges3 0.0128(7) 0.0146(7) 0.0052(6) -0.0005(5) -0.0008(5) -0.0002(5) Ca"1 0.041(2) 0.055(3) 0.012(1) -0.031(2) 0.000(1) 0.00e0) Ca"2 o.o47(21 0.0s8(2) 0.013(1) -o.o37(2) 0.002(1) - 0.006(1) Cabl 0.031(2) 0.026(1) 0.012(1) 0.012(2) 0.012(1) 0.007(1) Cao2 0.02q2) 0.034(2) 0.015(1) 0.0't7(2) 0.010(1) 0.013(1) Cao3 0.091(6) 0.087(6) 0.047(41 0.039(4) 0.023(4) 0.043(4) Cao4a 0.027(6) 0.022(7) 0.006(4) - 0.015(4) 0.008(3) -0.002(3) -0.03{1) Cao4b 0.06(1) 0.18(3) 0.041(8) - 0.01(2) -0.007(8) Cabsa 0.03s(6) 0.027(6) 0.009(4) -0.003(4) 0.005(3) -0.004(3) Cabsb 0.11(2) 0.17(s) 0.03(1) -0.05(3) 0.01(1) -0.02(2) Oo1 0.030(6) 0.09(1) 0.009(5) -0.033(7) 0.004(4) -0.006(5) Oo2 0.02e(6) 0.049(7) 0.001(3) 0.008(6) 0.000(4) -0.002(5) Oo3 0.027(6) 0.054(8) 0.009(4) 0.021(6) 0.002(4) -0.005(5) - Oo4 0.09(1) 0.08(1) 0.01510. 0.07(1) -0.004(6) 0.012(7) Oo5 0.13(2) 0.018(7) 0.029(8) -0.034(8) 0.04(1) -0.012(5) Oo6 0.028(s) 0.017(5) 0.011(4) 0.006(4) -0.001(4) 0.002(4) Oo7 0.030(7) 0.18(2) 0.026(6) 0.05(1) 0.012(5) 0.04(1) Oo8 0.14(2) 0.023(6) 0.023(6) -0.040(9) -0.021(8) 0.012(s) Oo9 0.062(8) 0.033(7) 0.010(4) 0.028(7) 0.003(5) 0.004(s) oo10 0.0s9(8) 0.04q8) 0.01510- -0.038(7) 0.007(5) -0.005(5) oA11 0.11(1) 0.025(7) 0.019(7) 0.008(6) 0.032(8) -0.004{5) oo12 0.053(8) 0.08(1) 0.008(4) -0.036(9) 0.008(s) - 0.012(6) oa13 0.043(8) 0.037(6) 0.01500' - 0.013(6) 0.007(s) 0.000(s) oo14 0.030(6) o.14(2) 0.024(6) -0.04s(9) -0.007(5) 0.024(8) O"1 0.024(6) 0.029(5) 0.01500' -0.002(5) -0.002(4) -0.001(4) o"2 0.023(5) 0.030(6) 0.00s(4) 0.005(5) 0.008(3) -0.003(4) o"3 0.072(9) 0.02s(6) 0.008(4) 0.02s(7) 0.009(s) 0.004(s) O14 0.09(1) 0.019(7) 0.025(6) 0.017(6) 0.02e(8) 0.00s(s) o"5 0.07(1) 0.07(1) 0.01500- - 0.0s(1) 0.001(6) 0.002(7) O"6 0.020(6) 0.053(8) 0.009(4) 0.011(5) -0.002(4) -0.015(s) O"7 0.030(7) 0.13(2) 0.011(5) 0.043(s) 0.004(4) 0.012(7)

Note-'Starred parametersare fixed, standard deviationsin parentheses,displacement parameters are ot the form exp[-2#(U rlf d' + U22l(82 + Usl2C2 + 2uehka' t + 2unhla'C + 2u2sklil C)1.

rings [Schliiflisymbol: (6.5'?.4) (6.5) (6.5.4)].Ba,CuSi,O, gugiaite (Ca-O) : 2.521 A, whereasin all other melilite- (Malinovskii, 1984) and KHoCoSi,O, (Ragimov et al., type structures (Ca-O) > 2.56 A. In addition, Rdthlis- 1980) have the same stoichiometry as melilite types and berger (1989) stated that in room-temperature electron possessesa two-dimensional TlSirO, network (Tl : diffraction experiments all known melilites of the type Cu,Co) forming four- and six-membered rings [SchHfli CarTlSirO,, except gugiaite(CarBeSi,O,) with the small- symbol: (6'z.4'z)(6'z.4)rl with Ba or K and Ho linking the est cation on Tl (Be), show satellite reflectionsindicating layers. incommensurate structures. According to Rdthlisberger (1989), no satellite reflections could be observed in mel- Bond-strengthcalculations for layer ilite structures of the type Sr,TlSirO? (Tl : Mg,Cu,Zn, topologieswith (T3O?)4+composition Fe,Mn,Cd). Table l3 indicatesthat in all TrO' sheetstruc- For all bond-strength calculations (Table 13) the em- tures the X position (X : Ca,Sr,Ba)has low calculated pirical constantsof Brown (1981) were used.Bond va- valence values. lencesdetermined from experimentally determined bond To estimate how well the TrO, sheetstructures fulfill a lengths are used to calculate atomic valences,which can balancedelectrostatic arrangement of atoms, a parameter be usedto evaluatea .On the other hand, > | 0 | is calculatedfor all structuresgiven in Table I 3. The prediction ofbond lengths is possible. Such calculations parameter > 16| is the sum of magnitudes of the differ- predict that eight-coordinate Ca should have averageCa-O encesbetween the predicted and ideal atomic valences. distances((Ca-O)) of approximately2.45 A. Kimata and Six atomic sites are distinguished; Tl, T2, X, and the Ohashi (1982) compared the Ca-O distancesof gugiaite anions OI, OU, and OIII. (OI, OII, and OIII are equiv- (CarBeSirOr) with those of melilite ([Ca,Na]r[Mg,Fe, alent to Ol, 02, and 03 in melilite structuresrefined in AlSil3O?), hardystonite (CarZnSirO,), (Ca,Alr- spacegroup P42,m.) OI links two T2 tetrahedra, oII is SiOr), and akermanite (CarMgSirOr) and noticed that in only coordinated to T2 and X, and OIII links Tl andT2. 854 ARMBRUSTER ET AL.: MELILITE-RELATED COMPOLINDS

Tmu 8, Selected interatomic distances (A) for the average low-CarZnGerOTstructure

ZnAl-Oa1 1.89(1) Zn^2-O^5 1.94(2) -oo9 1.95(1) -oo7 1.93(2) -oA11 1.98(1) -oo8 1.98(1) -oo14 1.93(1) -oo3 1.98(1) GeA3-Oo1 1.76(1) GeA4-Oo7 1.74(1) -Oo4 1.77(2) -o^10 1.78(1) -Oo5 1.68(1) -o^11 1.74(11 -Oo6 1.73(1) -oo12 1.72(1) GeAs-OA3 1.72(1) Gea6-O^2 1.71(1) -Oo9 1.77(11 -oo4 1.76(21 -oa10 1.76(21 -oo8 1.75(1) -oa13 1.73(1) -oo't4 1.72(1) ZnBl-OB3 1.96(1) -O"4 1.95(1) -O"6 1.8s(1) -Ot7 1.90(1) Ge"2-OB2 1.74(11 Ge83-O"1 1.72(11 -O14 1.77(1) -O13 1.7411) -O"5 1.74(2) -o"5 1.80(2) -O.7 1.72(11 -o"6 1.7s(1) Ca"1-Oa1 2.61(2) Ca.2-O"4 2.46(2) -Oo1 2.46(2) -O"5 2.ee(2) -Oo4 2.56(2) -O"8 2.48(2) -Oo5 2.37(2) -os12 2.63(1) -oo8 2.7612) -os13 2.75(11 -oo12 2.54(2) -o813 2.48(1) -oo12 2.8s(2) -o814 2.74(2) -oal3 2.65(1) -o"14 2.42(21 Caol-Oo2 2.65(1) Ca"2-Oo6 2.43(1) -Oo6 2.44(11 -Oo7 3.21(2) -Oo7 2.52(2\ -Oo9 2.54('tl Fig. 6. ORTEPplot of the structureof CarZnGe,25Sio75O7 -oo11 2.53(2) -oA10 2.s1(1) (spacegroup P2,/n) viewedalong [01]. Onlya singletetrahedral -o12 2.43(1) -Or1 2.54(1) layer (101)is shown.Compared to high and low CaZnGe.O, -O"3 2.s7(1) 'O"2 2.52(11 -O"5 2.64(2) -O"4 2.46(21 (Figs.2 and4), the displacement ellipsoids of all atomsare much -O"7 2.s4(2) -O"7 2.s7(2) smallerand approximately isotropic, reflecting the nonmodulat- Cao3-Oo2 2.65(2) ed characterof this structure.(Scale of displacementellipsoids -Oo3 2.2s(2) is 700/0.) -oa10 2.6112) -O"1 2.7s(2) -O"5 2.55(2) -o"6 2.28(21 Ca.4,-O^2 2.64(2) Cab4b-Oa2 3.16(6) structure cannot be resolved. As seenfrom the shapeof -Oo3 2.31(2) -Oo3 2.14(5) -OoO 2.46(2) -Oo6 2.30(41 anisotropic displacementellipsoids (Fig. 2), the major ef- -Oo9 2.90(2) -O"1 2.6't(4) fect of disorder or modulation in melilites is modeled in -O"1 2.47(21 -o"3 2.50(6) -o"3 2.25(2) 'o.4 2.13(4) the average structure as a rigid body libration of tetra- -O"4 2.64(2) hedral units around the c axis. This modeling has several Cabs"-Oo2 2.5o(2) Caosb-Oo2 2.45(5) effectson atomic coordinatesand on atomic valencescal- -Oo9 2.24(2) -Oo9 2.4818) -oo11 2.58(2) -oo11 2.04(6) culated from bond distances. -O"1 2.60(2) -o"1 3.22(9) l. In a projection of the melilite structure along c (Fig. -O"2 -o"2 2.40(21 2.40(5) 2), the T2-OI-T2 link appearsas a straightline, with Ol -o"3 2.8s(2) -o"6 2.13(7) -O"6 2.33(21 displaying strongly anisotropic displacementsperpendic- ular to the T2-Ol vector. This indicates that in the fine structurethe T2-Ol-T2 angleis bent within (001), caus- ing the real T2-Ol distanceto be significantly longer than If owrng to low symmetry one of these sites degenerates the apparent distance in the averagestructure. to two or more sites,average atomic valencesare consid- 2. It is also seenin Figure 2 that Ol and 03 displace- ered. Inspection of Table 13 reveals that the five struc- ment ellipsoids are elongated towards the center of the tures with the highest ) ld I values are exactly those struc- five-membered rings. Actually only 01 (constrained by tures for which Riithlisberger (1989) observed satellite symmetry) has its longestellipsoidal axis in (001), 45' reflections. If there is a misfit between the size of the from a; the longestaxis of the 03 displacementellipsoids interlayer cation and the tetrahedral sheet,the layers be- points towards the X site. Theseorientations indicate that come distorted, either in a disordered way, maintaining the fine structure has significantly shorter X-O distances P42,m symmetry, or in an ordered fashion, adopting a than the averagemodel. modulated structure(Seifert et al., 1987).By solving an 3. We assume that in the fine structure the Tl tetra- averagecrystal structure (without satellites),a difference hedron more or less maintains its P4 symmetry, but it is between an ordered (modulated) and disordered fine disordered by a rigid body libration about 4 axis. This ARMBRUSTER ET AL.: MELILITE-RELATED COMPOUNDS 855

Trau 10. Populationfactors, coordinates, and equivalentisotropic displacement factors for CarZnGe,2ssio7sO7.

Pop. z B- (A,) Zn1 1.0 0.82867(3) 0.09678(3) 0.11477(3) 0.724(3) Ge2 0.734(3)Ge 0.87207(3) 0 4s559(3) 0.20899(3) 0.481(3) 0.266Si Ge3 0.521(3)Ge 0.96891(4) 0.76878(4) 0.07667(4) 0.466(3) 0.479Si Ca4 1.0 0.4s74(1) 0.2821(1) 0.0282(1) 0.713(7) Ca5 1.0 0.2891(1) 0.5654(1) 0.2014(1 ) 1.201(9) ol 1.0 0.0094(2) 0.5682(2) o.1572(21 0.e2(3) 02 1.0 0.6321(2) 0.0625(2) 0.1343(2) 1.06(3) o3 1.0 0.6915(2) o.4s57(21 0.0501(2) 0.7s(3) o4 1.0 0.9672(2) -0.0987(2) 0.219't(21 0.74(3) o5 1.0 0.8564(2) 0.1810(2) -0.0651(2) 0.se(3) o6 1.0 0.9s4s(2) 0.2625(21 0.2764(21 0.97(3) 07 1.0 0.2959(2) 0.7292(2) o.4207(21 1.18(3) Note.'Bq :813r'>,(Z[Ur al ai a, q)], o(B*): Schomakerand Marsh(1983), standard deviations in parentheses.

effectcauses shorter Tl-O3 distancesin the averagemod- and the averagevalue for the two shorter axes of a dis- el compared to the actual structure. placementellipsoid (Table l4). Becausedisorder or mod- When we apply the above effectson bond-strengthcal- ulation affectsmainly Ol and 03, the parameter )dmax culations, it becomes obvious that average models of is the sum of dmax values for Ol and 03; standarderrors modulated or statistically disordered melilites are char- of 2dmax are large. Uncertainties in this value are related acteized by apparent low valencesfor X sites(calculated to the way in which main reflections accompanied by X-O distancesare too long), high valencesfor Tl and T2 satellitesare measuredand how the data are refined. Fig- positions (underestimationof Tl-O and T2-O), and by ure 8 displaysthe relation between2l6 | and Zdmax. As overbondingof Ol becauseof the underestimatedT2-Ol expected,the four melilite-like crystalswith a modulated distance.On first glanceit seemssurprising that 03 shows fine structure (open squaresin Fig. 8) display not only no overbonding effectsowing to underestimation of the the highestZlDl valuesbut also the greatestanisotropies distancesTl-O3 and T2-O3 in averagestructures. How- for the Ol and 03 displacementellipsoids. ever, the two X-O3 distances are both overestimated, Grice and Hawthorne (1989) applied the same bond- which balancesthe calculated03 valence.With the struc- valence model of Brown (1981) to leucophanite(Ca- tural constraints of spacegroup P42,m and a fairly rigid NaBeSirOuF)with spacegroup P2,2,2, and obtained a tetrahedral sheet,there is no degreeoffreedom to dimin- more balanced bond-strength pattern than in any meli- ish simultaneouslyoverbonding of Ol and to increasethe lite-structurewith spacegroup P42rm. The >ldl value is bond strength on the X site. 0.51. This low value can be explainedby distortions in The relation between calculated atomic valence,mon- the tetrahedral layer and by the more variable chemical itored in terms of the ) 16| parameter on one hand and composition, both of which cause lower symmetry. In the modulated or disorderedcharacter of an averagemel- leucophanite, Si occupies one Tl-type and one T2-type ilite structure on the other, can even semiquantitatively position. Be is ordered on an additional T2 site. One of be applied. For this purpose an additional parameter, the Si-Si bridging O atoms is additionally coordinated to )dmax, is introduced, where dmax is the difference be- two Na ions, and the other is coordinated to one Na and tween the root mean squareamplitude of the longestaxis one Ca ion, thus reducing the overbonding. The apical

TABLEI 1, Displacementparameters Uii(Ar) for Ca.ZnGe,,"Sio rrO,

Ur, U"" us Ure Ur. Urs Znl 0.0081(1) 0.0101(1) 0.0100(1 ) 0.0001(1) 0.0044(1) 0.0010(1 ) Ge2 0.00610) 0.0061(1) 0.0061(1) 0.00000 0.00250 0.00000 Ge3 0.0059(1) 0.0059(1) 0.0059(1) 0.00000 0.00240 0.00000 Ca4 0.0087(2) 0.0107(2) 0.0071(2) 0.0024(2) 0.0026(2) 0.0006(2) Ca5 0.0120(2) 0.0244(3) 0.0092(2) 0.0100(2) 0.0043(2) 0.0014(2) o1 0.0102(8) 0.0115(8) 0.0137(8) 0.0009(6) 0.0053(7) 0.oo24(7) 02 0.0104(8) 0.0179(9) 0.0142(9) 0.0030(7) 0.0071(7) 0.0081(7) o3 0.0086(7) 0.0112(8) 0.0084(8) -0.0001(6) 0.0033(6) 0.0005(6) o4 0.0116(8) 0.0085(8) 0.0077(7) 0.0016(6) 0.003s(6) 0.0001(6) o5 0.0120(8) 0.0164(8) 0.0103(8) 0.0014(7) 0.00ss(7) 0.0044{7) o6 0.0145(s) 0.0117(8) 0.0073(8) -0.0023(6) 0.0009(7) -0.0005(6) 07 0.0122(8) 0.0186(9) 0.0134(8) -0.0010(7) 0.0047(71 -0.0039(7) Note.'Standard deviations in parentheses, displacement parameters are of the form exp[-2rr(Urlfa'2 + Unkzt2 + UsPdz + 2uzhka'tr + 2unhldd + 2UBklUc"ll. 856 ARMBRUSTER ET AL.: MELILITE-RELATED COMPOUNDS

Trele 12. Seleeted interatomic distances (A) lor CarZnGe,,uSio ruO, Zn1-O2 1.893(2) Ge2-O1 1.757(2) Ge3-O4 1.702(21 -o4 1.893(2) -O2 1.709(2) -O1 1.729(2) -o5 1.924(21 -O3 1.712(2) -O5 1.687(2) -06 1.977(2) -06 1.704(21 -O7 1.658(2) Ca4-O2 2.290(2) Ca5-O1 2.41O(21 -o3 2.421(2) -O3 2.442(2) -o3' 2.47q2) -O4 2.412(21 -o4 2.377(2) -O5 2.805(2) (6242)(62 4)z (sn)(st), (6 s24)(6 s2)(652)(6 s 4) -06 2.378(21 -Os' 2.455(2) -o7 2.575(2) -06 2.744\2) Fig. 7. Topologiesof tetrahedrallayers. The centerof each -o7 2.871(2) tetrahedronis representedby a node,and the nodesare con- -o7' 2.4',t0(2) nectedto form a two-dimensionalnetwork of polygons.From Note.'Standard deviationsin parentheses. left to right: BarCuSirO,type, melilite type, new type with space gtorp P2r/n. anions (type OII in Table l3) displacedtoward the inter- CarZnGerO, structure displays a more balancedstructure layers tend to be underbondedin melilite-type structures. than that of the high modification. Overbonding of OI Two crystallographicpositions of this type are presentin (Table l3) is even lessthan that of melilite structuretypes, leucophanite,and one ofthem is occupied by F. and the averagebond strength ofthe four fully occupied Comparison of the ) lDI values of the high and low Ca sites is higher than in the average structure of high CarZnGerOT phases shows that the average low- Ca.ZnGe"O,.

TABLE13. Bond strengths(valence units) on the basisof the empiricalmethod of Brown (1981)

Compound T2 T1 otT2-Tz oil T2-X Olllr1-T2 >16| Reference BarCuSi,O, 4.09 1.74 1.99 2.14 1.89 2.03 0.64 Malinovskii(1984) 1.96 P42,m melilite-typecompounds (only Si or Ge on T2) ldeal 4.O 2.O 2.O 2.0 2.0 2.0 0.00 SrrMgSirO, 4.16 1.93 1.84 2.40 1.76 2.00 1.03 Kimata ('1983a) SrrMnSirO, 4.11 2.08 1.76 2.38 1.68 2.03 1.16 Kimata(1985) Ca"BeSi"O, 4.10 1.92 1.91 2.36 1.91 1.94 0.78 Kimataand Ohashi(1982) Ca,MgSirOz* 4.04 2.06 1.65 2.38 1.65 2.O1 1.19 Kimataand li (1981) CarcoSi20T' 4.27 2.17 1.64 2.46 1.68 2.04 1.62 Kimata (1983b) Ca,ZnSi,O,* 4.15 2.13 1.68 2.38 1.70 2.00 1.28 Louisnathan(1969) Ca2ZnGe2Or' 4.38 2.04 1.59 2.49 1.79 1.98 1.55 This paper SrrZnGerO, 3.92 1.96 1.67 2.32 1.72 1.90 1.15 Ochiet al. (1982) Ba,FeGe,O, 3.88 1.95 1.95 2.32 1.85 1.90 0.79 Malinovskiiet al. (1976) P421rnmelilite-type compounds (disorderedSi and Al on T2) ldeal 35 3.0 2.0 2.0 2.0 2.0 Sr,Al,SiO? 3.54 2.96 1.86 2.23 1.79 1.97 0.69 Kimata(1984) CarAlrSiO, 3.53 3.04 1.75 2.26 1.62 2.01 0.97 Kimataand li (1982) Average low CarZnGe"O,' This paper 4.16 2.23 1.63 2.37 1.73 2.O2 4.02 2.16 1.58 219 1.91 2.O3 4.30 2j7 1.87 2.25 1.60 2.22 4.O7 1.62 1.66 2.O0 (1.70) 1.76 2.O4 (1.78) 1.61 2.00 (1.4s) 2.06 (1.87) 2.06 (2.03) 2.O5 2.15 1.82 2.00 1.28 P2.,I n Ca"Zn(Ge,Sil This paper 4.14 2.25 1.72 2.'to "O, 2.O2 4.07 1.74 1.83 2.19 1.98 1.98 1.08

Note.' Ol symbolizesan oxygen which links two T2 tetrahedra and is equivalentto 01 in melilite-typestructures. Oll symbolizesan oxygen vertex which is directed towards the interlayercations (equivalentto 02 in melilite-typestructures). Olll symbolizesan oxygen which links T1 and T2 tetrahedra and is equivalentto 03 in melilite-typestructures. Bond strength values for partiallyoccupied Ca sites are given in parentheses.> l6 | is the difterence between the calculatedand the ideal valencevalue summed over T2, T1, X, Ol, Oll, and Olll. lf owing to lower symmetry more than one site is given, correspondingaverage values are taken for the summation. . Satellite reflections. ARMBRUSTER ET AL.: MELILITE-RELATED COMPOUNDS 857

The structurewith groupP2,/n (6.5r.4) n space and the o nR-- (6.5'z)(6.5.4) network hasbond strengthsthat are approx- + imately equal for all three types of O and the lowest 2 | d I o CoZn Ge value of all of the phases with the general formula o 06 CarZn(Ge,Si),O,.The displacementellipsoids are ap- 6 proximately isotropic. o c "" o a.o o4 CoCoSi Stability relations of melilite-related tr c: compoundsin the system CaO-ZnO-SiOr-GeO, -! CoMgSi ;i , 03 o There are three possibleexplanations ofhow interlayer a n E o.z CoZn Si cations with various radii can be accommodated by the o irznue CoBeSi t tetrahedral sheets:(l) preferenceof specific layer topol- cul - SrMnSi o SrntSi CoAt3i ogies as shown above, (2) variation in the stacking se- An 6rM quenceoftetrahedral sheets,and (3) distortion within the v: 06 08 1 1.2 14 16 tetrahedral sheets (lowering of symmetry and modula- sum of otomic volence dlfferences tion). I ldt The layertopologies (6,.4,)(6,.4),, (5") (5r), and (6.5,.4) Fig. 8. The differencesum of calculatedand ideal atomic (6.5'?)(6.5.4) have composition TrAr. Recentexperiments valences(> | 6I ) is plottedversus an anisotropyparameter of O I by Rdthlisberger (1989) indicate that a structure corre- and 03 displacementellipsoids of melilite structure tlpes spondingto that of BarCuSirO,[that has a (6,.4,) (6r.4), (P42,m).Modulated structures represented by opensquares have sheetl is not stable in the system CaO-ZnO-SiOr-GeO,. stronganisotropies for Ol and 03 in the averagestructures, resultingin Tl-O andT2-O distancesand Sinter experiments between 1320 and, 1420 K yielded a an underestimationof thusaffecting calculated atomic valences. phasehaving the composition BarZnSirO, and a powder pattern corresponding to that of BarCuSirOr. Thus we assumedthat the network type (6'z.4'z)(6'z.4), provides the largest eight-coordinated sites among the three kinds of modification. None of the sinter or hydrothermal exper- layerswith compositionTrOr. In BarCuSirO,the mean iments in the system CaO-ZnO-SiO,-GeO, by R6thlis- Ba-O distanceis 2.814 A lMalinovskii, 1984), and in berger(1989) produced this phase.It was only grown ep- KHoCoSirO, averageK-O distancesare 2.886 and 2.873 itaxially on high CarZnGerO, and with a flux method for A and averageHo-O distancesare 2.708 and 2.781 A which high CarZnGerO, may have been the phase pri- (Ragimov et al., 1980). marily formed. Perhapsthis stacking variant formed un- At high temperatures the melilite-type sheet (5') (5'), der conditions where the high phase was exposedto in- has the preferred topology for CarZnGerO, and tenseinternal strain, but where the activation energywas CarZnSirOr. However, if these structures occur metasta- not sufficient to causerearrangement of tetrahedral units bly at room temperature, the probable strong dynarnic to those of the phasewith spacegroup P2r/n that has the disorder of Ca is frozen at minima which do not obey (6.5'z.4)(6.5') (6.5.4) network. Thus, for strain minimi- spacegroup P42,m symmetry but adopt an incommen- zation a partly reconstructed(an A'A melilite fragment is suratearrangement that is also adopted by the tetrahedral maintained) new stackingvariant of the (54)(53), network layers. was adopted. Only weak Ca-O bonds must be broken for It is not clear whether the ABA'ABA' stacking variant such a reconstructive transformation. Weak satellite re- (low CarZnGerOr)of the (54) (53), network is a stable flections, large and strongly anisotropic displacementpa-

TABLE14. Root-mean-squaredisplacements (A) along ellipsoid axes for Ol and 03 of melilite-and gehlenite-typestructures

Compound u1(o1) u2(o1) u3(o1) dmax 01 u1(o3) u2(o3) u3(o3) dmax Og Orientation 45 a1 -45'a, 90 a, variable variable 25 a, 45 a2 -45 a2 90'4 variable variabfe 11V a2 9(rc 90c ec variable variable 75 c Sr,MgSi,O? 0.10 0.08 0.06 0.03 0.06 0.08 0.10 0.03 Sr,MnSirOT o.14 0.10 0.09 0.04 0.06 0.08 0.15 0.08 CarBeSirO, 0.14 0.08 0.08 0.06 0.08 0.09 o.12 0.03 Ca.MgSirO, o.27 0.14 0.09 0.15 0.10 o.12 0.23 o.12 Ca,CoSi,O, 0.29 0.11 0.07 0.20 0.10 0.13 0.29 0.17 Ca,ZnSirO? o.21 o.12 0.o7 0.11 0.o7 0.12 o.22 0.12 Ca2ZnGe20, 0.45 o.17 0.08 0.32 0.10 0.'t2 0.50 0.39 SrrZnGe"O, 0.16 0.09 0.09 0.07 0.08 0.10 0.1s 0.06 Sr,Al,SiO, o.12 0.07 0.09 0.04 0.07 0.09 0.13 0.05 CarAl2SiOT 0.13 0.11 0.09 0.03 0.09 0.11 0.15 0.05 Note. dmax(O1) : U1 - (U2 + U3ll2, dmax(O3): U3 - (U1 + U2)/2 orientationsof ellipsoidaxes labeled"variable" are differentfrom compound to comDound.For referencesconsult Table 13. 858 ARMBRUSTER ET AL. : MELILITE-RELATED COMPOUNDS rameters, and disordered Ca sites imply the modulated - (1985) The structural properties and mineralogicalsignificance of characterof this stackingvariant. The (6.5'z.4)(6.5) (6.5.4) synthetic SrrMnSlO, melilite with 4-coordinated manganese.Neues Mineralogie Monatshefte, 1985, 85-96. phase, grown Jahrbuch fiir which was at the lowest temperature, rep- Kimata, M., and Ii, N. (1981) The crystal structureof syntheticakerman- resentsa well-ordered structure without any indication of ite, CarMgSirOr. Neues Jahrbuch fiir Mineralogie Monatshefte, 1981. modulations. Transformation from a (5u)(53), to a (6.5'.4) l-10. (6.5'?)(6.5.4) tetrahedral network must occur by a recon- - (1982)The structural propertiesof syntheticgehlenite, CarAlrSiO,. 143, 254-267 . structive mechanism. Neues Jahrbuch fiir Mineralogie Abhandlungen, Kimata, M., and Ohashi, H. (1982) The crystal structure of synthetic The sequenceof CarZrGerO, polymorphs with de- gugiaite,CarBeSi,O,. Neues Jahrbuch fiiLrMineralogie Abhandlungen, creasingtemperature thus is (50)(5')r, P42,m (melilite- 143.2rV222. type structure),(5') (5'L, P2, (stackingvariant of the mel- Liebau, F. (1985) Structural chemistry ofsilicates: structure,bonding, and ilite structure),possibly metastable,and (6.5'z.4)(6.5'z) classification.Springer-Verlag, Berlin. (1969) crystal structure ofhar- (6.5.4), (new (5a) (53), Louisnathan, S.J. The refinement ofthe P2,/n structure type). The two dystonite CarZnSirO,. Zeitschrift fiir Kristallographie, 130, 427437. stacking variants may transform into modulated struc- Malinovskii, Yu.A. (1984) Crystal structure of Ba'CuSi'O'. Soviet Phys- tures upon rapid cooling. Finally, it should be empha- ics Doklady, 29, 70G708. sizedagain that the (6.5r.4)(6.5r) (6.5.4)layer structure Malinovskii, Yu.A., Pobedimskaya,E.A., and Belov, N.V. (1976) Syn- german- is not the lock-in phase produced from a high-tempera- thesis and X-ray analysisof two new iron-containing barium ates BarFeGerO, and FerNaBauGerO'o'4(OH,H'O). Kristallografiya, ture phasewith the melilite structure transforming through 2r, 1195-1197. an incommensurately modulated structure into a com- Ochi, Y., Tanaka, K., Morikawa, H., and Marumo, F. (1982) The crystal mensurate superstructure;rather, it has an entirely dif- structure of SrrZnGe,O,. Kobutsngahu Zasshi (Journal of the Miner- ferent topology. alogicalSociety ofJapan), 15, 331-341. Ragimov, K.G., Chiragov, M.I., and Mamedov, Kh.S. (1980) Crystal RnrrnrNcss crrED structure of the new synthetic silicate KHoCoSirO'. Soviet Physics Doklady, 25, 583-584. Bakakin, V.V., Belov, N.V., Borisov, S.V., and Solovelwa, L.P. (1970) Raithlisberger,F. (1989) Zusammenhangzwischen Chemismus, Stabiliilit The crystal structure of nordite and its relationship to melilite and und struklureller Variation der Melilite. Ph.D. thesis, Universitiit Bay- datolite-gadolinite.American Mineralogist, 55, I 167-1 181. reuth, FRG. Brown, LD. (1981) The bond valencemethod: An empirical approachto Schomaker,V., and Marsh, R.E. (1983) On evaluating the standard de- chemical structure and bonding. In M. O'Keeffe and A. Navrotsky, viation of U*. Acta Crystallographica,A39, 819-820. Eds , Structure and bonding in crystals II. Academic Press, New York. Seifert, F., Czank, M., Simons, B., and Schmahl, W. (1987) A cornmen- Dal Negro, A., Rossi, G., and Ungaretti,L. (1967) The crystal structure surate-incommensurate phase transition in iron-bearing akermanites. of meliphanite. Acta Crystallographica,23, 26C-264. Physicsand Chemistry of Minerals, 14, 26-35. Enraf Nonius ( I 983) Structuredetermination package(SDP). Enraf Noni- Shetdrick, G.M. (1976) SHELX76. Program for crystal structure deter- us, Delft, The Netherlands. mination. University of Cambridge, England. Grice, J.D., and Hawthorne, F.C. (1989) Refinementof the crystal struc- - (1986) SHELXS-86. Fortran-77 progmm for the solution ofcrystal ture of leucophanite.Canadian Mineralogist, 27, 193-197. structures from diffraction data. Institut fiiLr Anorganische Chemie der Hemingway,8.S.,Evans, H.T., Nord, G.L., Haselton,H.T., Robie,R.A., Universilal Gdttingen, FRG. and McGee, J.J. (1985) Akermanite: Phasetransitions in heat capacity Smith, J.V. (1953) Reexamination of the crystal structure of melilite. and thermal expansion, and revised thermodynamic data. Canadian American Mineralogist, 38, 643-661. Mineralogist, 24, 425434. Zucker, U.H., Perenthaler,E., Kuhs, W.F., Bachmann, R., and Schulz, Kimata, M. (1983a)The structural propertiesof syntheticSr-akermanite, H. (1983) PROMETHEUS: A program systemfor investigation of an- SrrMgSirO,. Zeitschrift fiir Kristallographie, l 63, 295-3O 4. harmonic thermal vibrations in crystals. Joumal of Applied Crystal- -(1983b) The crystal structure and stability of Co-akermanite, lography,16.35E. Ca,CoSi,O,, comparedwith the mineralogicalbehaviow of Mg cation. Neues Jahrbuch fiir Mineralogie Abhandlungen, 146, 221-241. - ( I 984) The structural properties ofsynthetic Sr-gehlenite,SrrAl'SiO'. M,c.NuscnrprREcETvED Ocrossn 30, 1989 Zeitschrift fiiLrKristallographie, 167, 103-1 16. Mexuscnrr"r ACCEFTEDApnIr 10, 1990